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Compost Mix Balancer
Enter the weight of each material in your pile in pounds. Your carbon-to-nitrogen ratio updates as you type, with a fix whenever it drifts off target.
Your pile
Weights in pounds. Leave anything you are not adding at 0.
Greens — nitrogen-rich
Browns — carbon-rich
How this is calculated
Like a lab worksheet — every step is shown so you can check the working:
- Weighted C:N ratio:
(w₁×r₁ + w₂×r₂ + …) ÷ (w₁ + w₂ + …)— each material’s weight in pounds times its C:N ratio, divided by the total weight. - Worked example: 10 lb kitchen scraps (20:1) + 30 lb dry leaves (60:1) →
(10×20 + 30×60) ÷ 40 = 2000 ÷ 40 = 50:1— too carbon-heavy. - Target zone: 25–30 parts carbon to 1 part nitrogen — the range where composting microbes work fastest.
- Fix for a low ratio (below 25):
dry leaves to add = (27 × total weight − weighted sum) ÷ (60 − 27). - Fix for a high ratio (above 30):
grass clippings to add = (weighted sum − 27 × total weight) ÷ (27 − 20).
What the ratio means
Composting microbes eat carbon for energy and use nitrogen to build protein. Give them roughly 25–30 parts carbon for each part nitrogen and they multiply fast, heating the pile and breaking material down in weeks rather than months.
Below 25:1 the pile turns slimy and smells of ammonia — add browns. Above 30:1 the pile sits cold and barely shrinks — add greens.
Sources & assumptions
Material C:N values are typical figures published by university extension composting guides (for example, Cornell’s composting resources and US cooperative extension factsheets); real batches vary with moisture, age and source. The calculator uses the common home-gardener method — a weight-weighted average of ratios — which is a planning estimate, not a laboratory analysis. The 25–30:1 target is the widely recommended range in extension composting literature.
New to composting?
Greens, browns, bins and turning schedules — the full beginner walkthrough.
Read the beginner’s guideCompost ratio FAQ
What is a good carbon-to-nitrogen ratio for compost?
Aim for 25–30 parts carbon to 1 part nitrogen by weight. In that range the microbes that do the composting have the balanced diet they need, so the pile heats up quickly and breaks down in weeks rather than months.
What happens if my pile has too much nitrogen?
Below about 25:1 the pile turns slimy, smells of ammonia and attracts flies, because the microbes cannot use all the nitrogen. Mix in browns — dry leaves, straw or shredded cardboard — and the smell usually fades within a day or two.
What happens if my pile has too much carbon?
Above about 30:1 the pile stays cold and barely shrinks, because the microbes run short of nitrogen. Add greens such as grass clippings, kitchen scraps or coffee grounds, and turn the pile to wake it up.
Should I measure materials by weight or volume?
This calculator uses pounds (weight), which is more accurate than volume because browns like leaves and straw are fluffy while greens like scraps are dense. If you only have buckets, weigh one full bucket of each material once and reuse those figures.
Are coffee grounds a green or a brown?
Coffee grounds are a green — roughly 20:1 — despite their brown colour. They heat a pile up nicely, so count them with your nitrogen-rich materials.
Can I compost meat, dairy and cooked food?
Best avoided in an ordinary home pile: they cause odours and attract rodents and other pests. Stick to fruit and vegetable scraps, coffee grounds, eggshells, leaves, straw, paper and cardboard, and save the rest for a municipal green-waste collection.