Free compost ratio calculator. Enter your greens and browns to get your pile's C:N ratio and the exact amounts to add for the ideal 30:1 mix.
Your results will appear here.
Enter your values and press Calculate.
The calculator applies the Cornell mixture formula: each material's wet weight is converted to dry weight (greens ~80% moisture, browns ~20%), then the pile's C:N ratio is the dry-weight-weighted average of the two materials' ratios. If the ratio misses the 30:1 target, the fix-it solver works backwards to compute the exact kilograms of greens or browns needed to correct it. Results are planning estimates, not professional agronomy advice.
Composting microbes need two things: carbon for energy and nitrogen to build protein. The carbon-to-nitrogen (C:N) ratio of your pile decides whether they thrive or stall.
The ideal starting ratio is about 30:1 - thirty parts carbon to one part nitrogen. Below ~20:1, excess nitrogen escapes as ammonia and the pile smells; above ~40:1, microbes run out of nitrogen and the pile sits cold and barely decomposes.
Think of it like a recipe: 'greens' (grass clippings, kitchen scraps, coffee grounds) supply nitrogen, 'browns' (dry leaves, straw, cardboard) supply carbon. Getting the ratio right is the difference between finished compost in 8 weeks and a smelly pile that never heats up.
The calculator uses the Cornell mixture formula, the same one university composting programs teach:
For each material: dry weight = wet weight × (1 - moisture). Carbon = dry weight × %C, nitrogen = dry weight × %N. Pile C:N = total carbon / total nitrogen.
Because the calculator asks for each material's C:N ratio directly, it simplifies this to a dry-weight-weighted average: pile ratio = (dry_greens × ratio_greens + dry_browns × ratio_browns) / total dry weight. Typical moisture is assumed: greens ~80% water, browns ~20%.
Worked example - 5 kg vegetable scraps (20:1) + 10 kg dry leaves (60:1): dry greens = 5 × 0.2 = 1 kg, dry browns = 10 × 0.8 = 8 kg. Ratio = (1 × 20 + 8 × 60) / 9 = 500 / 9 = 55.6:1 - too much carbon, so the pile needs greens.
The fix-it solver then works backwards: to reach 30:1, add dry greens = total_dry × (ratio - 30) / (30 - greens_ratio), converted back to wet weight. For the example: 9 × 25.6 / 10 = 23 kg dry = 115 kg of wet scraps. A lot - which is exactly why most home piles run carbon-heavy.
Greens are nitrogen-rich, moist materials; browns are carbon-rich, dry materials. Typical C:N ratios:
Greens: grass clippings 15-20:1, kitchen scraps 15-20:1, coffee grounds ~20:1, fresh manure 10-20:1. Browns: dry leaves 50-80:1, straw 40-100:1, shredded cardboard/newspaper 200-560:1, sawdust and wood chips 200-500:1.
The classic rule of thumb is 2-3 parts browns to 1 part greens by volume - but volume lies, because fluffy leaves and dense sawdust weigh very differently. That is why this calculator works in kilograms and each material's own C:N ratio: enter what you actually have, not a guess.
If your material is not on the list, use the closest match or the defaults (greens 20:1, browns 60:1) - the answer stays a useful estimate either way.
Weigh (or estimate) your greens and your browns in kilograms and enter them with their C:N ratios - the defaults (20:1 greens, 60:1 browns) suit most kitchen-scrap-and-leaf piles.
The calculator shows your pile's current C:N ratio and its status. If it is off target, it tells you exactly what to add and how much: 'Add 6.0 kg of greens' rather than a vague 'add more greens'. Re-run the numbers after adding to confirm you have landed near 30:1.
Tip: build the pile in alternating layers of greens and browns rather than dumping one big load of each - layering keeps air flowing and starts heating faster.
Ratio too high (too much carbon) - symptoms: the pile never heats up, decomposition crawls, materials look unchanged after weeks. Fix: add nitrogen-rich greens (grass clippings, kitchen scraps) in the amount the calculator gives, and turn the pile to mix.
Ratio too low (too much nitrogen) - symptoms: ammonia or rotten smell, slimy wet texture, flies. Fix: add carbon-rich browns (dry leaves, shredded cardboard), fluff the pile to add air, and stop adding greens until the smell clears.
Moisture mimics ratio problems: a pile below ~30% moisture stalls like a carbon-heavy one, and above ~65% it goes anaerobic and smells like a nitrogen-heavy one. Check moisture before blaming the ratio - squeeze a handful; it should feel like a wrung-out sponge with a drop or two of water.
