Compost Calculator

How the Compost Calculator works

Compost calculator: volume of a bin or heap

Composting is a volume game. A heap needs to be large enough to hold its own heat, fed with the right balance of carbon-rich browns and nitrogen-rich greens, and you need to know roughly what will come out the other end before you plan the beds it will feed. This calculator handles the arithmetic for all three.

Volume and heat. The usual rule is that a heap needs at least one cubic yard, or roughly one cubic metre (a 1 × 1 × 1 m bin), to insulate its centre and reach the 55 to 65 °C that kills weed seeds and breaks material down in weeks rather than a year. Smaller bins still work but stay cool, so they take longer and should not be given perennial weed roots or diseased material. The result flags whether your bin passes the 0.76 m³ threshold. Measure the inside of the bin and the height you actually fill to, not the height of the sides.

Greens and browns. Micro-organisms want roughly 25 to 30 parts carbon for each part nitrogen by weight. Greens such as grass clippings, kitchen scraps and manure are around 15 to 25:1; browns such as dry leaves, straw and cardboard run from 50:1 to several hundred. Mixing them by volume at about two or three parts browns to one part greens lands close to the target without weighing anything. The calculator splits the bin volume at your chosen ratio; use 2:1 for leafy browns and 3:1 for woody or paper browns. Too many greens gives a slimy, smelly heap; too many browns gives a dry heap that does nothing.

How much you get. Raw material shrinks by about half as water evaporates and carbon is respired away as CO₂, and often more if the browns were fluffy. The finished-compost line uses a 50 percent yield. Sieved, mature compost weighs around 500 to 600 kg per cubic metre when damp, which is the figure behind the weight estimate.

Where it goes. A 5 cm (2 inch) mulch is a good annual dressing for vegetable beds and borders, so the coverage line divides the finished volume by that depth. Halve the depth to cover twice the area with a lighter dressing, or use the raised bed calculator to see how many beds it will help fill at a 60/40 topsoil and compost mix.

Using the table. The carbon to nitrogen figures are typical ranges; they vary with moisture and age. Fresh leaves count as green, dry autumn leaves as brown. Sawdust and wood chips are so carbon-rich that a little goes a long way and they should be mixed with plenty of greens or left to rot separately.

Compost bin size table

Each bin is 1 m wide and 1 m high, with only the length changing.

Bin volume and finished compost by bin length, 1 m wide and high

Compost worked examples

The formula: bin volume = length × width × fill height; greens = volume ÷ (1 + browns ratio) and browns = volume − greens; finished compost ≈ volume × 0.5.

First example. A bin 1 m × 1 m × 1 m holds 1 m³, the usual minimum to compost hot. At a 2:1 brown to green ratio that is 0.67 m³ brown and 0.33 m³ green.

Second example. A 330-litre plastic dalek bin holds 0.33 m³, too small to heat up fully, and yields roughly a third to half its volume as finished compost.

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