Compost Calculator & Composter Size Calculator

Your compost mix

Amounts are entered as fresh weight.

Note: “Kitchen scraps, mixed” refers to a typical household mix of fruit scraps, vegetable scraps, grains, bread, coffee filters, and small cooked-food residues. It does not represent large amounts of raw meat, fats, oils, dairy, or commercial food waste streams. Any large amounts of individual materials should be added separately. Large amounts of fats or oils, or salty products should be disposed of and not composted. To calculate with volume rather than mass, see assumptions below regarding bulk density.

How often will you add this amount?

times per month

Available C:N ratio —

Add materials to calculate your mix.

Available carbon 0 g

Available nitrogen 0 g

Durvela recommendation

Aim for approximately 25–35:1 available carbon to nitrogen, with 30:1 as the ideal hot-composting target.

Balance using

Add materials to see a balancing recommendation.

Water to add

Add materials to see a water recommendation.

Recommended Durvela size

Add materials and enter how many times per month you will add this amount.

Why this calculator is different

Most compost calculators use total carbon. Durvela’s calculator estimates the carbon and nitrogen that are likely to be biologically available during the first week of active rotary hot composting.

Woody browns are excellent for moisture control, odour control, and structure, but much of their carbon is locked in lignin and does not behave like immediately available microbial food.

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Use the Durvela Size & Benefit Calculator to estimate the value of compost produced, landfill waste avoided, emissions reductions, and potential savings based on your household, garden, farm, or business.

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Reference assumptions

The values used in this calculator are designed specifically for active rotary hot composting systems such as the Durvela composter. Unlike traditional compost calculators, Durvela estimates only the portion of carbon and nitrogen that is likely to become biologically available during approximately the first week of active composting.

Typical C:N ratios

Kitchen scraps: ~20:1
Vegetable scraps: ~18:1
Fruit scraps: ~35:1
Raw meat scraps: ~6.6:1
Coffee grounds: ~20:1
Grass clippings: ~17:1
Brewer's yeast slurry: ~5:1
Wet brewery grain (spent malt): ~13:1
Spent hop cones: ~18:1
Dry leaves: ~60:1
Straw: ~80:1
Shredded paper: ~170:1
Cardboard: ~350:1
Wood chips / pellets / sawdust: ~400–500:1
Animal bedding (straw & manure): ~45:1

Why wood behaves differently

Wood products contain large amounts of lignin and crystalline cellulose. Much of this carbon is structurally protected and does not become rapidly available to compost microbes.

For this reason, Durvela treats woody materials as slow-release carbon sources rather than instantly available microbial fuel.

In the calculator, wood pellets are modeled at approximately 25% first-week carbon availability during active hot composting, reflecting the slow-release nature of woody carbon.

Why bulk density matters

One kilogram of compost feedstock can occupy very different volumes depending on the material. Dense food scraps may occupy less than 1.5 litres per kilogram, while fluffy dry leaves or straw may occupy more than 10 litres per kilogram.

Rather than assuming every kilogram occupies the same space, the Durvela calculator uses material-specific bulk density values to estimate the actual volume inside the composter. This improves the size recommendation because composter capacity is limited by occupied volume, not just weight.

Kitchen scraps: ~650 kg/m³ (40.6 lb/ft³, 1 kg ≈ 1.5 L / 0.40 gal)
Coffee grounds: ~550 kg/m³ (34.3 lb/ft³, 1 kg ≈ 1.8 L / 0.48 gal)
Wood pellets: ~650 kg/m³ (40.6 lb/ft³, 1 kg ≈ 1.5 L / 0.40 gal)
Sawdust: ~250 kg/m³ (15.6 lb/ft³, 1 kg ≈ 4.0 L / 1.06 gal)
Dry leaves: ~80 kg/m³ (5.0 lb/ft³, 1 kg ≈ 12.5 L / 3.30 gal)
Yard and Garden Waste: ~250 kg/m³ (15.6 lb/ft³, 1 kg ≈ 4.0 L / 1.06 gal)
Straw: ~60 kg/m³ (3.7 lb/ft³, 1 kg ≈ 16.7 L / 4.41 gal)

Durvela recommendations are based on calculated feedstock volume, recommended brown additions, material-specific volume settling during approximately four weeks of active composting, and working fill space for mixing and airflow. The calculator checks both the loose volume of a single addition and the time-weighted active volume held over the composting period.

Wet food scraps and grass: roughly 40–50% retained volume after active composting
Coffee grounds, garden trimmings, and brewery grain: roughly 50–55% retained volume
Dry leaves, straw, and shredded paper: roughly 25–35% retained volume once wetted, mixed, and settled
Cardboard and animal bedding: roughly 40–55% retained volume
Sawdust, pellets, chips, and woody material: roughly 65–85% retained volume

For repeated additions, the calculator estimates each load separately by age. Newly added material counts near its loose starting volume, while older material counts at a lower retained volume according to the material's settling curve.

Primary reference sources

Cornell Composting Science & Engineering: compost C:N ratios and lignin effects
Texas A&M AgriLife Extension: Composting Fundamentals
USDA composting guidance documents
Published compost microbiology and lignocellulosic decomposition literature
The Composting Handbook: a how-to and why manual for farm, municipal, institutional and commercial composters; Edited by Robert Rynk

Balance your greens and browns.