Compost Ratio Calculator

A ratio above 35:1 means the pile is carbon-heavy relative to nitrogen, which slows down the microbial activity that drives decomposition — a straw or sawdust-dominated pile can sit for many months with little visible breakdown. Adding more green material like grass clippings or food scraps, as the calculator's suggestion specifies, brings the blend closer to the faster-decomposing range.

An ammonia smell is a classic sign of a low C:N ratio, meaning too much nitrogen-rich green material relative to carbon-rich brown material. The excess nitrogen is off-gassing rather than being consumed by microbes, and mixing in more dry leaves, straw, or cardboard usually resolves the smell within a turn or two.

Composting is a process of decomposition and volume reduction — microorganisms break down organic matter, water evaporates, and the pile compacts as larger pieces collapse into finer material. The roughly 40% finished-volume estimate reflects that a full compost bin typically yields well under half its starting volume once the process completes.

It's a widely used general target for hot, actively managed compost piles aiming for the fastest breakdown, but slower cold-composting methods can tolerate a wider range without failing outright. If you're not turning the pile regularly or tracking temperature, the ratio matters less than simply keeping the pile moist and aerated over a longer timeline.

They're representative averages for each material category, but real-world values vary — fresh grass clippings from different lawns, or cardboard with more or less ink and tape, can shift several points from the reference number used here. Treat the resulting blended ratio as a reasonable estimate for adjusting your mix rather than a lab-precise measurement.