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Compost ratio calculator

What is going in the pile

36.2:1

Too much carbon. At 36.2:1 this pile is carbon-heavy. It will break down, but slowly and without heating up much. Add a nitrogen-rich green.

Adding about 34.8 lb (10.4 gallons) of fresh grass clippings would bring it into the 25–30:1 range.

How are you measuring?
What each material contributes
MaterialOwn C:NWeightCarbonNitrogenShare of N
Dry autumn leaves (brown)30–80:120.05 lb8.181 lb0.1364 lb40.5%
Fresh grass clippings (green)9–25:133.42 lb4.01 lb0.2005 lb59.5%

The C:N column shows the published spread, not a single value. Cornell’s own wording is that its figures “should be viewed as representative ranges, not as universal values”, and Nebraska calls its table “only guidelines”. The arithmetic above uses the middle of each range; a pile built from the top of one range and the bottom of another will land somewhere else, which is why the target is a band rather than a number.

Ratios from the Cornell Waste Management Institute composting tables and the University of Nebraska-Lincoln, Garden Compost G2222. Materials marked as estimates appear in no allowed source.

A compost pile that will not heat up, or that smells of ammonia, almost always has a carbon to nitrogen problem. This calculator estimates the ratio of a pile from what you are putting in it, and works out how much of one material would bring it into range.

It does the arithmetic properly, which matters more here than it sounds. The common shortcut — averaging the C:N ratios of the ingredients — gives the wrong answer, often wildly wrong. See the second question below for why.

How to use it

  1. Choose how you are measuring. Volume is the realistic option for most people: buckets, or liters. Weight is there if you know it.
  2. Add each material from the dropdown and enter how much of it is going in. The note under each one flags anything worth knowing about handling it.
  3. Add as many rows as you need with the button underneath, and remove any you change your mind about.

The result gives the pile's ratio, a verdict against the 25–30:1 target band, and — when the pile is out of range — how much of a suitable green or brown would fix it.

How it is calculated

Not by averaging ratios. The pile's ratio is the total mass of carbon divided by the total mass of nitrogen, so each material has to be resolved into those two masses first:

dry mass = as-is mass x dry matter %
nitrogen = dry mass x nitrogen % of dry matter
carbon   = nitrogen x that material's own C:N ratio

Then:

pile C:N = total carbon / total nitrogen

That is why data/compost-materials.ts carries a nitrogen percentage and a dry matter percentage for every material as well as its C:N ratio. Volume entries are converted to weight first, using each material's bulk density — five pounds per cubic foot for dry leaves, forty for coffee grounds.

The suggested correction solves for the mass x of a chosen material that lands the pile on target:

(C + x.cx) / (N + x.nx) = target,  so  x = (C - target.N) / (target.nx - cx)

A worked example

Thirty pounds of dry leaves and ten pounds of fresh grass clippings.

Leaves: 30 lb as-is, 85% dry matter = 25.5 lb dry. At 0.8% nitrogen that is 0.204 lb of nitrogen, and at 60:1 that implies 12.24 lb of carbon.

Grass: 10 lb as-is, 25% dry matter = 2.5 lb dry. At 2.4% nitrogen that is 0.06 lb of nitrogen, and at 20:1, 1.2 lb of carbon.

  • Total carbon: 12.24 + 1.2 = 13.44 lb
  • Total nitrogen: 0.204 + 0.06 = 0.264 lb
  • Ratio: 13.44 / 0.264 = 51:1

Carbon-heavy, so it will be slow. Grass is three quarters water, which is why ten pounds of it contributes so little nitrogen by weight — and why a balanced pile needs a surprising amount of it.

Common materials

MaterialTypeC:N ratio
SawdustBrown500:1
Wood chipsBrown400:1
CardboardBrown350:1
Shredded newspaperBrown175:1
Pine needlesBrown80:1
StrawBrown75:1
Dry autumn leavesBrown60:1
Shrub trimmingsBrown50:1
Fruit wasteGreen35:1
Horse manure with beddingGreen25:1
Grass clippingsGreen20:1
Vegetable scrapsGreen20:1
Coffee groundsGreen20:1
Cow manureGreen20:1
SeaweedGreen19:1
Alfalfa mealGreen12:1
Chicken manureGreen7:1

Tips and common mistakes

Shred the browns. Whole leaves mat into layers that shed water; a run over with a mower first makes a visible difference to how fast a pile works.

Mix, do not layer. Neat alternating layers look tidy and compost unevenly. Turning the pile once it has cooled is what mixes it properly.

Aim for a wrung-out sponge. Moisture matters as much as the ratio. A dry pile stalls whatever its chemistry says, and a sodden one goes anaerobic.

Size matters for heat. Around a cubic yard is roughly the minimum for a pile to hold its own heat. Smaller piles still compost, just cold and slow.

Keep meat, dairy and oils out, and keep perennial weed roots and seed heads out unless the pile genuinely gets hot — a cool pile will happily propagate them for you.

Common questions

What is the ideal C:N ratio for compost?
Roughly 25 to 30 parts carbon to one part nitrogen. Below that band there is surplus nitrogen with no carbon to build into microbial bodies, so it escapes as ammonia and the pile smells. Above it, the microbes run short of nitrogen and the pile breaks down slowly without heating up. Both piles eventually become compost; the balanced one just does it in weeks rather than a year.
Why can I not just average the C:N ratios of my materials?
Because a ratio is not a quantity. Take equal weights of sawdust at 500:1 and coffee grounds at 20:1. Averaging the ratios gives 260:1. But the sawdust carries almost no nitrogen to average with, while the coffee grounds bring ten times as much, so the actual mix comes out around 62:1. Working from masses of carbon and nitrogen is the only way to get this right, and it is what this calculator does.
What counts as a brown and what counts as a green?
Browns are dry and carbon-rich: autumn leaves, straw, cardboard, sawdust, woody trimmings. Greens are moist and nitrogen-rich: grass clippings, vegetable scraps, manure, fresh weeds. Coffee grounds are a green despite the name and colour, which catches people out regularly. The material's ratio matters more than its appearance.
Is the two-parts-brown-to-one-part-green rule wrong?
Not wrong, just imprecise. It is a volume rule standing in for a mass calculation, and it works because typical browns are light and dry while typical greens are heavy and wet. It breaks down with unusual materials: three buckets of sawdust and one of grass is nowhere near balanced, and neither is two buckets of straw with one of chicken manure.
Why does my compost smell of ammonia?
Too much nitrogen relative to carbon. The microbes cannot use it all, so the surplus leaves as ammonia gas, which is both the smell and a waste of the nitrogen you wanted to keep. Mix in a carbon-rich brown such as shredded leaves, straw or cardboard, and turn the pile to get air into it.
My pile is not heating up. What is wrong?
Usually one of three things, in this order: too much carbon and not enough nitrogen, too dry, or too small. Run your materials through the calculator first. If the ratio is fine, squeeze a handful — it should be as damp as a wrung-out sponge. If that is fine too, the pile may simply be too small to hold heat; a cubic yard is about the minimum for a hot pile.
Do I need to weigh everything?
No. The calculator accepts volume, in buckets or liters, and converts using each material's bulk density, because nobody weighs their compost. Weight is available if you happen to know it, which is sometimes the case for a bought bag of manure or alfalfa meal.

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Written and checked by Muzamil Ali. Results are estimates — see the disclaimer.