C:N Ratio Results
All C:N values are on a dry-weight basis per NRAES-54 and CREF Appendix B. Actual ratios vary by material freshness, fertilization, and species. See full methodology.
Carbon to Nitrogen Ratio Calculator
Blend your compost ingredients to hit the ideal 25–35:1 C:N ratio. Uses 45+ feedstocks from NRAES-54 and CREF Appendix B.
Understanding the Carbon to Nitrogen Ratio in Composting
If there's one number worth getting right before you build a pile, it's the carbon-to-nitrogen (C:N) ratio. It decides whether the microbes doing the actual work have what they need — or whether they stall out short on nitrogen, or gas off the excess as ammonia. The extension research keeps landing in the same place: 25:1 to 35:1[4] is the sweet spot for fast, hot composting, and 30:1 is the number you'll see cited most.[3]
Why C:N Ratio Matters
Microbes doing the decomposing need roughly 30 parts carbon for every 1 part nitrogen — that's just the C:N ratio baked into their own cell biomass. Get your pile into that range and decomposition takes off, with temperatures climbing into the thermophilic zone (130–160°F). Drop below 20:1 and you'll smell it: excess nitrogen gasses off as ammonia, which is exactly why a pile loaded with grass clippings or fresh manure gets that sharp odor. Go above 40:1 and things grind to a crawl, because now nitrogen is what's holding the microbes back.
Browns vs. Greens: A Simplified Mental Model
The popular "3 parts browns to 1 part greens" rule gets you into the neighborhood of that 30:1 target, but it's too rough for anything you'd want to calculate precisely. "Browns" range from dry leaves (40–80:1) to straw (50–150:1) to cardboard (150–500:1). "Greens" span fresh grass clippings (9–25:1), food scraps (15–25:1), and manures (5–25:1). The calculator beats the rule of thumb because it works from the actual ratio of each material, not just whether it looks brown or green.
How This Calculator Works
This calculator runs on the dry-mass weighted harmonic mean formula from the Cornell Waste Management Institute. It converts each ingredient's volume to dry mass first, using bulk density and moisture data from NRAES-54 and CREF, then blends the C:N ratio from there. Simple arithmetic averaging — a common mistake — gets this wrong, because it ignores how much each material actually weighs relative to the rest.
A worked example
Say you combine 4 cubic feet (about 30 gallons) of dry autumn leaves with half a cubic foot (about 3.7 gallons) of used coffee grounds. Run that through the calculator and here's what happens: it converts each volume to dry mass first. Leaves are light and fairly dry, so that whole pile only works out to about 10 lb of dry material. Coffee grounds are dense and wet, so the much smaller volume still comes out to about 8 lb dry. Weight each material's C:N by its dry mass — a harmonic mean, not a plain average — and the leaves at 60:1 and the grounds at 22:1 land you around 34:1, right in the target zone. You needed roughly eight times more leaves by volume than grounds to get there, which is exactly the density-and-moisture gap the color-coded "3 parts browns to 1 part greens" rule misses — and why plugging in real volumes beats eyeballing it.
Once you've got your C:N ratio dialed in, check it against the Moisture Calculator to make sure you're in that 40–65% sweet spot, then track progress with the Pile Temperature Tracker.
Frequently Asked Questions
What is the ideal C:N ratio for composting?
Aim for 25:1 to 35:1 to start — 30:1 is the number you'll see most often. Piles in that window decompose fast. Go below 20:1 and you'll likely smell ammonia as the extra nitrogen escapes; go above 40:1 and things slow way down because the microbes run out of nitrogen to work with.
Why does the calculator use gallons instead of pounds?
Because that's how most people actually measure this stuff — by the bucket or wheelbarrow, not the scale. Behind the scenes, the calculator converts your gallons to dry mass using bulk density and moisture figures from the CREF feedstock database, then runs the C:N formula. Switch to metric mode and you can enter liters instead.
What is the Dry/Typical/Wet condition toggle?
C:N and moisture shift depending on how fresh or dried-out a material is — spring grass clippings you just mowed (Wet) aren't the same as clippings that sat in the sun for a week (Dry). The toggle moves the selected feedstock between the minimum (Dry), typical (Typical), and maximum (Wet) published values from NRAES-54 and CREF.
How do I calculate C:N ratio by hand?
You want the dry-mass weighted harmonic mean here, not a simple average: first find the dry mass of each material (volume × bulk density × (1 − moisture fraction)), then compute R_mix = (sum of dry masses) / (sum of dry_mass_i / R_i). Averaging the C:N ratios directly is the mistake most people make, and it throws the result off.
My C:N is too high — what should I add?
A high C:N — anything above 35:1 — means you've got too many "browns" in the mix. Balance it with nitrogen-rich "greens": fresh grass clippings, vegetable scraps, coffee grounds, chicken manure, or blood meal. You're aiming for 25–35:1.
My C:N is too low — what should I add?
A low C:N — under 20:1 — means "greens" are dominating. Bring in carbon-rich "browns": dry leaves, straw, cardboard, newspaper, or wood chips. Leave it too low and the excess nitrogen turns into ammonia odor and starts attracting pests.
Are C:N ratios the same as "browns to greens" ratios?
Not quite. Browns-to-greens is a handy shorthand (often given as 3:1 by volume), but C:N ratio is an actual chemical measurement. It's more accurate because it accounts for the real carbon and nitrogen content of each specific material, not just a rule of thumb.
What does the moisture percentage in the results mean?
That's the weighted average moisture across your whole pile. A healthy, active pile sits at 40–65%. Come in under 40% and the calculator tells you how much water to add; go over 70% and you'll want to mix in dry carbon materials instead.