Compost Calculator
Work out how much brown (carbon-rich) material to mix with your green (nitrogen-rich) material, your total pile volume, and roughly how much finished compost to expect.
Common Green & Brown Compost Materials
Approximate carbon-to-nitrogen (C:N) ratios — lower numbers are "greener," higher numbers are "browner."
| Material | Type | Approx. C:N Ratio |
|---|---|---|
| Fresh grass clippings | Green | 15–20:1 |
| Vegetable / fruit scraps | Green | 15–20:1 |
| Fresh animal manure | Green | 10–20:1 |
| Dry leaves | Brown | 50–80:1 |
| Straw | Brown | 40–100:1 |
| Cardboard / paper | Brown | 200–350:1 |
| Sawdust / wood chips | Brown | 200–500:1 |
Target compost pile C:N ratio: roughly 25–30:1 for efficient decomposition.
How This Calculator Works
From green material on hand to an estimated finished compost volume.
Understanding Compost: Greens, Browns, and Balance
Composting works because a diverse community of microorganisms breaks down organic matter, and like any living system, those microbes need a reasonably balanced diet to work efficiently. Nitrogen-rich "green" materials — fresh grass clippings, kitchen scraps, fresh manure — supply the protein-building nitrogen microbes need to grow and reproduce. Carbon-rich "brown" materials — dry leaves, straw, cardboard, sawdust — supply the energy-rich carbon that fuels their activity and adds bulk that keeps the pile aerated. Get the balance right, and decomposition proceeds efficiently with minimal odour; get it badly wrong in either direction, and the pile either stalls (too much carbon) or turns wet and smelly (too much nitrogen).
This calculator uses a practical, volume-based approach: tell it how much green material you have on hand, choose a brown-to-green ratio, and it calculates how much brown material to add, your total starting pile volume, and roughly how much finished compost to expect once decomposition completes.
Why a 2:1 to 4:1 Brown-to-Green Ratio
The often-cited "ideal" carbon-to-nitrogen ratio for compost is around 25–30 parts carbon to 1 part nitrogen by weight — but that's a chemical ratio, not the same as a simple volume ratio of materials, since different materials pack different amounts of actual carbon and nitrogen into the same volume. In practice, most home and farm composting guidance simplifies this into a volume-based rule of thumb: roughly 2 to 4 parts brown material to 1 part green material by volume tends to land close enough to the ideal chemical ratio for most common material combinations, which is why this calculator offers that range as adjustable presets rather than requiring you to look up exact C:N values for every material.
How to Use This Calculator
- Choose hot or cold composting, based on how actively you plan to manage and turn the pile.
- Enter the amount of green material you currently have, in any consistent unit — buckets, wheelbarrow loads, or kilograms.
- Select your brown-to-green ratio — 3:1 is a solid general default; lean toward 4:1 for very wet or nitrogen-heavy green material like fresh manure.
- Tap "Calculate Compost Mix" to see the brown material needed, total starting pile volume, estimated finished compost volume, and expected maturity timeline.
Hot vs Cold Composting
Hot Composting
Hot composting involves actively managing the pile — maintaining a good brown-to-green balance, keeping it appropriately moist, and turning it regularly (often every one to two weeks) to introduce oxygen and redistribute heat. Done well, internal pile temperatures can reach 55–65°C, which speeds decomposition dramatically and also helps kill weed seeds and pathogens. Finished compost from a well-managed hot pile can be ready in as little as 6 to 8 weeks.
Cold Composting
Cold composting is the low-effort approach: materials are simply added to a pile or bin over time with little to no turning or active management. It requires far less labour but takes considerably longer, typically 6 months to a year, and internal temperatures rarely get hot enough to reliably kill weed seeds or pathogens.
Signs Your Compost Mix Needs Adjusting
- Strong ammonia or rotten-egg smell: Usually indicates too much green material or too little airflow — add more brown material and turn the pile.
- Pile isn't heating up or breaking down: Often means too much brown material, insufficient moisture, or not enough nitrogen — add more green material and check moisture.
- Pile is too dry: Add water while turning, aiming for a moisture level similar to a wrung-out sponge.
- Pile is soggy and compacted: Add more brown material to absorb excess moisture and improve airflow.
Materials to Avoid in Standard Composting
- Meat, dairy, and oily foods: Decompose slowly, attract pests, and can create strong odours in standard piles.
- Diseased plant material: Cold composting in particular may not reach temperatures high enough to kill plant pathogens, risking reintroducing disease when the compost is used.
- Treated or painted wood: Can introduce chemicals into the finished compost.
- Pet waste from carnivorous animals: Can carry pathogens unsuitable for compost used on food crops.
Using Finished Compost
Finished compost should be dark, crumbly, and smell earthy rather than sour or ammonia-like, with no recognizable original material remaining. It works well mixed into topsoil before planting, as a top-dressing mulch around established plants, or blended into potting mixes — improving soil structure, water retention, and nutrient availability well beyond what its modest N-P-K content alone would suggest.
Final Thoughts
Getting the green-to-brown balance roughly right from the start saves considerable troubleshooting later — a well-balanced pile mostly takes care of itself, while a poorly balanced one stalls, smells, or takes far longer than necessary. Use this calculator each time you start a new pile, and adjust the ratio based on how your specific mix of materials behaves in practice.
Frequently Asked Questions
A commonly recommended starting ratio is roughly 2 to 3 parts brown (carbon-rich) material to 1 part green (nitrogen-rich) material by volume, which tends to produce a reasonably balanced carbon-to-nitrogen ratio for active decomposition.
Green materials include fresh grass clippings, vegetable and fruit scraps, and fresh manure, which are relatively high in nitrogen. Brown materials include dry leaves, straw, cardboard, and sawdust, which are relatively high in carbon.
Finished compost is typically 40 to 60 percent smaller in volume than the original raw material pile, as microbial decomposition breaks down organic matter and moisture evaporates over time.
Hot composting, with a well-balanced mix, adequate moisture, and regular turning, can produce finished compost in as little as 6 to 8 weeks. Cold composting, with less maintenance and turning, typically takes 6 months to a year.
Too much brown material relative to green slows decomposition considerably, since the pile lacks enough nitrogen to support an active microbial population, often resulting in a dry pile that breaks down very slowly.
Excess green material relative to brown often creates a wet, dense, poorly aerated pile that can turn anaerobic, producing unpleasant odours and slowing beneficial decomposition rather than speeding it up.
Turning introduces oxygen that active decomposing microbes need, and speeds up the composting process considerably. Hot composting methods typically require turning every one to two weeks, while cold composting can be left largely undisturbed at the cost of a much longer timeline.
These are generally not recommended for standard home or farm composting, since they decompose slowly, attract pests, and can create strong odours. They're better suited to specialized composting systems designed to handle them safely.