Compost Mix Calculator
Find exactly how much brown material to add to your compost pile given your green waste weight, material types, and desired moisture level. Use it whenever you start a new batch to hit the ideal 30:1 carbon-to-nitrogen ratio.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Healthy compost needs a carbon-to-nitrogen (C:N) ratio of roughly 30:1. Green materials (grass clippings, food scraps) are nitrogen-rich with low C:N ratios, while brown materials (dry leaves, newspaper, sawdust) supply carbon with high ratios. Treating each material by its C:N ratio alone (i.e. assuming similar nitrogen content per pound), the mix reaches 30:1 when greens × (30 − green C:N) = browns × (brown C:N − 30), so: brownNeeded = greenWeight × (30 − greenCN) / (brownCN − 30). The result is then increased by (1 + moisture target) because wetter piles need extra structural browns to keep air flowing. Lower-ratio greens (coffee grounds 12:1) need more browns than higher-ratio greens (grass 20:1), and high-carbon browns (sawdust 500:1) are needed in much smaller amounts than leaves (60:1). Real materials differ in nitrogen content and moisture, so treat the result as a starting mix and adjust by smell and temperature.
How to use
Suppose you have 10 lbs of fresh grass clippings (20:1), you plan to add dry leaves (60:1), and you want 50% moisture. brownNeeded = 10 × (30 − 20) / (60 − 30) × (1 + 0.5) = 10 × 10/30 × 1.5 = 5.0 lbs of leaves. With 10 lbs of kitchen scraps (15:1) and newspaper (150:1) instead: 10 × 15/120 × 1.5 ≈ 1.9 lbs of newspaper.
Frequently asked questions
What is the ideal carbon-to-nitrogen ratio for compost and why does it matter?
The ideal C:N ratio for active composting is between 25:1 and 30:1. Below this range the pile smells of ammonia because excess nitrogen escapes as gas; above it decomposition slows dramatically because microbes run out of nitrogen to build proteins. Hitting the sweet spot keeps the pile hot (above 130 °F) and produces finished compost in as little as four to eight weeks. Getting the ratio right from the start is far more efficient than trying to correct an imbalanced pile mid-process.
How does moisture content affect how much brown material I need in my compost pile?
Moisture acts as the medium through which microbes move and feed, so the ideal pile is about 50–60% moisture — similar to a wrung-out sponge. When you target higher moisture, more brown material is needed to create air pockets and prevent the pile from becoming anaerobic and slimy. The calculator's moisture factor increases the brown material estimate proportionally, so a wetter target always calls for more structural carbon. If your pile smells like rotten eggs, it is almost certainly too wet and needs more browns turned in immediately.
Why do different green materials need different amounts of brown to balance them in compost?
Different green materials have different C:N ratios — this calculator uses about 15:1 for kitchen scraps, 20:1 for fresh grass clippings and 12:1 for coffee grounds. The lower the C:N ratio of your greens, the more carbon-rich browns you must add to bring the blend up to 30:1. The calculator accounts for this through the (30 − green C:N) term. Using the wrong factor — for example, treating fruit waste the same as grass — can leave your pile nitrogen-starved or nitrogen-flooded.