ClutchCalcs

Garden

Compost Mix Calculator

Hot composting (the kind that heats to 130-160°F, kills weed seeds, and breaks down in 6-12 weeks instead of a year) needs the right balance of carbon-rich "browns" and nitrogen-rich "greens." Too many browns and the pile is dormant. Too many greens and it goes anaerobic, slimy, and smells like death. The target is roughly 25:1 to 30:1 carbon-to-nitrogen ratio by weight. This calculator takes the volumes of browns and greens you have on hand and tells you what your actual C:N ratio works out to — with advice on whether to add more browns, more greens, or call it good and start the pile.

Browns vs greens (and their C:N ratios)

Browns (high carbon):

  • Dry leaves: C:N 60:1
  • Straw: 80:1
  • Cardboard / paper: 350:1 (very high carbon)
  • Sawdust: 500:1 (extreme carbon)
  • Wood chips: 400:1
  • Newspaper: 175:1

Greens (high nitrogen):

  • Grass clippings: C:N 15:1
  • Vegetable scraps: 20:1
  • Coffee grounds: 20:1 (despite being brown in color, these are nitrogen-rich)
  • Fresh manure (chicken, cow, horse): 15:1
  • Garden weeds: 20:1
  • Fruit waste: 35:1

Mix volume-wise about 3 parts browns to 1 part greens for a balanced pile. The math gets more nuanced when using high-carbon materials like cardboard — a 3:1 of cardboard to grass is way too carbon-heavy.

Hot compost vs cold compost

Hot composting: hits 130-160°F in the first week, kills weed seeds and pathogens, finished compost in 6-12 weeks. Requires balanced C:N (25-30:1), pile size at least 3 ft x 3 ft x 3 ft (for thermal mass), turning every 3-7 days for oxygen, and consistent moisture ("damp sponge" feel).

Cold composting: just pile up organic matter and wait. Finishes in 6-24 months. Easier, less effort, but weed seeds and pathogens survive. Better for small-volume kitchen scraps over winter.

For a hot pile: layer browns and greens in 3-inch-deep alternating layers, water as you build, turn weekly. Should hit thermophilic (130°F+) by day 3-5.

How to use this calculator

  1. Pick brown type and volume in gallons.
  2. Pick green type and volume in gallons.
  3. Output: your C:N ratio + advice on adjusting.
  4. Volume conversion: a 5-gallon bucket of dry leaves is ~5 gallons; a 5-gallon bucket of fresh grass is ~5 gallons. Don't worry about compaction — the calculator uses approximate densities.

Common scenarios

Fall leaf-pile composting, 30 gal dry leaves + 10 gal kitchen scraps. Ratio works out to about 35:1 — a bit high. Add another 5-10 gal of kitchen scraps or coffee grounds and you're in the sweet spot.

Spring grass-clipping composting, 30 gal grass + 30 gal dry leaves. Ratio ~30:1 — nearly ideal. Layer 6-inch grass and 6-inch leaves alternately, turn weekly. Will heat up in 3-4 days.

Cardboard-heavy pile: 20 gal shredded cardboard + 10 gal manure. Ratio ~60:1 — way too carbon-heavy. Either add 20+ more gal of nitrogen (more manure or grass) or accept it'll take 12-18 months instead of hot composting.

FAQ

What's the ideal C:N ratio? +
25:1 to 30:1 by weight. Below 20:1: pile gets smelly, anaerobic, loses nitrogen as ammonia. Above 35:1: cold, slow, doesn't heat enough to kill weed seeds. The microbes that decompose the pile need a balanced food source — too much of either side starves them.
Why is sawdust 500:1? +
Sawdust is nearly pure cellulose with very little nitrogen. Compounds beautifully in a balanced mix (fluffs the pile, provides carbon backbone) but if you dump straight sawdust on plants, it actually steals nitrogen from the soil as it slowly decomposes. Mix with high-nitrogen materials before composting.
Can I compost meat / dairy / fats? +
Backyard piles: no. They attract rodents and produce nasty smells. Use a Bokashi system (anaerobic fermentation with EM-1 inoculant) for meat/dairy scraps, or municipal composting if your city offers it. Worm bins (vermicomposting) also avoid meat/dairy.
How wet should the pile be? +
Damp sponge — when you squeeze a handful, you get a drop or two of water. Too dry: decomposition stops. Too wet: anaerobic, smelly. Water the pile when you turn it; cover with a tarp if heavy rain is coming.
How often should I turn the pile? +
For hot composting: every 3-7 days for the first 3 weeks, then weekly until finished. Turning introduces oxygen (aerobic decomposition) and moves uncooked material into the hot center. A pitchfork or compost aerator tool works for backyard piles; tumblers do it via cranking.
How do I know when compost is done? +
Looks like dark, crumbly soil. No recognizable original ingredients (except maybe a few twigs). Smells earthy, not rotten. Pile temperature has dropped to ambient (not actively decomposing). Take 6-12 weeks for hot, 6-24 months for cold.
What about disease and weed seeds? +
Hot composting (130°F+ for 3+ days) kills most weed seeds and pathogens. Cold composting preserves them. For disease-resistant compost, monitor temperature with a long compost thermometer. Don't compost diseased plant material (tomato blight, etc.) unless you can verify hot temps.
How much compost do I need? +
For garden beds: 1-2 inches of compost worked into the top 6 inches of soil = significant improvement. A 100 sq ft garden bed at 1-inch depth = ~8.3 cubic feet = ~3 wheelbarrow loads. Plan accordingly when building piles to match consumption.