Compost Carbon-to-Nitrogen Ratio Calculator

How this calculator works

Microbes that break down compost need carbon for energy and nitrogen to build proteins. A mix with roughly 25 to 30 parts carbon to 1 part nitrogen by weight (the C:N ratio) heats and decomposes quickly. Each material you add has a typical C:N ratio and a typical moisture content. The calculator converts each pound to dry weight, then estimates the blend's overall ratio as total dry weight divided by the total of each material's dry weight over its own ratio. This assumes carbon is a similar share of the dry weight in most organic materials, a common simplification that works well enough for planning a pile.

A worked example

Ten pounds of fresh grass clippings (ratio about 17, 80 percent water) and ten pounds of straw (ratio about 80, 12 percent water) give about 2 pounds and 8.8 pounds of dry material. Dividing 10.8 by (2 ÷ 17 + 8.8 ÷ 80) gives a blend of about 47 to 1, which is too carbon-heavy and will break down slowly. Raising the grass to 40 pounds in total brings the blend to about 29 to 1, which is in the ideal range, so a lot of fresh greens is needed to balance a heavy load of straw.

Using the result

• Below 20: too much nitrogen, which can cause ammonia smells. Add browns.
• 25 to 30: ideal for fast composting.
• Above 40: slow decomposition. Add greens.

Moisture matters as much as ratio. A pile that is too dry stops working, and a pile that is soaking wet goes anaerobic and smells. A good test is to squeeze a handful: it should feel like a wrung-out sponge and release a drop or two of water.

Where these numbers come from

C:N ratios and moisture values are midpoints of the ranges published in university composting references, such as Cornell's composting resources, and they vary a lot by source and condition. Your actual materials may differ, so treat the result as an estimate. Do not compost meat, dairy, pet waste or diseased plants in a home pile.