Compost Ratio Calculator
How to Use the Compost Ratio Calculator Select all the brown (carbon) and green (nitrogen) materials in your compost pile, enter the approximate weight in pounds for each material, and click Analyse. The calculator estimates your C:N ratio, rates the quality of the mix, estimates time to finished compost, and gives specific guidance on what […]
How to Use the Compost Ratio Calculator
Select all the brown (carbon) and green (nitrogen) materials in your compost pile, enter the approximate weight in pounds for each material, and click Analyse. The calculator estimates your C:N ratio, rates the quality of the mix, estimates time to finished compost, and gives specific guidance on what to add or reduce to reach the ideal 25:1 to 30:1 ratio.
The target carbon-to-nitrogen ratio for fast, hot composting is 25:1 to 30:1. Browns provide carbon (dry leaves, cardboard, straw). Greens provide nitrogen (fresh grass, food scraps, coffee grounds). A practical rule of thumb: 3 parts browns to 1 part greens by volume.
Why C:N Ratio Matters
Carbon provides the energy source for the microorganisms that decompose organic matter. Nitrogen provides the protein building blocks those microorganisms need to reproduce and grow. When the ratio is correct, decomposition is fast, the pile heats up internally to 55-65 degrees Celsius – hot enough to kill weed seeds and pathogens – and the process is odour-free. Too much nitrogen (too many greens) produces ammonia smells and a slimy, wet pile. Too much carbon (too many browns) results in slow, cold decomposition taking many months without intervention.
Common Composting Materials and Their C:N Ratios
High-carbon browns include dry autumn leaves (C:N 60:1), cardboard and paper (C:N 350:1), straw (C:N 75:1), sawdust from untreated wood (C:N 400:1), and wood chips (C:N 400:1). High-nitrogen greens include fresh grass clippings (C:N 20:1), vegetable and fruit kitchen scraps (C:N 15:1), coffee grounds (C:N 20:1), fresh garden weeds (C:N 25:1), and chicken manure (C:N 10:1). The very high C:N ratios of wood-based materials explain why compost piles with a lot of sawdust or cardboard break down so slowly – they need very significant amounts of nitrogen-rich greens to compensate.
The Single Most Important Composting Habit
Turn your pile regularly. Every time you turn a compost pile, you introduce oxygen that accelerates microbial activity – the aerobic bacteria responsible for fast decomposition need oxygen and cannot function without it. A pile turned every 1-2 weeks can produce finished compost in 6-8 weeks. The same pile left untouched takes 6-12 months. A well-managed compost pile at the right ratio should smell earthy, like forest soil. Any unpleasant smell means the pile needs adjustment – either more browns to reduce nitrogen, more turning to restore oxygen, or water if it has dried out.
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Frequently Asked Questions
What can I not put in a home compost bin?
Do not compost: meat, fish, and bones (attract pests and produce odours in home bins), dairy products and oily foods, diseased plants (pathogens may survive), weeds that have set seed, coal ash, chemically treated wood products, and cat or dog waste (requires commercial composting temperatures to safely neutralise pathogens). Everything else – all plant-based food scraps, garden waste, paper, and cardboard – is generally appropriate.
How long does compost take to be ready?
Hot composting with the right C:N ratio and weekly turning: 6-8 weeks. Passive cold composting with monthly or less turning: 6-18 months. Vermicomposting (worm bin): 3-6 months. Finished compost is dark brown or black, crumbly in texture, smells earthy, and has no recognisable original materials. If you can still identify what went in, it is not finished yet.
Why is my compost pile not heating up?
The most common causes: not enough nitrogen (add more greens – grass, food scraps, coffee grounds), pile is too dry (water it until it feels like a wrung-out sponge), pile is too small (a minimum of 3 cubic feet is needed to generate and retain heat), or not enough oxygen (turn the pile). If your pile smells bad but is not heating, it is likely too wet and anaerobic – turn it and add more dry browns.