Water Filtration System Buying Checklist
A filter cannot remove what it was never chosen for. Buy from a test of your water and the contaminant you need gone, and the rest of the decision makes itself.
Kit coverage
0%
0 of 28 items handled · tick states are saved on this device only.
Identify the source and the concern
You cannot choose a filter until you know what you are filtering out.
- COREName the source: mains, bore, rain tank, dam or a mixed supply Amazon
- COREList the actual concerns: taste, chlorine, sediment, lead, hardness or microbes Amazon
- A laboratory or meter reading for anything you cannot taste, like nitrate or metals Amazon
- Whether the target is drinking water only or all water entering the house Amazon
Match the media to the contaminant
Each media removes a specific set of problems and nothing else.
- COREActivated carbon for taste, odour, chlorine and many organic chemicals Amazon
- Sediment cartridge for silt, rust and particles ahead of finer filters Amazon
- COREReverse osmosis membrane for dissolved solids, heavy metals and fluoride Amazon
- UV lamp for bacteria and viruses in a microbial-risk supply Amazon
Point-of-entry or point-of-use
Where the filter sits decides exactly what it protects.
- COREPoint-of-use under-sink or tap filter for drinking and cooking water only Amazon
- Countertop or pitcher filter where plumbing a system is not possible Amazon
- Point-of-entry whole-house filter for sediment, chlorine in every tap and shower Amazon
- COREA staged combination where one unit cannot cover both sediment and polish Amazon
Size flow rate and capacity
A filter sized too small throttles pressure; too big wastes money.
- COREPeak flow demand: the taps that may run at once, matched to the filter's rated litres per minute Amazon
- Rated capacity in litres before the cartridge needs changing Amazon
- COREWorking water pressure and any pressure drop across the unit Amazon
- Waste ratio for reverse osmosis: how much water goes to drain per litre made Amazon
Verify certification, not claims
A certification number is the only filter promise you can trust.
- CORECertification to a named standard for the exact contaminant you target Amazon
- The reduction percentage and the standard cited, not a general 'purifies' line Amazon
- COREFood-grade and potable-contact materials on every wetted part Amazon
- The specific model, not the brand family, appears on the certification listing Amazon
Price the ongoing cartridges
The unit is a one-off. The filters are a subscription for life.
- COREReplacement cartridge price and the change interval in months or litres Amazon
- Five-year cartridge cost worked out before you buy the unit Amazon
- CORECartridge availability and standard sizing versus a proprietary format Amazon
- Whether a blocked-filter indicator or pressure drop tells you when to change Amazon
Check the install realities
Plumbing, space, power and drain decide whether the plan survives delivery.
- COREUnder-sink or plant-room space measured for the housing and cartridge change Amazon
- Inlet and outlet plumbing size, and adapters for the entry point Amazon
- Power for UV or a pump booster, and a drain and space for reverse-osmosis waste Amazon
- A bypass valve or service fittings so a filter change is not a water outage Amazon
Water filtration is the one preparedness purchase people most often get backwards. They browse systems, compare finishes and prices, and only then ask what the filter actually removes. The correct order starts at the other end. Find out what is in your water, choose the media that targets it, and only then pick a unit.
There is no universal best filter. Activated carbon polishes taste, chlorine and some chemicals. A sediment cartridge catches rust and silt. Reverse osmosis strips dissolved solids and heavy metals. UV light inactivates microbes. Each is excellent at its job and useless at the others. Buying the wrong one means paying for water treatment you do not get.
Two quiet realities decide satisfaction after purchase. Certification gives you proof rather than promises, and cartridge cost turns a cheap unit into an expensive habit. A filter you stop changing because the replacements are dear is a filter that is quietly making your water worse.
This checklist runs the buying decision in the right order: identify the source and concern, match the media, choose point-of-entry or point-of-use, size flow and capacity, verify certification, price the ongoing cartridges, and finish with install realities. For keeping the system fresh after you own it see our water filter replacement checklist. Every buyable line links to current options on Amazon.com, and Spare Theory may earn a commission on those links. We never set or verify Amazon's prices.
