Weigh or estimate greens
Kitchen caddy empties and grass clippings are nitrogen-rich greens.
Compost Mix Calculator
Enter greens, browns, moisture and bin size to see your carbon-to-nitrogen balance, score and practical next steps for a healthy heap.
Calculator
Tap a screen and type with the keypad (or your keyboard), choose an option, then press =. The answer shows on the calculator screen, with full details underneath.
Garden Plant Finder
COMPOST MIX · GPF
How to measure
Follow these steps for accurate inputs and better results.
Add browns by weight
Aim for more browns than greens by weight for a stable pile.
Check moisture
Squeeze a handful — a few drops is perfect.
Turn and wait
Use the time estimate as a guide; turning speeds results.
Why this matters
A balanced mix heats properly, smells earthy (not rotten), and finishes faster.
Too many greens create sludge and odours; too many browns stall the pile.
Understanding C:N helps you troubleshoot instead of guessing.
Common mistakes
All grass clippings
Matted greens go anaerobic — mix with shredded cardboard.
Soggy, sealed piles
Without air, compost smells and slows. Turn and add browns.
Huge woody chunks
Chip or shred browns so microbes can work faster.
Adding diseased plants
Avoid persistent weeds with seed and seriously diseased material in cold heaps.
Never turning
Occasional aeration dramatically improves speed and quality.
Tips from gardening experts
Layer like lasagne
Alternate greens and browns rather than dumping one material.
Chop materials
Smaller pieces compost faster.
Cover the heap
A lid or tarp helps manage moisture in wet UK weather.
Finish with screening
Sieve mature compost for pots; return coarse bits to the heap.
Frequently asked questions
What is a good C:N ratio for compost?
Around 25–30:1 is a common target. Our calculator estimates C:N from your green and brown weights.
What counts as green material?
Nitrogen-rich soft waste: grass clippings, fruit/veg scraps, coffee grounds, fresh weeds (without seed).
What counts as brown material?
Carbon-rich dry waste: leaves, cardboard, paper, straw, wood chips, woody prunings.
Why does my compost smell?
Usually too wet or too many greens. Add browns and turn for oxygen.
How wet should compost be?
Like a wrung-out sponge — damp, not dripping.
How long does composting take?
Often 2–6 months depending on mix, turning, particle size and bin size.
Can I compost citrus and onions?
In moderation in a hot, active heap. Large amounts can slow worms in cool bins.
Should I add activators?
Fresh greens or a shovel of mature compost often work as well as shop activators.
Can I compost cooked food?
Avoid cooked food and meat in open heaps — they attract pests. Use a suitable closed system if allowed locally.
When is compost ready?
When it is dark, crumbly and earthy-smelling, with few recognisable scraps.
Sources and further reading
Want more detail? These pages from the Royal Horticultural Society (RHS), the UK’s gardening charity, explain the topic in depth.
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Downloads
Greens & browns checklist
Printable dos and don’ts for the compost bin.
C:N quick guide PDF
Reference sheet (placeholder).
Get seasonal gardening advice
See what to sow, plant and prune each month in the Garden Plant Finder seasonal calendar.
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Use Plant Finder filters for soil, sun, wildlife and flowering month — then plan quantities with these calculators.