Free Compost Calculator
Enter your garden bed area and desired compost depth to see the cubic feet and cubic yards to order — then switch to pile mode to check your carbon-to-nitrogen ratio and find out whether your mix will heat up fast, updated live, as you type.
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Estimates only. Compost volumes and C:N ratios vary by material density, moisture content, and particle size. Add 10% to volume orders to allow for settling.
Results are estimates. Consult a professional.
How the compost calculator works
The calculator handles two separate compost jobs. In bed application mode it tells you how many cubic feet and cubic yards of finished compost to buy for a garden bed at a given application depth. In pile ratio mode it calculates the carbon-to-nitrogen (C:N) ratio of your compost pile from the weights and carbon or nitrogen percentages of your materials — telling you whether the mix will decompose quickly or need adjustment.
Formulas used
Step-by-step worked examples
Priya has a 4 ft × 8 ft raised vegetable bed she wants to top-dress before spring planting. She plans to apply 3 inches of compost and work it in lightly. How much compost does she need?
Step 1 — area: 4 × 8 = 32 ft².
Step 2 — volume: 32 × (3 ÷ 12) = 8 ft³.
Step 3 — cubic yards: 8 ÷ 27 = 0.30 yd³.
Tom has 10 lb of dry straw (~50% carbon content) and 5 lb of fresh grass clippings (~3% nitrogen content). What is the pile's C:N ratio, and is it in range?
C:N = (10 × 50) ÷ (5 × 3) = 500 ÷ 15 = 33.3:1. This is just above the ideal 25–30:1 range. The pile will still compost, just slightly slowly.
Carbon and nitrogen content of common compost materials
Published C% and N% values are averages — actual content varies by species, maturity, and moisture. Use these as reasonable starting points. The C:N column is the ratio of a single material on its own; most brown materials have very high C:N ratios, which is why they need to be balanced with greens.
| Material | Type | C% (approx.) | N% (approx.) | C:N (solo) |
|---|---|---|---|---|
| Dry straw | Brown | 50% | 0.5% | ~100:1 |
| Wood chips | Brown | 50% | 0.2% | ~250:1 |
| Dry leaves (shredded) | Brown | 50% | 1% | ~50:1 |
| Cardboard (plain) | Brown | 45% | 0.2% | ~225:1 |
| Grass clippings (fresh) | Green | 20% | 3% | ~7:1 |
| Vegetable scraps | Green | 15% | 2.5% | ~6:1 |
| Coffee grounds | Green | 50% | 2% | ~25:1 |
| Aged manure (cow) | Green | 12% | 1.5% | ~8:1 |
Values are typical published averages from Cornell Waste Management Institute and USDA composting guides. Coffee grounds are unusual: high C% but also significant N%, giving a near-ideal ratio on their own.
Building and managing an active compost pile
C:N ratio is the most important factor, but three other variables determine whether your pile heats up in weeks or months:
- Moisture — the pile should feel like a wrung-out sponge. Too dry and microbial activity stalls; too wet and anaerobic bacteria take over (rotten-egg smell).
- Aeration — turn the pile every 1–2 weeks to introduce oxygen. Aerobic decomposition is faster and odour-free.
- Particle size — shredded or chopped material decomposes faster than whole leaves or large chips because microbes have more surface area to work.
Compost calculator terms
How accurate is this compost calculator?
The bed application formula is exact geometry — area times depth divided by 12 gives the precise volume in cubic feet, and dividing by 27 converts exactly to cubic yards. If your measurements are accurate, the volume is accurate. The main real-world variable is compost density, which affects weight but not volume. Suppliers price and sell by volume, so the ft³ and yd³ figures are directly usable.
The C:N ratio result is exact given the inputs, but the inputs carry inherent uncertainty. Published carbon and nitrogen percentages for organic materials are population averages from lab testing; a batch of particularly wet grass clippings or very dry straw will differ. For home composting, this level of precision is more than adequate because you are targeting a range (25–30:1), not a precise point. Being within 5 ratio units of the target is fine in practice.
USDA Natural Resources Conservation Service — Composting guidelines