At a glance
- A filter is chosen by contaminant, not by brand or price. Chlorine, sediment, lead and bacteria each need different media.
- Certification to a named standard is proof. A vague 'purifies water' claim is marketing.
- The cartridge is a subscription. Price the replacement filters over five years, not the tap price of the unit.
- A whole-house system and a under-sink filter solve different problems. Where you filter decides what you filter.
The full list
The same list the builder above ticks. Use it for printing or when JavaScript is off.
Identify the source and the concern
You cannot choose a filter until you know what you are filtering out.
- COREName the source: mains, bore, rain tank, dam or a mixed supply Amazon
- COREList the actual concerns: taste, chlorine, sediment, lead, hardness or microbes Amazon
- A laboratory or meter reading for anything you cannot taste, like nitrate or metals Amazon
- Whether the target is drinking water only or all water entering the house Amazon
Match the media to the contaminant
Each media removes a specific set of problems and nothing else.
- COREActivated carbon for taste, odour, chlorine and many organic chemicals Amazon
- Sediment cartridge for silt, rust and particles ahead of finer filters Amazon
- COREReverse osmosis membrane for dissolved solids, heavy metals and fluoride Amazon
- UV lamp for bacteria and viruses in a microbial-risk supply Amazon
Point-of-entry or point-of-use
Where the filter sits decides exactly what it protects.
- COREPoint-of-use under-sink or tap filter for drinking and cooking water only Amazon
- Countertop or pitcher filter where plumbing a system is not possible Amazon
- Point-of-entry whole-house filter for sediment, chlorine in every tap and shower Amazon
- COREA staged combination where one unit cannot cover both sediment and polish Amazon
Size flow rate and capacity
A filter sized too small throttles pressure; too big wastes money.
- COREPeak flow demand: the taps that may run at once, matched to the filter's rated litres per minute Amazon
- Rated capacity in litres before the cartridge needs changing Amazon
- COREWorking water pressure and any pressure drop across the unit Amazon
- Waste ratio for reverse osmosis: how much water goes to drain per litre made Amazon
Verify certification, not claims
A certification number is the only filter promise you can trust.
- CORECertification to a named standard for the exact contaminant you target Amazon
- The reduction percentage and the standard cited, not a general 'purifies' line Amazon
- COREFood-grade and potable-contact materials on every wetted part Amazon
- The specific model, not the brand family, appears on the certification listing Amazon
Price the ongoing cartridges
The unit is a one-off. The filters are a subscription for life.
- COREReplacement cartridge price and the change interval in months or litres Amazon
- Five-year cartridge cost worked out before you buy the unit Amazon
- CORECartridge availability and standard sizing versus a proprietary format Amazon
- Whether a blocked-filter indicator or pressure drop tells you when to change Amazon
Check the install realities
Plumbing, space, power and drain decide whether the plan survives delivery.
- COREUnder-sink or plant-room space measured for the housing and cartridge change Amazon
- Inlet and outlet plumbing size, and adapters for the entry point Amazon
- Power for UV or a pump booster, and a drain and space for reverse-osmosis waste Amazon
- A bypass valve or service fittings so a filter change is not a water outage Amazon
There is no best filter, only the right contaminant
The single most expensive mistake in water filtration is buying a system for a problem you do not have. A reverse-osmosis unit will not fix a supply that merely tastes of chlorine any more than a carbon block will remove dissolved salts.
Filters are defined by their media, and each media has a narrow field of work. Carbon adsorbs chlorine and many organic chemicals. Sediment cartridges strain particles. Reverse osmosis strips dissolved solids and metals. UV inactivates microbes. Nothing does all of it well.
So the buying starts with a question mark about your own water. Is the worry taste, a visible sediment, a lead in old pipework, or a bacterial risk from a tank or dam? That answer, not the shop shelf, chooses the media.
