Compost mix calculator
Balance your greens and browns.
A compostor is happiest and makes the best compost when carbon (browns) and nitrogen (greens) is balanced and the compost is moist (but not wet). Add your compost mix below and the calulator will tell if (and how much) browns and water you need to add to your food scraps for active composting in the Durvela rotary hot compostor.
Your compost mix
Amounts are entered as fresh weight.
Durvela recommendation
Aim for approximately 25–35:1 carbon to nitrogen available over the first 14 days, with 30:1 as the operating target.
Add materials to see a balancing recommendation.
Water to add
Add materials to see a water recommendation.
Odour & ammonia check
Calculate a mix to see operating guidance.
Estimated monthly finished compost
Add materials and enter how many times per month you will add this amount.
Recommended Durvela size
Why this calculator is different
Most compost calculators use total carbon. Durvela’s calculator estimates the carbon and nitrogen likely to become biologically available during the first 14 days 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.
Wondering if composting saves money?
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.
Use the Size & Benefit CalculatorReference 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 uses a 14-day cumulative availability window for both carbon and nitrogen. This better represents an actively aerated thermophilic cycle while avoiding the excessive woody additions produced by a first-week-only model.
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.
Wood pellets are modeled at approximately 50% cumulative carbon availability over the first 14 days of active hot composting. This is an operating estimate, not a claim that half of the pellets will disappear in 14 days.
The recommendation is also limited so the total starting C:N ratio does not exceed approximately 45:1. This protects against an overly woody mix that heats slowly, ties up nitrogen, and takes longer to mature.
Feedstocks and operating conditions vary. Calibrate these assumptions against actual temperature, moisture, odour, ammonia, and laboratory data whenever possible.
Temperature, moisture & odour
Aim for approximately 55–60% moisture: damp like a wrung-out sponge, with enough open structure for airflow.
Ammonia odour indicates that available nitrogen is being lost. Aerate and cool the material first, then add reserved brown material in small increments if the ammonia persists.
Sour or rotten odours usually indicate low oxygen or excessive moisture. Restore airflow and structure before assuming the mix simply needs more carbon.
Thermophilic temperatures support fast decomposition and sanitation, but avoid prolonged operation above 65°C. Turn or increase aeration when necessary.
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