Compost Calculator

Why the carbon to nitrogen ratio decides everything

Composting is a microbial meal. The microbes eating your kitchen scraps and leaves need carbon for energy and nitrogen to build protein, and they need those two in roughly the right proportion. Get it right and the pile heats up, breaks down in a few months, and smells like forest floor. Get it wrong and you get either a slimy pile that reeks of ammonia or a dry heap that sits unchanged for a year.

The target is around 30 parts carbon to 1 part nitrogen by weight. Penn State Extension's home composting guide gives a preferred range of 25 to 30 to 1, and the EPA's approaches to composting describes the same balance in kitchen terms: about three parts browns to one part greens by volume.

Those two statements are not in conflict. Browns like dry leaves and cardboard are bulky and light, so three buckets of leaves weigh far less than one bucket of wet grass clippings. This calculator works in pounds so the arithmetic holds up, and you can weigh a bucket once to learn what your own materials weigh.

What goes wrong when the ratio is off:

• Too much nitrogen (ratio under about 20 to 1): the pile goes anaerobic, smells like ammonia or sewage, and turns slimy. The excess nitrogen escapes as gas.
• Too much carbon (ratio over about 40 to 1): the pile stays cool and stalls. The microbes run out of nitrogen and stop working, and the pile sits for a year.
• Right in the range: the pile heats to 130°F to 150°F within a few days and finishes in 3 to 6 months with regular turning.

How this calculator works

Each material carries a typical carbon percentage and a nitrogen percentage by weight. Mixing two materials is just a weighted average of each element.

Total carbon is the pounds of greens times their carbon percentage, plus the pounds of browns times theirs. Total nitrogen works the same way. Divide carbon by nitrogen and you have the ratio of the pile.

To find how much of one material to add, solve for the amount that brings the ratio to your target. If the pile has too much nitrogen, you need more browns, and the amount is:

browns to add = (target x nitrogen - carbon) / (brown carbon% - target x brown nitrogen%)

If the pile has too much carbon, you need more greens, and the formula mirrors it:

greens to add = (carbon - target x nitrogen) / (target x green nitrogen% - green carbon%)

The percentages behind each material come from the classic compost tables, the same values the Cornell composting program's carbon to nitrogen calculator uses. They are averages for dry-weight composition, so treat them as good estimates rather than laboratory results. Your own leaves and grass will differ a little with species, season and how wet they are.

Worked example: grass clippings and dry leaves

The most common pile in a suburb is bagged grass clippings and raked leaves. Say you have 20 pounds of each, and you want 30 to 1.

• Grass clippings: 45 percent carbon, 2.0 percent nitrogen, a ratio near 22 to 1
• Dry leaves: 50 percent carbon, 0.8 percent nitrogen, a ratio near 62 to 1
• Carbon in the mix: (20 x 0.45) + (20 x 0.50) = 9 + 10 = 19 lb
• Nitrogen in the mix: (20 x 0.02) + (20 x 0.008) = 0.4 + 0.16 = 0.56 lb
• Ratio of the pile: 19 / 0.56 = about 34 to 1

That is close but a touch heavy on carbon, so the calculator returns 14.7 more pounds of grass clippings to land exactly on 30 to 1. Check it: adding 14.7 pounds of grass adds 6.6 pounds of carbon and 0.29 pounds of nitrogen, taking the pile to 25.6 pounds of carbon and 0.85 pounds of nitrogen, which is 30.1 to 1.

Note the practical implication. Equal weights of grass and leaves are not a balanced pile. You need roughly 1.7 pounds of grass for every pound of leaves you add, which is why a pile built from equal buckets of each tends to run a little short on nitrogen.

Worked example: kitchen scraps and shredded cardboard

Kitchen scraps are wet and nitrogen-heavy, which makes them the most likely material to turn a pile sour. Cardboard is the fix.

• Food scraps: 48 percent carbon, 2.5 percent nitrogen, a ratio near 19 to 1
• Shredded cardboard: 43 percent carbon, 0.1 percent nitrogen, a ratio near 430 to 1
• 30 pounds of food scraps and 5 pounds of cardboard
• Carbon: (30 x 0.48) + (5 x 0.43) = 14.4 + 2.15 = 16.55 lb
• Nitrogen: (30 x 0.025) + (5 x 0.001) = 0.75 + 0.005 = 0.755 lb
• Ratio of the pile: 16.55 / 0.755 = about 22 to 1

That pile is too hot on nitrogen and will smell. The calculator returns about 15 pounds more shredded cardboard to reach 30 to 1, which is roughly two pounds of cardboard for every three pounds of food scraps you keep adding.

Cardboard is a strong brown because its nitrogen content is so low. Pound for pound it corrects excess nitrogen about 1.5 times as effectively as dry leaves. Remove the tape, staples and any glossy coating first, and tear or shred it so it does not mat down into an airless layer.

Carbon to nitrogen ratios of common materials

The full list this calculator draws from, with the carbon and nitrogen percentages behind each ratio:

Horse manure at 32 to 1 is the odd one out. On its own it is already almost perfectly balanced, which is why aged manure is such a good compost ingredient. Wood chips and sawdust at the other end are so carbon-heavy that they stall a pile unless you add a lot of nitrogen with them, and they break down slowly because of the lignin.

Fixing a pile that smells, dries out or stalls

Once the ratio is right, three other variables decide whether the pile works.

• Moisture, 40 to 60 percent. Squeeze a handful. One or two drops should appear. Dry piles stall; soggy piles go anaerobic and smell like sulfur. Fix a dry pile with water and a turn, and a wet pile by mixing in dry browns.
• Air. Turning the pile every one to two weeks for the first month keeps oxygen in. Compaction is the reason a pile goes anaerobic even with the right ratio.
• Particle size. Chop or shred what you add. A pile of whole branches composts in years; the same material chipped composts in months, because the microbes can only work on the surface.

If the pile smells like ammonia, add browns and turn. If it smells like rotten eggs, it is too wet and has gone anaerobic, so turn it and add dry material. If it does not heat up at all, it is short on nitrogen or water, or the pile is smaller than the 3-foot minimum.

What not to add to a backyard pile: meat, fish, dairy, grease, pet waste, and diseased plants. They either attract animals or carry pathogens that a home pile cannot reliably kill. For a full list of what belongs in a home compost bin, the EPA's composting pages are the place to check.