UK composting context: greens, browns, and the RHS approach
This calculator finds the carbon-to-nitrogen (C:N) ratio of your compost pile based on the materials and quantities you enter. A balanced C:N ratio produces fast decomposition without the common problems of a pile that is too wet, too dry, or smells of ammonia.
In UK composting terminology, "greens" are nitrogen-rich materials (fresh grass clippings, vegetable peelings, fruit waste, coffee grounds) and "browns" are carbon-rich materials (dry autumn leaves, cardboard, straw, paper). The Royal Horticultural Society (RHS) recommends a ratio of approximately 25-30:1 C:N and advises a mix of roughly equal volumes of greens and browns by weight, though in practice most UK gardeners add more browns than greens to avoid the pile becoming waterlogged.
Around 40% of UK households compost at home. Most UK councils also offer green waste kerbside collections, and from 2025 all English councils must collect food waste separately under revised waste regulations. Home composting reduces the volume of organic waste going to landfill and produces a free soil conditioner.
C:N ratios of common UK materials
| Material | C:N ratio | Category | Notes |
|---|---|---|---|
| Straw | 80:1 | Brown (carbon) | Good bulking agent |
| Cardboard (shredded) | 350:1 | Brown | Remove tape and staples |
| Autumn leaves | 40-80:1 | Brown | Shred to speed breakdown |
| Grass clippings | 15-25:1 | Green (nitrogen) | Layer thinly to avoid matting |
| Vegetable peelings | 15-20:1 | Green | Most common UK kitchen waste |
| Coffee grounds | 20:1 | Green | Worms love them |
| Chicken manure | 7:1 | Very green | Use sparingly; very high nitrogen |
Troubleshooting your pile
- Smells of ammonia: too much nitrogen (greens); add more browns (cardboard, dry leaves)
- Smells of rotten eggs: pile is too wet and anaerobic; turn it and add dry browns
- Not decomposing: too much carbon (browns) or too dry; add greens and water
- Pest problems: avoid cooked food, meat, dairy; use a closed bin with a tight-fitting lid
Hot vs. cold composting
Hot composting (turning the pile regularly, maintaining 55-65 degrees C) produces finished compost in 6-8 weeks and kills weed seeds. Cold composting (leaving the pile undisturbed) takes 6-18 months but requires no effort. For flat or balcony composting, a worm bin (vermicomposting with red wigglers) works year-round in UK temperatures and produces both worm castings and liquid feed. Enter your materials above to check whether your ratio is balanced before you start.