Compost Calculator
Balance your green and brown materials for the ideal 25-35:1 carbon-to-nitrogen ratio.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator checks whether your compost pile has the right carbon-to-nitrogen (C:N) ratio for efficient decomposition. The ideal range is 25-35:1. Here is how to use it:
- Choose your green material. Greens are nitrogen-rich: grass clippings, fresh food scraps, coffee grounds, and animal manure. They feed the microbes that break down your compost.
- Enter your green amount. Use consistent units (cups, buckets, or pounds). The calculator uses the ratio between green and brown amounts, not the absolute values, so any unit works as long as you use the same one for both.
- Choose your brown material. Browns are carbon-rich: dry leaves, straw, cardboard, wood chips, and paper. They provide structure and energy for the composting process.
- Enter your brown amount. The calculator shows your resulting C:N ratio and whether it falls in the optimal 25-35:1 range. Adjust the amounts until the ratio is optimal.
About the Compost Calculator
The carbon-to-nitrogen ratio describes how much carbon material exists relative to nitrogen material in your compost pile. A ratio of 25-35:1 is optimal because microorganisms that decompose organic matter consume about 25-30 parts carbon for every 1 part nitrogen. If the ratio is too low (too much nitrogen), the pile becomes slimy and smelly. If the ratio is too high (too much carbon), decomposition slows dramatically because the microbes are starved for nitrogen.
Different materials have very different C:N ratios. Food scraps and fresh grass clippings are nitrogen-rich at around 15-20:1. Dry leaves are a moderate brown at 60:1. Cardboard and wood chips are extreme browns at 350-400:1 and need a lot of nitrogen-rich material to balance them. The calculator uses a weighted average of your chosen materials.
Frequently Asked Questions
What is the ideal carbon-to-nitrogen ratio for compost?
The ideal C:N ratio for active composting is 25-35:1. At this ratio, microbial activity is at its peak and the pile heats up quickly, killing weed seeds and pathogens. Below 20:1 (too much nitrogen), the pile produces ammonia gas and develops an unpleasant smell. Above 40:1 (too much carbon), decomposition slows to a crawl and can take years instead of months. Finished compost that is ready to use has a C:N ratio of around 10-15:1 because microbes consume much of the carbon during decomposition.
How much brown material should I add to grass clippings?
Grass clippings have a C:N ratio of about 20:1, which is already close to optimal. Adding dry leaves (60:1) in a roughly 3:1 green-to-brown ratio produces a C:N ratio around 30:1. In practical terms, for every bucket of fresh grass clippings, add about a third of a bucket of dry leaves. If you are using straw (75:1) instead of leaves, a 4:1 ratio of grass to straw works well. Pure grass clipping piles tend to mat and smell; mixing in some browns prevents this.
Can I compost cardboard?
Yes, cardboard is an excellent compost ingredient, but its extreme C:N ratio of around 350:1 means you need a lot of nitrogen-rich material to balance it. Remove tape, staples, and glossy coatings first. Shred or tear cardboard into small pieces to speed decomposition. Plain brown cardboard works well; avoid heavily inked or coated cardboard. In a standard compost bin, use cardboard sparingly as one of several brown materials, balancing it with large amounts of food scraps or fresh green material.
How do I know when my compost is ready?
Finished compost is dark brown, crumbly, and smells like fresh earth or a forest floor. You should not be able to identify the original materials. It should not be hot anymore (finished compost has cooled) and should not smell like ammonia or rotten material. A simple test is to put some in a sealed plastic bag for a week. If it smells bad when opened, it is not finished and needs more time. If it smells earthy, it is ready. Most active compost piles take 2-3 months in summer and longer in cold weather.