Compost Calculator
Composting converts organic waste into a valuable soil amendment while diverting material from landfills, where it would otherwise decompose anaerobically and generate methane, a potent greenhouse gas. The science of composting centres on the carbon-to-nitrogen (C:N) ratio of your input materials. Greens (nitrogen-rich materials such as food scraps and grass clippings) have low C:N ratios, typically 15:1 to 20:1. Browns (carbon-rich materials such as dry leaves, cardboard, and straw) have high C:N ratios from 60:1 to 350:1. Blending these in the right proportions achieves the 25:1 to 30:1 optimal range that microorganisms need to break down organic matter efficiently. This calculator takes your weekly inputs for food scraps, grass clippings, and dry leaves, computes the blended C:N ratio, tells you whether you need more greens or more browns to balance the pile, estimates monthly and annual compost yield, and calculates how much CO2 equivalent you divert from landfill. The CO2 diversion figure is based on EPA food-waste data (2.0 kg CO2e per kg of food waste diverted from landfill).
C:N ratio: -- — status: --. Annual CO2 diverted: -- kg.
How the C:N ratio is calculated
Each material contributes carbon and nitrogen in proportion to its weight and C:N ratio. Nitrogen contribution equals the material weight; carbon contribution equals weight multiplied by C:N ratio. The blended ratio is total carbon divided by total nitrogen.
Total carbon = (food scraps x 15) + (grass x 20) + (leaves x 60)
Total nitrogen = food scraps + grass + leaves
Blended C:N = total carbon / total nitrogen
Monthly yield (lbs) = weekly input x 4.33 x 0.50
Annual yield (lbs) = weekly input x 52 x 0.50
Annual CO2 diverted (kg) = food scraps (lbs/wk) x 52 x 0.4536 x 2.0
Worked example (defaults)
Food scraps 7 lbs, grass 5 lbs, leaves 3 lbs per week:
- Total carbon = (7 x 15) + (5 x 20) + (3 x 60) = 105 + 100 + 180 = 385
- Total nitrogen = 7 + 5 + 3 = 15
- C:N = 385 / 15 = 25.7:1 (balanced)
- Weekly input = 15 lbs; annual yield = 15 x 52 x 0.5 = 390 lbs
- Annual CO2 diverted = 7 x 52 x 0.4536 x 2.0 = 329.82 kg
Balancing recommendations
If C:N is below 25 (too much nitrogen), the calculator recommends adding dry leaves. Amount to add (lbs/week) = (target C:N x total N - total C) / (60 - target C:N), using a target of 28. If C:N is above 30 (too much carbon), it recommends adding food scraps. Amount to add = (total C - target C:N x total N) / (target C:N - 15).
C:N ratios of common composting materials
| Material | Type | C:N ratio (approx) |
|---|---|---|
| Food scraps (fruit/veg) | Green | 15:1 |
| Grass clippings | Green | 20:1 |
| Coffee grounds | Green | 20:1 |
| Fresh manure | Green | 15:1 |
| Dry leaves | Brown | 60:1 |
| Straw | Brown | 75:1 |
| Cardboard (shredded) | Brown | 350:1 |
| Newspaper | Brown | 175:1 |
C:N ratios are reference values from EPA and USDA composting guides. Actual ratios vary with material maturity, moisture content, and species. Use these as planning guides, not exact measurements.
Food waste and landfill emissions
Food waste is the single largest category of material sent to US landfills by weight, according to the EPA. When buried in landfill, food decomposes anaerobically and produces landfill gas, approximately 50% methane and 50% CO2 by volume. The EPA's Waste Reduction Model (WARM) estimates that diverting one metric ton of food waste from landfill avoids approximately 2.0 metric tons of CO2 equivalent emissions.
Only food scraps are counted in the CO2 diversion calculation in this tool, as grass clippings and leaves produce minimal methane in landfill compared to food waste. The CO2 figure represents avoided emissions only; it does not account for any emissions associated with transporting or managing compost inputs.
Composting: frequently asked questions
What is the ideal carbon-to-nitrogen ratio for compost?
Composting scientists and the USDA recommend a C:N ratio between 25:1 and 30:1 for optimal decomposition. Within this range, microorganisms have enough carbon for energy and enough nitrogen to build proteins, so the pile heats up well and breaks down efficiently. A ratio below 25:1 means too much nitrogen (often signalled by ammonia smell), while a ratio above 30:1 means too much carbon and the pile will decompose very slowly.
Why does food waste produce more methane in landfill than other waste?
Food scraps are rich in biodegradable organic matter. In landfill, this material decomposes in anaerobic (oxygen-free) conditions, producing methane (CH4), which is approximately 28 times more potent as a greenhouse gas than CO2 over a 100-year period (EPA). When food waste is composted instead, it decomposes aerobically, producing CO2 and water rather than methane, significantly reducing the climate impact of disposal. The EPA estimates food waste in landfill generates approximately 2.0 kg CO2e per kg of food waste.
Why do I need both greens and browns in compost?
Greens (nitrogen-rich materials like food scraps, grass clippings, and fresh plant matter) provide the protein and moisture that microorganisms need to thrive. Browns (carbon-rich materials like dry leaves, cardboard, and straw) provide structure, absorb excess moisture, prevent compaction, and supply the carbon that microorganisms use as an energy source. Without enough browns, the pile becomes wet, smelly, and slow. Without enough greens, decomposition stalls for lack of nitrogen.
How long does composting take?
Hot composting in an actively managed pile with the correct C:N ratio, adequate moisture, and regular turning can produce finished compost in 2 to 6 weeks. Passive cold composting in a backyard bin typically takes 6 to 12 months. The volume reduction estimate used in this calculator (50% of input weight) reflects typical finished compost yield for a well-managed pile. Actual yield depends on moisture content, particle size, and management intensity.
Can I compost all food scraps?
Most fruit and vegetable scraps, coffee grounds, tea bags, eggshells, and bread can be composted. The EPA and USDA advise against adding meat, fish, dairy, and oily foods to backyard compost piles, as these can attract pests and produce odours. These materials can be processed in municipal food-waste programmes or industrial composting facilities. Garden and yard waste, shredded cardboard, and paper are generally suitable for all composting methods.
Official sources
- EPA Composting at Home: epa.gov/recycle/composting-home.
- USDA Agricultural Research Service, composting guide: nal.usda.gov/legacy/afsic/composting.
- EPA Waste Reduction Model (WARM), food waste emission factor: epa.gov/warm.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. C:N ratios and CO2 factors are scientific reference values; actual results depend on pile management, moisture, and particle size.