Ratio gets the glory, but moisture does half the work. The sweet spot is 50-60% moisture - the famous 'wrung-out sponge' test.
Below 30%: bacteria go dormant and the pile stalls, whatever the ratio. Fix: water it in layers as you turn. Above 65%: air is squeezed out, anaerobic microbes take over, and you get the rotten-egg smell. Fix: add dry browns and turn to aerate.
Rain is the usual culprit for wet piles - cover the pile in wet seasons. In dry climates, a pile under a tree can dry out in days; water it when you turn it.
Hot composting (55-65 C core temperature) finishes in 6-8 weeks and kills most weed seeds and pathogens - but only if the pile is at least ~1 cubic meter, has the right 30:1 ratio, and gets turned every week or two. This calculator is tuned for hot composting.
Cold composting (just piling it up and waiting) works with any ratio and no turning, but takes 6-12 months and will not kill weed seeds. If you are cold composting, the ratio still matters for smell and speed, just less urgently - aim roughly for the same 30:1 and let time do the work.
Keep these out of a home pile whatever the ratio: meat, fish, dairy and oily foods (smell + pests + pathogens), pet waste from dogs and cats (parasite risk), diseased plants (home piles do not reliably get hot enough to kill them), and treated or painted wood (chemicals).
Eggshells, coffee filters, plain cardboard, yard trimmings and vegetable scraps are all fine. When in doubt, the question is not the C:N ratio - it is whether the material brings pests, pathogens or chemicals. Leave those out.
Use this quick reference table of common compost materials when planning your pile. Green materials (nitrogen-rich): fresh grass clippings 20:1, coffee grounds 20:1, vegetable scraps 25:1, fresh leaves still green 25:1, alfalfa hay 12:1. Brown materials (carbon-rich): dry autumn leaves 60:1, straw 80:1, shredded newspaper 175:1, cardboard 350:1, wood chips and sawdust 400:1. Aim for an overall mix near 30:1.
To calculate your compost ratio from your ingredients, add up the carbon of all ingredients and divide by the total nitrogen. Example: 3 kg of dry leaves (carbon ~45%, nitrogen ~0.75%) plus 1 kg of coffee grounds (carbon ~50%, nitrogen ~2.5%) gives roughly (1.35 + 0.5) / (0.0225 + 0.025) = 1.85 / 0.0475 = about 39:1, slightly carbon-heavy, so add a little more green material.
About 30:1 - thirty parts carbon to one part nitrogen. That is the sweet spot where composting microbes have enough of both to work fast. Below ~20:1 the pile smells of ammonia; above ~40:1 it stays cold and decomposes very slowly.
Roughly 2-3 parts browns to 1 part greens by volume. But volume is unreliable - fluffy leaves and dense sawdust weigh very differently - so this calculator works in kilograms with each material's own C:N ratio for a real answer.
Add carbon-rich browns: dry leaves, straw or shredded cardboard. Enter your current mix in the calculator above and it will tell you exactly how many kilograms of browns to add to reach 30:1. Fluff the pile as you add them to restore airflow.
It stays cold and decomposes very slowly, because microbes run out of nitrogen. Add nitrogen-rich greens - grass clippings, kitchen scraps or coffee grounds - in the amount the calculator recommends, and turn the pile to mix them in.
With the right 30:1 ratio, moisture and turning, hot composting finishes in 6-8 weeks. Cold composting (pile it and wait) takes 6-12 months. A wrong ratio, dry pile or no turning can stall either method.
For hot composting, yes - every 1-2 weeks to add oxygen, which keeps the aerobic microbes working and prevents smells. For cold composting, turning is optional; it just speeds things up.
No - meat, dairy and oily foods attract pests, smell, and can harbor pathogens that home piles do not reliably kill. Stick to vegetable scraps, coffee grounds, eggshells, yard waste and plain cardboard.
Squeeze a handful: it should feel like a wrung-out sponge, releasing a drop or two of water. Too dry and the pile stalls; dripping wet and it goes anaerobic and smells. Aim for 50-60% moisture.
The green to brown ratio for composting refers to nitrogen-rich green materials (grass, coffee grounds, scraps) versus carbon-rich brown materials (dry leaves, straw, cardboard). By volume, most gardeners use roughly 1 part green to 2-3 parts brown to land near the ideal 30:1 C:N ratio. If the pile smells, add brown material; if it is cold and inactive, add green material.