Compost Calculator Tool

Free Compost Calculator — Get Your Carbon to Nitrogen Ratio Right Every Time

The difference between a hot, fast-working compost heap and a cold, slimy, smelly one usually comes down to one thing — the balance of your ingredients. Too many grass clippings and kitchen scraps with not enough cardboard and dry material, and you'll end up with a soggy anaerobic mess. Too much cardboard and straw with not enough greens, and your heap will sit there doing very little for months. Our free Compost Calculator takes the guesswork out of it entirely.

Just dial in your weekly waste volumes, tell us your bin size, and we'll calculate your carbon to nitrogen ratio, tell you whether your mix is right, and give you a personalised recipe to fix it if it isn't.

How the Compost Calculator Works

Every compostable material has a carbon to nitrogen ratio — a measure of how carbon-rich or nitrogen-rich it is. Kitchen scraps, grass clippings and fresh weeds are nitrogen-rich greens with a low C:N ratio. Cardboard, dry leaves, straw and wood chip are carbon-rich browns with a high C:N ratio. The sweet spot for a healthy, active compost heap sits between 25:1 and 35:1 — and our calculator tells you exactly where your mix lands.

• Green materials — vegetable peelings, grass clippings, coffee grounds, fresh weeds, fruit scraps, soft plant stems and animal manure, each with their real C:N values built in
• Brown materials — cardboard, newspaper, dry leaves, straw, wood chip, paper bags, egg boxes and sawdust, all accurately weighted
• Weekly volumes in litres — use the sliders to dial in roughly how much of each material you add per week
• Bin size — choose from a small 220L bin up to a large hotbin or heap at 1000L

What Your Results Tell You

• Your C:N ratio — the key number that determines how well your heap will work
• Weekly input volume — total litres going in per week across all materials
• Time to fill your bin — in both weeks and months, so you can plan when you'll have compost ready
• Browns to greens ratio — a simple volume ratio to help you keep things balanced week to week
• A colour-coded balance bar — instantly shows whether you're too green, too brown, or in the ideal zone
• Personalised advice — tells you exactly what to add more of and why, based on your specific mix

Understanding the Ideal C:N Ratio

A ratio of 25:1 to 35:1 gives the microorganisms in your heap the right balance of energy (carbon) and protein (nitrogen) to work efficiently. Below 20:1 and the heap goes anaerobic — you'll get slime, bad smells and slow decomposition. Above 50:1 and there's not enough nitrogen to fuel the microbes, so everything grinds to a halt and the pile just sits there drying out.

The good news is that once you know your ratio, fixing it is straightforward. Too green? Add a layer of torn cardboard or dry leaves. Too brown? Throw in some grass clippings or kitchen scraps. Our calculator tells you which way to go and roughly how much to add.

Tips for a Faster, Healthier Compost Heap

• Layer your materials — alternating thin layers of greens and browns works better than dumping everything in at once. Aim for roughly 2–3 parts browns to every 1 part greens by volume
• Keep it moist but not wet — your heap should feel like a wrung-out sponge. Too dry and decomposition stalls; too wet and it goes anaerobic
• Turn it regularly — turning your heap every 2–3 weeks introduces oxygen, which dramatically speeds up decomposition. A well-managed heap can produce finished compost in as little as 8–12 weeks
• Chop and shred materials — smaller pieces break down much faster. Shredding cardboard, chopping up stems and running over leaves with a mower all speed up the process significantly
• Avoid these in a standard bin — cooked food, meat, fish, dairy, diseased plants and persistent weeds like bindweed or couch grass. These either attract pests or survive the composting process and spread when you use the finished compost
• Add an activator if you're struggling — a handful of fresh nettles, comfrey leaves or a small amount of chicken manure can kick-start a slow heap by boosting the nitrogen content rapidly

Why Good Compost Matters on the Allotment

Home-made compost is one of the most valuable things you can add to your growing space. It improves soil structure in both sandy and clay soils, adds a slow-release feed that plants can access all season, suppresses weeds when used as a mulch, and builds the biological life in your soil that makes everything grow better. A well-managed heap can produce several hundred litres of finished compost a year from materials that would otherwise go in the bin — reducing your waste and your growing costs at the same time.