Compost Calculator (C:N Ratio & Finished Volume)

Greens, browns, and the 30:1 target

Compost microbes want carbon and nitrogen in balance. Extension teaching aims for roughly 25–30:1 carbon to nitrogen by the mix. Greens (grass, food scraps, fresh manure) are nitrogen-rich; browns (leaves, straw, chips, cardboard) are carbon-rich. This checker weights feedstock C:N ratios by volume parts — a planning shortcut, not a lab assay. Example: 2 parts dried leaves (C:N 60) plus 1 part grass (C:N 17) averages about 45.7 — too brown — so add greens such as grass clippings until you land in the target band.

The feedstock table

Representative ratios in our data: grass 17, food scraps 15, fresh manure 15, dried leaves 60, straw 80, wood chips 400, shredded cardboard 350. Chips and cardboard are extreme browns — use them as a minority or they stall the pile. Fresh manure is hot chemistry; keep it in a managed hot pile and follow waiting periods before food crops.

Pile size and the one-third rule

Starting volume shrinks as water leaves and organics mineralize. We estimate finished compost at one-third of the starting pile (inside the common 30–40% band). A 3×3×3 ft pile is 27 cu ft — about a cubic yard — a practical minimum for a hot pile that holds heat. Smaller heaps still make compost; they just run cooler and slower.

Hot vs cold timelines

Weekly turns and good moisture (wrung-out sponge) can finish in a couple of months in warm weather. Neglected cold piles may take half a year or more but still yield useful humus. Smell of ammonia or rotten eggs usually means too wet or too green — add browns and aerate. A pile that never heats often needs more greens, more mass, or water. Cover the top in heavy rain so nutrients do not wash into the driveway; uncover or fluff when the center feels like a wrung sponge again.

Using finished compost

Screen chunky bits back into the next pile. Top-dress gardens about an inch (see the soil amendment calculator for cubic feet), or blend into potting mixes. Skip meat, dairy, and diseased plants in home piles. Chicken manure belongs in a hot pile with enough browns — treat it as a green/nitrogen source, not a free mulch against lettuce stems. Finished compost should smell earthy, not sour; if it heats again in a bag, it was not quite done — spread it thin and let it finish in place.

Troubleshooting tip: if your C:N checker says “add greens” but the pile is slimy, fix moisture and air first — drowning greens do not help. If it says “add browns” and the pile is dry and cold, water lightly as you layer leaves so microbes can work the new carbon. Keep a simple log of what you added; next season you will remember whether the “too brown” warning matched reality. Share surplus finished compost with neighbors — it beats sending leaves to the landfill.

Frequently asked questions