Compost C:N Ratio Calculator
Balance your compost pile like a pro. Add your materials with their weights, and the calculator finds your mix's carbon-to-nitrogen ratio â and tells you how to fix it.
Calculate your pile's C:N ratio
Enter wet (as-weighed) weights â the calculator converts to dry weight using typical moisture content.
What is the C:N ratio?
The carbon-to-nitrogen (C:N) ratio describes the proportion of carbon to nitrogen in organic matter. Every material you add to a compost pile contains both: carbon-rich "browns" like dry leaves and straw, and nitrogen-rich "greens" like grass clippings and food scraps. The ratio of the whole mix determines how happily the microbes that do the composting can work.
Think of it as a diet for bacteria and fungi. Carbon is their energy source â the fuel that powers decomposition. Nitrogen is the building block they need to grow and reproduce. When the balance is right, microbe populations explode, the pile heats up to 130â160°F (55â70°C), and your scraps turn into finished compost in weeks instead of months.
Why 25â30:1 is the ideal range
Microbes consume carbon and nitrogen in roughly a 25-to-30 to 1 proportion. Research on aerobic composting consistently shows that a starting mix around 25â30:1 produces the fastest, hottest decomposition with the least nitrogen loss.
Go too high in carbon (say 50:1 or more â a pile of mostly dry leaves and wood chips) and the microbes run out of nitrogen. They can only work with the nitrogen available, so the pile sits cold and decomposes at a glacial pace. Go too low (below about 20:1 â a bin of pure grass clippings) and there's more nitrogen than the microbes can use. The excess escapes as ammonia gas, which is why nitrogen-heavy piles smell like a barnyard and lose the very nutrient you're trying to keep.
The calculator's verdict bands reflect this: mixes in the 25â30:1 sweet spot get the green light, while mixes that drift too far either way get specific advice on which materials to add.
Greens vs. browns
| Material | Type | Typical C:N | Notes |
|---|---|---|---|
| Fresh grass clippings | Green | ~20:1 | Use thin layers to avoid matting |
| Fruit & veggie scraps | Green | ~25:1 | Bury in the pile to deter pests |
| Coffee grounds | Green | ~20:1 | Count as green despite the brown color |
| Chicken manure | Green | ~10:1 | Very hot â balance with plenty of browns |
| Dry leaves | Brown | ~60:1 | The classic compost brown |
| Straw | Brown | ~80:1 | Adds structure and airflow |
| Shredded newspaper / cardboard | Brown | ~170:1 | Avoid glossy or heavily inked paper |
| Wood chips / sawdust | Brown | ~400:1 | Extremely carbon-heavy; use sparingly |
A common beginner mistake is counting coffee grounds as a brown because they look brown â they're actually nitrogen-rich. Color is a rough guide at best; the table above (and the calculator's built-in values) is the reliable reference.
How the calculator does the math
C:N ratios are defined on a dry-weight basis, but nobody dries their grass clippings before weighing them. So the calculator takes your wet weights and converts them using typical moisture content for each material â grass clippings are roughly 80% water, dry leaves about 30%, sawdust under 10%.
For each material it computes the dry weight, multiplies by the material's carbon and nitrogen fractions, then sums across all materials. The final ratio is total carbon divided by total nitrogen. Because it's a weighted average, adding a little of a very nitrogen-rich material like chicken manure moves the needle far more than adding the same weight of leaves.
Tips for a faster, hotter pile
- Chop or shred materials. Smaller pieces mean more surface area for microbes â a shredded pile can finish twice as fast as one built from whole leaves and branches.
- Aim for wrung-out-sponge moisture. Squeeze a handful: a few drops of water should come out. Too dry and microbes stall; too wet and the pile goes anaerobic and smelly.
- Turn every 1â2 weeks. Turning reintroduces oxygen, which hot composting microbes need constantly. A turned pile typically finishes in 6â8 weeks in warm weather.
- Layer greens and browns. Alternate 2â3 inch layers rather than dumping one material in bulk â it keeps the ratio even throughout the pile.
- Don't stress about perfection. Anything roughly between 25:1 and 40:1 will compost fine; the extremes are what cause real problems. The calculator helps you avoid those.
Frequently asked questions
What is the ideal C:N ratio for compost?
About 25â30:1. In this range, composting microbes have enough carbon for energy and enough nitrogen to build protein, so the pile heats up quickly and decomposes fast.
What happens if the C:N ratio is too high?
A carbon-heavy pile (above ~40:1) decomposes very slowly because microbes run out of nitrogen. Add nitrogen-rich greens such as grass clippings, food scraps, or manure to bring the ratio down.
What happens if the C:N ratio is too low?
A nitrogen-heavy pile (below ~20:1) loses excess nitrogen as ammonia, causing strong odors. Mix in carbon-rich browns such as dry leaves, straw, or shredded cardboard to balance it.
Do I enter wet or dry weights in the calculator?
Enter wet (as-weighed) weights. The calculator converts each material to dry weight using typical moisture content values, then computes the blended carbon-to-nitrogen ratio.
What are greens and browns in composting?
Greens are nitrogen-rich materials like fresh grass clippings, food scraps, and manure. Browns are carbon-rich materials like dry leaves, straw, wood chips, and newspaper. Mix both to hit the target C:N ratio.