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.
About this calculator
Healthy compost requires a carbon-to-nitrogen (C:N) ratio of roughly 30:1. Green materials (grass clippings, food scraps) are nitrogen-rich, while brown materials (cardboard, dry leaves) supply carbon. The calculator uses each material's inherent C:N factor to determine how many pounds of browns are needed to balance your greens, then adjusts for moisture because wetter piles need slightly more structural brown matter to maintain airflow. The core formula is: brownNeeded = (greenWeight × greenCN × 30 / brownCN − greenWeight) × (1 + moistureFactor). A result near zero means your materials are already well-balanced; a large positive number means you need to add significant browns before turning the pile.
How to use
Suppose you have 10 lbs of grass clippings (greenType C:N factor ≈ 0.067), you plan to add straw as your brown material (brownType factor ≈ 2.0), and you want moderate moisture (moistureContent = 0.1). Plug in: brownNeeded = (10 × 0.067 × 30 / 2.0 − 10) × (1 + 0.1) = (10.05 − 10) × 1.1 = 0.05 × 1.1 ≈ 0.1 lbs. That tiny result shows grass clippings paired with straw are nearly in balance, so only a handful of extra straw is required.
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 wildly different C:N ratios — fresh grass clippings sit around 15:1 while coffee grounds are closer to 20:1 and fruit scraps even lower. 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 by multiplying your green weight by a material-specific factor before computing the required browns. Using the wrong factor — for example, treating fruit waste the same as grass — can leave your pile nitrogen-starved or nitrogen-flooded.