(Enter your compost bin dimensions and the brown and green materials you have available. The calculator shows your carbon-to-nitrogen ratio, how to balance it, and your estimated time to finish compost based on your method.)
The difference between a compost pile that produces rich, finished compost in 8–12 weeks and one that just sits there for two years is almost entirely the carbon-to-nitrogen ratio.
Too much brown material (high carbon) and the pile stays cold and slow — there’s not enough nitrogen for microbes to work fast. Too much green material (high nitrogen) and it gets slimy, smelly, and anaerobic. The ratio matters, and this calculator helps you hit it.
Editor’s Note: The ideal C:N ratio for hot composting is 25–30 parts carbon to 1 part nitrogen by weight. In practical terms: roughly 3 parts brown material (dry leaves, straw, cardboard) to 1 part green material (fresh grass clippings, kitchen scraps, fresh plant material). The calculator accounts for the actual C:N ratio of your specific materials — because dry leaves have a C:N of 60:1 while grass clippings are 15:1, so equal volumes of each produce very different results.
Brown vs. Green — Understanding What Each Does
| Material | Brown or Green | Approx C:N Ratio | Notes |
|---|---|---|---|
| Dry fallen leaves | Brown | 60:1 | Best bulk carbon source — shred for faster breakdown |
| Straw (not hay) | Brown | 80:1 | No weed seeds if straw (seed-free); hay has seeds |
| Cardboard (shredded) | Brown | 350:1 | Very high carbon — use sparingly; remove tape |
| Wood chips | Brown | 400:1 | Very slow to break down; use in thin layers |
| Fresh grass clippings | Green | 15:1 | Best nitrogen activator — add in thin layers to prevent matting |
| Kitchen vegetable scraps | Green | 15:1 | Bury in centre to deter pests |
| Fresh plant material | Green | 15–30:1 | Includes crop waste, spent plants |
| Coffee grounds | Green | 20:1 | Despite brown appearance — acidic, excellent nitrogen |
| Manure (chicken) | Green | 7:1 | High nitrogen activator — powerful in small amounts |
| Manure (horse) | Green | 25:1 | More balanced — good bulk green material |
Hot vs Cold Composting — Choosing Your Method
Hot composting (active method): Build a pile at least 3×3×3 feet using the 3:1 brown-to-green ratio, keep it moist (wrung-out sponge consistency), and turn every 3–5 days. Pile heats to 130–160°F — kills weed seeds and pathogens. Produces finished compost in 8–12 weeks. Requires more active management but the result is significantly faster.
Cold composting (passive method): Add materials as you have them, don’t worry about ratios, turn occasionally. No heat — weed seeds survive, pathogens may survive, breakdown is slow. Produces finished compost in 6–18 months. Low effort — ideal if time matters more than speed.
For most home gardeners, a hybrid approach works well: maintain one active hot pile for kitchen and garden waste that you manage through the growing season, and one passive pile for bulk leaf fall in autumn.
Finished compost — dark brown, earthy smell, crumbly, unrecognisable original materials — is one of the most valuable amendments for any garden. Once you have it, use our raised bed soil calculator to determine how many cubic feet of compost to add to each bed and our fertilizer calculator to understand how home compost changes your fertiliser needs.
Frequently Asked Questions
My compost pile smells bad — what's wrong?
Bad smell from compost is almost always too much nitrogen (green material) and not enough carbon (brown material), combined with poor aeration. The pile has gone anaerobic.
Fix: add brown material (dry leaves, straw, cardboard) in generous quantities, turn the pile to introduce oxygen, and check that it is not waterlogged. A properly balanced, turned pile should smell earthy at most — not bad. If it smells like ammonia, the ratio is heavily nitrogen-skewed; add browns. If it smells like rotten eggs, it is waterlogged and anaerobic; add browns and turn.
Can I compost diseased plants?
In a hot compost pile that reliably reaches 130–160°F: yes, most pathogens are killed at those temperatures. In a cold compost pile: no — disease pathogens and fungal spores survive cold composting and re-infect the garden when the compost is applied. When in doubt, bag and discard diseased material rather than risk spreading the pathogen. This applies especially to tomato blight material, club root brassicas, and any plant showing viral symptoms.
Free Tools & Guides:
Final Thoughts
We hope this calculator demystifies the ratios behind good composting and makes it easier to turn your garden and kitchen waste into the most valuable amendment you can make for free.
Home compost applied annually to vegetable beds is genuinely transformative for long-term soil health.
For the complete soil care picture, our vegetable gardening guide and soil pH calculator work together with composting for a complete fertility system.
Share this free tool with a fellow gardener who has a compost pile that just sits there year after year without producing anything usable — and tell us in the comments how it helped. Happy growing!