When you cannot name the contaminant, you cannot choose the filter honestly. A simple test or a meter reading turns a guess into a specification, and a specification makes the range of right products very small.
The taste test is data: A chlorine smell points at carbon. Gritty water points at sediment. A metallic taste in an old building points at lead and calls for reverse osmosis. Your senses narrow the field before a kit confirms it.
Choose the filter by the contaminant you actually have. Carbon, sediment, reverse osmosis and UV each do one job, and none does them all.
Match media to problem, then combine
Once the concern is known, pick the media that is certified to reduce it. If several concerns coexist, the honest answer is usually a staged system rather than one magic cartridge. A sediment pre-filter protects a carbon block, which protects a reverse-osmosis membrane.
That staging is not a complication. It is how real systems last. Sediment removed first means the finer filters behind it do not clog in a month. Coarse to fine is the correct order, and it makes every later stage cheaper to run.
Microbial risk is a separate branch. Carbon and reverse osmosis do not reliably sterilise water. If the supply could carry bacteria, from a rain tank, a dam or flood-water ingress, the answer is UV or a rated absolute-microfiltration membrane, plus a source test.
Be wary of any single unit that claims to remove everything. Good systems are honest about what each stage does. Read the specification stage by stage and check each stage against your list of concerns.
Coarse to fine: Always order stages from the biggest particles to the smallest. A cheap sediment cartridge in front of an expensive membrane is the best money in the system.
Pick the media certified for each concern, and stage them coarse to fine. Sediment first protects the finer filters and cuts the whole running cost.
Where you filter is what you filter
The placement decision is really a scope decision. A point-of-use filter on the kitchen tap treats only the water you drink and cook with. A point-of-entry filter on the incoming main treats everything, including showers, laundry and the hot side.
They solve different problems. If the worry is drinking-water quality, a point-of-use unit is simpler, cheaper and does not touch the whole house. If the worry is chlorine or sediment damaging appliances, skin or plumbing, only whole-house treatment helps.
A common smart pairing is a whole-house sediment and carbon stage to protect the building, plus a point-of-use reverse-osmosis unit at the kitchen tap for the drinking glass. Each stage does the job that suits its position.
Decide the scope before the model. Trying to solve a whole-house sediment problem with a tap filter, or a drinking-water concern with a shower filter, wastes the purchase entirely.
The hot-water trap: Many point-of-use filters fit the cold line only. Never route hot water through a carbon or reverse-osmosis unit that is not rated for it, and know which taps the filter actually covers.
Point-of-use treats your glass. Point-of-entry treats the house. Choose the scope first, and often the right answer is a stage at both.
Flow and capacity are a sizing problem
A filter that is too small for your flow quietly ruins the experience. Water pressure drops at the tap, the kettle takes forever, and a shower becomes a trickle. Fine media resists flow by nature, so the rated litres-per-minute has to meet your real peak demand.
Peak demand is the taps that might genuinely run together, not one trickle. For a drinking tap, a modest flow is fine. For a whole-house unit, the sizing has to cover a bath and a shower at once without choking them.
Capacity is the other half. Every cartridge is rated for a volume before it should be replaced. An undersized unit with a small capacity means changing filters constantly, and a filter you are always replacing is a filter you will be tempted to skip.
For reverse osmosis, add the waste ratio and the storage tank to the sizing. These units make purified water slowly and send a portion to drain, so the flow at the tap depends on the tank as much as on the membrane.
Pressure drop is normal, near-zero is not: Every fine filter drops some pressure. A reverse-osmosis system may need a booster pump to restore it. Check the rated drop before blaming your mains.
Match the filter's rated flow to your real peak draw and its capacity to a sane change interval. Undersizing gives you low pressure or endless cartridge swaps.
Certification is proof, not marketing
The filter market is full of soft claims. Words like purifies, restores and energise mean nothing you can verify. The one trustworthy signal is a certification to a named standard, for the specific contaminant, on the specific model number you are buying.
