Compost Ratio Calculator

The microorganisms that break down organic matter need carbon for energy and nitrogen to build protein. When the ratio between the two is right, they multiply fast enough to generate heat. A hot pile (one that reaches 130 to 160 degrees) breaks down faster, kills weed seeds, and produces a more finished product. A cold pile is still composting. It will get there. It just takes longer, often a year or more.

The target C:N range for a hot pile is roughly 25:1 to 35:1. Below that and you have too much nitrogen. The pile may smell of ammonia and will not heat efficiently. Above that and you have too much carbon. Decomposition slows to a crawl because the microbes do not have enough nitrogen to reproduce. Most of the common mistakes on both ends are fixable just by adjusting what goes in.

During peak growing season, regular trimming is part of managing a productive garden. Deadheading, suckering, removing yellowing leaves, pinching back overgrown stems. All of it happens on a weekly to biweekly schedule when plants are growing fast. Fast growers like vertical zucchini generate a steady supply of removed leaves and pruned laterals throughout summer, sometimes every week when the vine is running hard. That material goes straight to the pile as greens.

Fresh trimmings from healthy plants have a C:N ratio in the range of 15 to 25, depending on how woody the stems are. Most fall squarely in the greens category. The exception is end-of-season cutbacks, when stalks have dried and hardened. Those lean toward browns. A simple check: fresh and pliable is green, dried and brittle is brown. Large woody stems from tomatoes or peppers should be chipped or left to dry before adding, or they will take a long time to break down whole.

Regular trimming also matters for plant health. Removing crowded or interior growth improves airflow through the canopy, which is one of the primary ways to reduce fungal disease pressure. The trimming habit and the compost habit reinforce each other: the garden produces the raw material, and the pile turns it back into something the garden can use.

Most things from a vegetable garden or kitchen are fair game. A few categories cause real problems. Meat, fish, bones, dairy, and cooking fats attract pests and create serious odor before they break down — even small amounts are enough to draw animals. Salty foods like chips or pickles are toxic to worms at the concentrations that form in a pile, and bread tends to attract fruit flies, so bury it deep or skip it.

Diseased plant material is the other main one. If the plant had powdery mildew, late blight, or anything that spread fast through the bed, bag it rather than composting it. Disease spores can survive a pile that never gets fully hot, and plants that have gone to seed carry the same risk. A hot pile handles both; a cold one does not. Same logic applies to invasive plants — if you are not confident the pile heats consistently, do not risk it.

Treated wood and sawdust from treated lumber belong nowhere near a pile. The preservatives are toxic to soil organisms and persist in the finished compost. Sawdust from untreated firewood is a high-carbon brown and works well. Pet waste from dogs and cats carries pathogens; chicken manure composts quickly and is fine.

A compost pile needs mass to hold heat. The minimum for reliable hot composting is roughly 3 feet by 3 feet by 3 feet, about 1 cubic yard. Smaller than that and the pile loses heat faster than it generates it. Larger piles can work but require more turning to move air to the center.

The pile should be as damp as a wrung-out sponge. Squeeze a handful and you should get a few drops, not a stream. Too dry and microbial activity stalls. Too wet and the pile goes anaerobic (airless) and starts to smell. In dry weather, water the pile when you turn it. In wet weather, cover it or build it under a simple roof.

Turning introduces oxygen, which the aerobic microorganisms need to stay active. In an active hot pile, turn every three to five days and the pile will reheat after each turn as the microbes get fresh air and unexposed material. A pile managed this way can produce finished compost in six to eight weeks. A pile that is never turned will still break down but may take a year or more.

Finished compost looks like dark, crumbly soil and smells like earth, not rot. If you can still identify what went in (if there are recognizable stems, peels, or chunks), it is not done. Sift it and put the unfinished pieces back in the next pile.