Calculator
Example data table
| Scenario | Batch size | Green | Brown | Targets | Expected output |
|---|---|---|---|---|---|
| Backyard bin | 120 L, volume basis | Kitchen scraps (C:N 15, 75% moisture) | Dry leaves (C:N 60, 15% moisture) | C:N 30, moisture 55% | Greens ≈ 38 L, browns ≈ 82 L |
| Straw-heavy | 80 L, volume basis | Grass (C:N 17, 80% moisture) | Straw (C:N 80, 12% moisture) | C:N 30, moisture 55% | More greens needed, plus water adjustment |
Formula used
This calculator treats each material’s C:N ratio as a dry-matter property. Using dry masses (D), the blend C:N becomes:
- Rg, Rb, Ra are the C:N ratios for green, brown, and additive.
- Dry mass is computed from moisture: D = WetMass × (1 − Moisture).
- For volume basis, wet mass comes from density: WetMass = Volume × Density.
- The solver finds green and brown amounts that hit the target C:N within your batch size.
How to use this calculator
- Choose unit system and blend basis that match your workflow.
- Enter batch size, target C:N, and target moisture.
- Select presets or type values for greens and browns.
- Optional: add a fixed amount of bulking material.
- Press calculate, then build layers using the output.
- Do a squeeze test and adjust water or browns.
Target C:N and heat management
A blend near 25–35:1 gives microbes enough nitrogen to multiply while still having carbon for energy. Below that, the pile can turn wet and smelly; above it, decomposition slows and temperatures stay low. The calculator converts each ingredient to dry matter, then balances carbon and nitrogen within your batch size. For finishing compost, slightly more carbon can reduce odors.
Volume basis versus weight basis
Volume blending is practical for bins and barrels, but it depends on bulk density, which shifts with shredding, compaction, and moisture. Weight blending is more repeatable because a kilogram or pound stays constant even when the material fluffs up. If you measure by buckets, keep densities realistic and avoid packing one ingredient harder than another.
Moisture control for steady airflow
Moisture affects both biology and oxygen. Around 50–60% moisture supports fast composting without waterlogging pores. The calculator estimates moisture from wet mass and each ingredient’s percentage, then suggests water to add or the excess equivalent. Confirm with a squeeze test: it should feel like a wrung sponge, not drip. If it clumps, add browns; if dusty, mist while turning.
Bulking additives and structure
Bulking materials such as wood chips, coarse stems, or shredded twigs increase pore space and reduce anaerobic pockets. They also tend to have high C:N, so even a small fixed amount can shift ratios. The optional additive field lets you lock a structural component into the batch and solve the remaining mix around it. For very wet greens, a bulking layer can prevent matting and make turning easier.
Interpreting the output and adjusting
Use the “green : brown” ratio as a mixing guide, then watch the pile for feedback. Ammonia odor or slimy texture usually means too many greens or too much water; add browns and turn. A cool pile with intact leaves often needs more greens, smaller particle size, and a larger batch. Recalculate after ingredient changes and record your best blends later.
FAQs
How do I choose a target C:N ratio?
For active composting, many piles perform well around 25–35:1. Use the lower end for wetter, nitrogen-rich inputs and the higher end for drier, woody mixes. Temperature and odor are your feedback signals.
Should I use volume or weight basis?
Use volume if you measure with buckets and can estimate density reasonably. Use weight if you have a scale and want repeatable batches. Weight basis is less sensitive to fluffing, compaction, and particle size changes.
Why does the calculator ask for density?
Density is needed to convert volume to wet mass, so moisture and dry matter can be estimated consistently. If you do not know density, switch to weight basis or start with a preset and refine after one batch.
How do I handle very wet kitchen scraps?
Increase browns and add a coarse bulking ingredient for airflow. Mix thoroughly, then recheck with a squeeze test. If liquid drips, add more dry leaves, shredded cardboard, or straw until it feels like a wrung sponge.
What if the tool says no positive blend found?
Your materials and targets may be incompatible, especially with extreme C:N values or large fixed additives. Adjust the target C:N, change one material, or reduce the additive amount. Small changes usually restore a workable solution.
Can I download results for records?
Yes. After calculating, use the CSV button for spreadsheet tracking or the PDF button for a printable recipe. Saving your best blends helps you repeat successful batches and troubleshoot seasonal ingredient changes.