Independent certification bodies test a product against a defined protocol and publish a reduction percentage and a list of what was tested. That is a claim you can hold someone to. A picture of a mountain stream on the box is not.
Read the certification carefully. It names the model, the standard and the contaminant reduced. A brand family being 'certified' says little if the exact unit on your shortlist is not on the list. Match the model number, not the logo.
For any filter on drinking water, also confirm potable-contact and food-grade materials on the wetted parts and housings. Certification of the media is about performance; material grading is about the filter itself not adding to the problem.
Check the listing yourself: Certification bodies publish searchable product lists. Spend two minutes confirming your model appears there before paying. It filters the snake oil out instantly.
Trust only a certification naming the standard, the contaminant and your exact model. Vague purifying language is advertising, not evidence.
The cartridge is the real purchase
The unit you buy at the till is the smallest part of the cost. The filter cartridges are a subscription that runs for as long as you keep the system. Judge the purchase on the five-year cartridge bill, not the sticker on the housing.
This is where a cheap unit can be a trap. Proprietary cartridges that only one brand sells, at a premium, with no generic option, quietly tax you every few months. A system using standard, widely available cartridges stays cheap and easy forever.
Availability matters as much as price. If the correct cartridge is hard to source locally, you will run past its change date, and an expired filter is worse than none at all because it can harbour growth and pass the contaminants it captured.
Before buying, work the number: cartridge price multiplied by changes per year multiplied by five. Add it to the unit price. That total is what the system actually costs, and it should drive the choice between two otherwise similar units.
The abandoned filter: The most common filter failure is not a broken part. It is a filter nobody changed because the replacement cost too much or could not be found. Buy so that never happens to you.
Price the cartridges over five years, not the tap price of the unit. Standard, available filters keep the true cost low; proprietary ones do not.
Install realities before the box arrives
Plenty of perfect filtration decisions die at the plumbing. Before ordering, confirm the physical fit. Is there room under the sink or in the plant space for the housing and, crucially, for the room to change the cartridge, not just install the unit once?
Check the pipe sizes and the entry point. A whole-house filter needs a suitable spot on the incoming main, ideally after any pressure tank and before the hot system. Adapters, a bracket and a bypass valve are small extras that should be bought with the unit, not chased later.
Some technologies need services. UV requires power and a lamp-change plan. Reverse osmosis needs a drain for its waste stream and space for a storage tank. Confirm each is available at the chosen point before, not after, delivery.
Finally, fit a bypass or easy service fittings. A filter you cannot isolate is a filter that turns a routine cartridge change into a whole-house water shut-off. A two-dollar valve is the difference between a ten-minute job and an afternoon.
Measure the cartridge room: Room to install is not room to service. Leave vertical clearance above a canister to unscrew the cap and pull a wet filter out. Check that before you commit to the cupboard.
Confirm space, pipe size, power and drain before you buy. Fit a bypass valve so a cartridge change is never a water outage.
Media by contaminant and typical placement
| Concern | Media that works | Best placement | Verify by |
|---|---|---|---|
| Chlorine taste and odour | Activated carbon | Whole-house or point-of-use | Carbon certification |
| Silt, rust, particles | Sediment cartridge | Point-of-entry, first stage | Micron rating |
| Lead and heavy metals | Reverse osmosis | Point-of-use drinking tap | Standard + model listing |
| Dissolved solids, salt | Reverse osmosis | Point-of-use | TDS before and after |
| Bacteria and viruses | UV or absolute membrane | After finer filtration | Microbial test |
| Hardness and scale | Softener or scale control | Point-of-entry | Hardness test |
Confirm each product's certification covers your exact contaminant and model. No single medium removes every concern; combine stages coarse to fine.
Key terms
- Point-of-entry
- A filter fitted on the incoming main so it treats all water entering the house, including hot, shower and appliance lines.
- Point-of-use
- A filter at a single tap or appliance, usually the kitchen cold line, treating only the water drawn there.
