What is the ideal C:N ratio for compost?

The ideal range is 25:1 to 30:1. Below 20:1 the pile may smell of ammonia and lose nitrogen; above 40:1 decomposition slows significantly because microbes run out of nitrogen.

What are "brown" and "green" materials?

Browns are carbon-rich materials like straw, cardboard, wood chips, and dry leaves (high C:N). Greens are nitrogen-rich materials like grass clippings, food scraps, and manure (low C:N).

How is the C:N ratio calculated?

The calculator estimates carbon mass and nitrogen mass for every material, then divides total carbon by total nitrogen. This mass balance is more accurate than averaging the individual C:N ratios. Preset percentages are approximate dry-matter values, so use lab-tested values with the custom option when available.

My ratio is too high — how do I fix it?

Add more nitrogen-rich (green) materials such as grass clippings, coffee grounds, or fresh manure to bring the ratio down toward 30:1.

My ratio is too low — how do I fix it?

Add more carbon-rich (brown) materials such as straw, dry leaves, or cardboard to raise the ratio toward 25:1.

Can I enter a custom material?

Yes. Select "Custom material" and enter its dry-matter carbon and nitrogen percentages. The calculator derives its C:N ratio and includes both elemental masses in the mixture total.

Compost Ratio Calculator

Add your compost materials and weights to instantly calculate the overall carbon-to-nitrogen ratio of your mix.

Add Material

Material

Dry Weight

Add at least one material to see the C:N ratio result.

Overall C:N Ratio

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: 1 nitrogen

Too low (smelly) Ideal zone Too high (slow)

0 25–30 ideal 80+

Materials

Total Weight

Browns : Greens

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Material Breakdown

Material	Weight	C:N	% of mix	Remove

Summary

The Compost Ratio Calculator helps gardeners and farmers achieve the ideal carbon-to-nitrogen (C:N) ratio in their compost pile. By entering each material and its dry weight, the tool estimates total carbon and total nitrogen, then divides those totals to calculate the mixture ratio. A ratio between 25:1 and 30:1 is considered optimal for fast, hot composting.

How it works

Select a compost material from the dropdown or choose "Custom" to enter carbon and nitrogen percentages.
Enter the dry weight (in pounds or kilograms) of that material.
Click "Add Material" to add it to your mix.
Repeat for all materials in your compost pile.
The calculator instantly shows the overall C:N ratio and whether it falls in the ideal range.
Adjust quantities or remove materials to fine-tune your mix.

Use cases

Balance brown (carbon-rich) and green (nitrogen-rich) materials for fast decomposition.
Troubleshoot a slow or smelly compost pile by checking the C:N ratio.
Plan a new compost batch before collecting materials.
Teach students or gardeners about the science of composting.
Optimize compost for specific end uses like vegetable gardens or lawn topdressing.
Compare different material combinations to minimize inputs.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu