How the compost ratio works
A healthy pile balances nitrogen-rich greens (kitchen scraps, fresh grass) with carbon-rich browns (dry leaves, cardboard, straw). At a common 3:1 browns-to-greens volume ratio, 2 cu ft of greens needs 2 × 3 = 6 cu ft of browns, for an 8 cu ft pile in total.
Volume ratios are a practical stand-in for the carbon-to-nitrogen balance microbes actually want, roughly 30:1 by weight. Two more examples: 4 cu ft of greens at 2:1 needs 8 cu ft of browns, and 3 cu ft at 3:1 needs 9 cu ft.
The cubic feet calculator is the natural next step once you have this figure.
Why browns outweigh greens
Browns supply carbon and air pockets that keep the pile from going slimy and smelly, while greens drive the heat and decomposition. If your pile smells, add more browns. Sizing the bed those scraps came from? Use the plant spacing calculator.
Materials vary, so treat the ratio as a starting point. Fresh grass clippings and coffee grounds are much wetter and more nitrogen-dense than vegetable peelings, while sawdust and cardboard are extremely carbon-heavy and need extra moisture to break down. A pile smaller than about 27 cubic feet (3 × 3 × 3 ft) struggles to hold heat, which slows everything down in cold weather.
The square feet to acres calculator answers the question that usually follows this one.
How to use this calculator
Enter your greens volume in cubic feet and your target ratio (3 is a good default). The result shows browns needed plus the total pile size. Need finished compost as fill? See the soil calculator.
Shred bulky browns first; smaller pieces can halve the time to finished compost. Aim for damp-sponge moisture and turn the pile every week or two.
Frequently asked questions
- How many browns for 2 cu ft of greens?
- At a 3:1 ratio, 6 cubic feet of browns, making an 8 cubic-foot pile.
- What ratio should I use?
- By volume, roughly 2:1 to 3:1 browns to greens works well for most home compost piles.
- What counts as greens and browns?
- Greens are wet and nitrogen-rich (scraps, grass); browns are dry and carbon-rich (leaves, cardboard, straw).
- My pile smells — what do I do?
- It usually means too many greens. Mix in more browns and turn the pile to add air.
- How long does compost take to finish?
- A balanced pile that is turned regularly can finish in two to three months in warm weather, while a cold pile left alone may take a year. Shredding materials and keeping the pile above 27 cubic feet speeds it up noticeably.