Compost Ratio Calculator

Successful composting depends on the right balance of carbon-rich “brown” materials and nitrogen-rich “green” materials.

The ideal carbon-to-nitrogen (C:N) ratio is approximately 25:1 to 30:1. This ratio provides the optimal environment for decomposition microorganisms.

Why the ratio matters:

• Too much carbon (too many browns): Decomposition slows dramatically. The pile may sit for months with little change.
• Too much nitrogen (too many greens): The pile becomes slimy, compacted, and smelly (ammonia odor).
• Just right: The pile heats up quickly, breaks down efficiently, and produces rich, dark compost.

Common brown materials (carbon-rich):

• Dry leaves: C:N ratio ~60:1
• Straw: C:N ratio ~80:1
• Newspaper: C:N ratio ~175:1
• Cardboard: C:N ratio ~350:1
• Wood chips and sawdust: C:N ratio ~400:1

Common green materials (nitrogen-rich):

• Kitchen scraps (fruit/veg): C:N ratio ~15:1
• Grass clippings: C:N ratio ~17:1
• Coffee grounds: C:N ratio ~20:1
• Manure (cow, horse): C:N ratio ~20:1
• Fresh garden waste: C:N ratio ~25:1

Getting from a C:N ratio to a wheelbarrow, in two steps

The published C:N figures are ratios by dry weight, and nobody weighs dry leaves. Two conversions stand between the table and the pile.

First, work out the dry weight ratio. This is not an average of the two C:N figures. Carbon and nitrogen have to be totalled separately, which for materials of similar carbon content gives:

Brown weight per 1 green = ((Target − Green C:N) ÷ (Brown C:N − Target)) × (Brown C:N ÷ Green C:N)

Second, convert weight to volume using how much dry matter each material packs into a given space. This is where the familiar rule of thumb comes from and also where most of the uncertainty lives: a cubic foot of dry leaves holds far less dry matter than a cubic foot of wet kitchen scraps, and shredding or wetting either one changes it substantially.

Run kitchen scraps and dry leaves through both steps and you land on very close to 3 parts brown to 1 part green by volume, the number most composting guides quote. Run grass clippings and leaves through the same two steps and you get roughly 0.7 parts brown to 1 part green, less than a quarter of it.

The reason is density, and it runs the opposite way to intuition. Fresh grass clippings are wet and heavy in the hand, but almost all of that weight is water: a cubic foot holds under 3 lb of actual dry matter. Kitchen scraps hold three times as much. Since the C:N arithmetic works on dry matter, a bucket of scraps brings far more nitrogen to the pile than a bucket of grass does, and needs far more brown to balance it.

Treat the classic 3:1 as a starting point for the kitchen-scrap pile it was written for, not a law. If you are composting a season of lawn clippings, following it will give you a cold heap of leaves that sits until spring.

Additional requirements:

• Moisture: The pile should feel like a wrung-out sponge.
• Air: Turn the pile every 1-2 weeks to introduce oxygen.
• Size: Minimum 3×3×3 feet (1×1×1 m) to generate enough heat.