- Reverse osmosis
- A semi-permeable membrane process that strips dissolved solids and heavy metals, producing purified water and a waste stream to drain.
- Absolute rating
- A filter's guaranteed maximum particle passage size, as opposed to a nominal average; the figure to trust for microbial control.
- TDS
- Total dissolved solids, measured in parts per million; a quick before-and-after number that shows whether a membrane is working.
- Waste ratio
- For reverse osmosis, the volume of water sent to drain for each litre of purified water produced; a running cost and plumbing constraint.
Tips and common mistakes
Field-tested tips
- Test or at least sample your water before you shop. A named contaminant turns an endless catalogue into three candidate products.
- Buy standard-size cartridges wherever possible. A 10-inch generic fits many housings and will always be in stock at a fair price.
- Fit a pressure gauge before and after the filter at install. A rising differential across a clean unit tells you the media is spent before flow drops.
- Write the cartridge change date and the model number inside the housing lid. It is the note you will need at 3am with a wrench.
Common mistakes
- Buying a filter for a contaminant the water does not have, chosen from the shelf instead of a test.
- Trusting vague purifying claims with no certification naming the standard, the contaminant and the model.
- Judging a system on the unit price and being taxed later by dear, proprietary or hard-to-find cartridges.
- Ordering a whole-house problem solved by a tap filter, or a drinking problem solved by a shower head, and covering the wrong scope.
Frequently asked questions
What is the best water filtration system for a house?
There is no single best; it depends on your water. For a mains supply that only tastes of chlorine, a certified carbon filter is enough. For old pipework with lead, a point-of-use reverse-osmosis unit at the drinking tap. For a tank or dam with microbial risk, UV plus staged filtration. Start with a test, name the contaminant, then pick the media certified to reduce it.
Do I need a whole-house filter or just an under-sink one?
It depends on the goal. If you only want better drinking and cooking water, an under-sink point-of-use filter is simpler and cheaper. If sediment or chlorine is harming appliances, plumbing, skin or laundry, you need point-of-entry treatment on the main. Many homes sensibly use a whole-house sediment and carbon stage plus a point-of-use unit at the kitchen tap.
Do water filter certifications actually matter?
Yes, they are the only reliable measure of performance. A certification to a named standard tests the product and publishes a reduction percentage for specific contaminants. Independent lists let you confirm your exact model was tested. Vague claims like purifies or restores are not evidence, so always match the model number to a certification listing before buying.
How much does a water filter cost to run?
Judge the running cost, not the purchase price. Multiply the replacement cartridge cost by the number of changes each year, then by the years you expect to keep the system. Proprietary cartridges can dwarf the unit price over five years, while standard-size filters stay cheap and available. A system you can afford to keep changing is the one that actually works.
Can one filter remove everything from my water?
No single media removes every concern well. Carbon handles chlorine and some chemicals, sediment cartridges catch particles, reverse osmosis strips dissolved solids and metals, and UV deals with microbes. Real systems often stage these coarse to fine, with a pre-filter protecting the finer ones. Any unit claiming to do everything should be checked against a certification for each contaminant you care about.
What size water filter do I need?
Size on flow and capacity, not on the number of people alone. Match the filter's rated litres per minute to the taps that may run together, and choose a capacity that gives a sensible change interval. Whole-house units need enough flow for a shower and a bath at once. Undersizing gives low pressure at the tap or constant cartridge changes, and both make a good filter frustrating.
About this checklist
- Prepared by
- Spare Theory editorial · research-based
- Last checked
- 2026-10-16
How we choose what earns a spotWe build the buying decision around contaminant first, media second, and unit last, following standard water-treatment selection practice. Certification guidance reflects independent product-certification schemes, and running-cost advice treats cartridges as the dominant lifetime expense. We name no brand as best, state no prices, and recommend laboratory testing and qualified installation for potable and plumbing work.
Reference basisNSF International and water filter certification standard summariesUS Environmental Protection Agency drinking water contaminant information