List what goes into the heap with the mass of each material and the tool works out the blended carbon-to-nitrogen ratio — the single number that decides whether a pile heats up and breaks down or turns into a wet, smelly mass. Typical composition values are prefilled and every one of them is editable, because a heap of your grass is not a heap from a table.
Enter the mass of at least one material to see the blend.
How it works
A compost heap works best between about 25 and 35 parts carbon to one part nitrogen. Below that band there is surplus nitrogen, which escapes as ammonia — that is the smell. Above it, microbes run short of nitrogen and the pile sits cold for months.
The blend cannot be an average of the C:N values, because a kilogram of sawdust and a kilogram of grass carry wildly different amounts of dry matter. The tool converts each material to dry matter, then to nitrogen mass (dry matter × N%), then to carbon mass (nitrogen × C:N), and divides the totals. Averaging 17 for grass and 442 for sawdust would suggest 230:1; the honest arithmetic gives 59:1, because the wet grass brings almost all the nitrogen.
Moisture is shown alongside because it is the second thing that goes wrong. A heap wants roughly 45–60% water — damp like a wrung-out sponge. Grass clippings arrive at 82% water and cardboard at 8%, so a mix that is right on carbon can still be too wet or bone dry. Pick a material in the last selector and the tool solves how much of it would bring the ratio to 30:1.
Practical examples
Grass clippings alone
100 kg of fresh clippings is 17:1 and 82% water — far too much nitrogen and far too wet. The tool suggests about 25.8 kg of straw to reach 30:1.
A balanced garden heap
300 kg grass, 100 kg dry leaves and 30 kg straw blends to 30.2:1 with 65% moisture: the ratio is ideal and the pile is a touch wet, so a few more dry leaves would help.
Autumn leaves on their own
200 kg of dry leaves sits at 54:1. It will compost eventually, but adding about 380 kg of fresh grass brings it to 30:1 and turns a year into a few months.
Poultry manure needs a lot of straw
100 kg of poultry manure is only 10:1 — extremely nitrogen-rich. Balancing it takes roughly 111 kg of straw, which is why deep-litter systems use so much bedding.
Frequently asked questions
What is the C:N ratio and why does 30:1 matter?
It is the mass of carbon per unit of nitrogen in the material. Composting microbes consume roughly thirty parts carbon for every part nitrogen they use, so a mix near 30:1 feeds them exactly what they need and heats up fastest.
What counts as a brown and what as a green?
Greens are moist, nitrogen-rich materials: fresh grass, kitchen scraps, manure. Browns are dry and carbon-rich: leaves, straw, cardboard, sawdust. The labels are a rule of thumb — the numbers in the tool are what actually decides the blend.
Why is my heap smelling of ammonia?
Almost always too much nitrogen, too much water, or both. Ammonia is nitrogen leaving the pile, which is a loss of fertiliser value. Mixing in dry, coarse browns fixes the ratio and lets air in at the same time.
Why is nothing happening in my pile?
A high ratio (all leaves, wood chips or cardboard) starves the microbes of nitrogen, and a dry pile stalls too. Check both the ratio and the moisture figure before adding an activator.
How accurate are the prefilled values?
They are typical figures from composting handbooks, and real materials vary a lot — poultry manure is published anywhere from 7:1 to 15:1 depending on bedding and diet. Treat the result as a guide and edit any value you know better for your own materials.
Should I enter fresh weight or dry weight?
Fresh weight, as you would put it on the heap. The tool subtracts the water itself using the moisture percentage, which is why moisture is one of the editable fields.
How do I weigh a heap in practice?
Nobody weighs a garden heap exactly. Use volumes you know — a full wheelbarrow of fresh clippings is roughly 40–60 kg, of dry leaves 20–30 kg — and work in wheelbarrow multiples. The ratio matters far more than the absolute mass.
What moisture should I aim for?
Around 45–60%. Squeeze a handful: it should feel like a wrung-out sponge, with a drop or two of water at most. Wetter than that and the pile goes anaerobic; drier and it stops.
Can I compost cooked food, meat or dairy?
They compost, but they attract animals and can go anaerobic in a garden-scale heap, so most guidance keeps them out of open piles. The calculator has no view on this — it does arithmetic, not pest control.
Does particle size matter as much as the ratio?
It matters a great deal for speed. Shredded material has far more surface area, so a chopped 30:1 mix can finish in weeks while the same mix in whole branches takes a year. The ratio sets the ceiling; particle size and turning decide how close you get.
Can I use the result for a bokashi bin or a worm farm?
Only loosely. Worm bins and fermentation systems have their own preferences, generally more carbon-rich and drier than a hot pile. This tool models a thermophilic heap.
Is my mix uploaded anywhere?
No. The calculation runs entirely in your browser.
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