Match the Bin to the Waste You Actually Make

Litres, not vibes: what the bin actually holds

The inner dimensions multiplied give the raw capacity, and that figure should govern every purchase decision. A bin described as one cubic metre is a thousand litres. The difference between that and the flimsy box on the garden-centre shelf is usually a dozen litres of self-delusion.

Measure the inside, not the marketing. Panel walls, angle braces and the lid's dead space all eat volume the brochure quietly forgets. The calculator takes your honest inner numbers and does not flinch.

Height is the most under-sized dimension. A heap under about a metre loses heat faster than it generates it. Short bins force the whole system to rely on ambient weather instead of its own metabolism. Full height in every direction is what makes a pile cook.

Then there is settling, the tax every heap charges. Eight hundred litres of spring material becomes five hundred by summer as air leaves and biology compacts. The capacity figure is therefore the volume you fill to, and the yield figure is what remains when the dust settles.

• Inner dimensions decide capacity. The brochure decides disappointment.
• A cubic metre means a thousand litres. Use the number before you buy.
• Expect roughly a third of raw volume as finished compost.

The browns tax: every green litre brings a partner

Green waste never arrives alone. Nitrogen-rich material needs carbon to balance it, and the working rule by volume is roughly two parts browns to one part greens. Forty weekly litres of grass therefore summons eighty of leaves, torn cardboard or straw.

That ratio rewrites the fill maths completely. The bin fills at greens times three, not at the grass rate alone. Gardeners who plan for the green volume only will find the heap a wet, reeking half-truth every single summer.

The browns budget is where autumn earns its keep. A bin of leaves harvested in October pays for three months of May's grass mulch. Gardens with no leaf supply need a cardboard or wood-shavings route instead, and that is a purchasing plan, not an afterthought.

Browns also set the ceiling on what the heap can accept at pace. A generous store of carbon makes every flush of grass a fixable recipe. A bare store makes every flush an emergency, which is why experienced composters treat browns with the respect the calculator gives them.

Fill time: the number that reveals your real season

Dividing capacity by weekly input converts the vague promise of composting into a calendar. Six hundred and forty litres at one hundred and twenty weekly fills in five weeks. In a garden with a lawn, that is an early June reality.

The reverse season is equally real. Winter's fifteen weekly litres needs forty-two weeks for the same bin, and the heap looks abandoned precisely when nobody can see it working. Both seasons are normal; only the expectation is adjustable.

This is the case for the two-bin system, made in numbers rather than gardening folklore. One bin fills while the other cooks. The fill-rate figure sizes each of the pair, and a full-and-resting rhythm beats the single bin's guilty half-empty cycle.

Where the calendar shows a genuine mismatch, change the shape of the system rather than forcing the habit. A garden that fills a cubic metre in three spring weeks may need exactly that capacity for the grass season. Nothing wrong with it is fixed by a smaller bin.

From full bin to finished compost, with the shrinkage honoured

The yield estimate is honest arithmetic with a range. Raw material loses the trapped air immediately, and the soft fractions digest down over months. What remains as stable compost sits near a third of the starting volume. Two thousand litres in is about six to seven hundred out.

That third explains the composter's eternal question: where did my bin go. It did not go anywhere. It worked. Gardeners who know the shrinkage in advance plan the next cycle instead of mourning the last one.

The finished volume still earns a final step: screening. A full bin of rough compost screens down to a finer tilthy fraction plus a reserve of half-rotten material. That reserve returns to the next heap rather than the bed. Expect the plant-ready amount to be a generous two-thirds of the shrunk figure.

Matching yield to demand closes the loop. A raised bed's annual top-up of ninety litres takes about three hundred litres of finished-from-raw compost. That makes the waste stream and the soil budget two ends of the same calculator. The raised-bed planner's figure is the demand side of that equation.

Assumptions, limits and the biology this maths cannot see

The model counts volumes, and volume is the right currency for bin sizing. It cannot taste the heap. Moisture, aeration, particle size and the actual carbon-nitrogen chemistry decide whether these litres cook or sulk. The calculator assumes you give them both the balance and the turning.

The browns ratio you enter is treated as a recipe, not a measurement. Real material varies wildly: wet leaves packed down are nothing like torn dry ones, and grass mowed after rain is nearly half water. Adjust the ratio by the feel of the heap, not just the figure on the bag.

The yield third is a rule of thumb that flatters slow heaps. Cold, neglected piles can finish closer to a quarter while hot, turned batches hit a half with good structure and shorter cycles. The estimate frames expectations; your own bin's history refines it.

Finally, this tool sizes one container. Multi-bay systems, tumblers and worm farms each bring their own geometry and processing speed. The worm-versus-tumbler matcher here covers the two bins biology does for you, which is a different calculation entirely.