Compost Ratio Calculator

How to Use the Compost Ratio Calculator

• Estimate the greens you have — kitchen scraps, fresh clippings, spent coffee grounds — in cubic yards.
• Estimate the browns — dry leaves, shredded cardboard, straw — in the same unit.
• Press Calculate to read the pile volume and the current brown-to-green ratio.
• Use 'extra browns for 3:1' as a shopping list: that is the carbon to add to reach the target, worked in as you build rather than dumped on top at the end.
• Confirm with your senses — a balanced pile smells earthy and feels like a wrung-out sponge, with no free water dripping when you squeeze a handful.

The Compost Ratio Formula, Explained

Worked Example: A Fall Leaves-and-Scraps Pile

• You have one cubic yard of kitchen scraps and fresh clippings, plus three yards of shredded leaves.
• Ratio: 3 ÷ 1 = 3, exactly the 3:1 volume target.
• Total pile: 4 cubic yards — comfortably past the minimum size microbes need to build heat.
• Add a second yard of scraps instead, and the ratio falls to 1.5:1 — headed for a stink.
• The calculator then asks for 3 × 2 − 2 = 4 more yards of browns, far more than most people expect.
• That correction figure is the difference between a pile that cooks and one that turns anaerobic and sour — and it is also your cue to split the build across two bays.

Common Browns and Greens

Key Terms

Greens

Nitrogen-rich wet materials — scraps, fresh clippings, manure — that fuel microbial heat.

Browns

Carbon-rich dry materials — leaves, straw, cardboard — that build structure and hold air spaces.

C:N ratio

Carbon to nitrogen by weight; microbes start best near 25–30:1 and the ratio drifts as greens burn off.

Anaerobic

Oxygen-starved conditions in a soggy, overly green pile — sour or ammonia smells are the tell.

Turning

Mixing the pile to restore oxygen; a balanced pile reheats within a day of turning.

What This Means in a Real Compost Pile

• Too many greens is by far the most common failure: if a pile smells sour or ammoniated, add browns — usually about twice as much by volume as your gut says.
• Pile size matters as much as ratio: under 3 × 3 feet it cannot hold heat, and over 5 feet the center goes anaerobic. The calculator's total doubles as a size check.
• Sawdust is a trap ingredient: at 325:1 it stalls decomposition, and raw sawdust worked into beds temporarily locks up soil nitrogen as it breaks down.
• Ratio is a starting point, not a contract — piles finish at anywhere from 20:1 to 40:1; you are mainly trading speed for patience the further you drift.
• Moisture is ratio's hidden twin: even a perfect 3:1 pile goes dormant if it dries out, so water each new layer lightly as you build.

Common Mistakes & Fixes

• Counting coffee grounds as a brown because of their color — the single most common ratio error in small kitchen-driven piles.
• Balancing by smell alone: an earthy pile can still be creeping toward 45:1 and quietly stalling.
• Adding meat, dairy or oily foods, which sour the pile's chemistry and recruit pests instead of microbes.
• Leaving browns unshredded — whole leaves mat into waterproof layers that leave the pile soggy outside and bone-dry inside.
• Buying a tumbler before measuring volume: most tumblers hold under 4 cubic feet, one haul of leaves past that overwhelms the ratio you can correct inside.

When to Use This Ratio (and When Not To)

Compost Ratio Calculator: Frequently Asked Questions