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Compost C:N Ratio Blender Calculator

Calculate compost C:N ratio, moisture, green/brown balance, and correction amounts from common compost materials like leaves, grass, food scraps, cardboard, straw, and manure.

compost calculatorcarbon nitrogen ratioC:N ratiogreens and brownscomposting

Compost C:N ratio blender

Blend greens and browns by weight or volume to estimate carbon-to-nitrogen ratio, moisture, and what to add to balance the pile.

Current C:N ratio Estimated blended carbon-to-nitrogen ratio using dry-weight-adjusted material estimates.
30.1:1
Near active compost range
Moisture estimate Wet minus dry material weight divided by wet weight. Active compost often likes about 40–60% moisture.
62.5%
Target usually 40–60%
Total pile amount Approximate total wet and dry material amount. Volume mode uses approximate material densities.
17 gal
Dry weight 10.3 lb
Green vs brown dry weight Approximate share of dry material coming from nitrogen-rich greens and carbon-rich browns.
37.9% green
62.1% brown
Suggested correction Estimated amount of the selected correction material to move the blend toward the target C:N ratio.
0 gal
Add Fresh grass clippings
After correction Estimated C:N ratio if the suggested correction amount is mixed in.
30.1:1
Target 30:1
MaterialAmountC:NMoistureRole
Dry fallen leaves1060:120%brown
Fresh grass clippings517:180%green
Fruit/vegetable food scraps215:180%green
Compost C:N values and densities are approximate. Real materials vary by moisture, age, particle size, manure source, and how tightly volume is packed. Use smell, temperature, and texture to adjust.

Score from the build-order list

This calculator is item #37 — Compost C:N Ratio Blender from the build-order list, with score:

Vol / Comp / Ratio = 3 / 1 / 3.0

It is the third calculator from the research-prioritised group after the Cabinet Door Paint and Primer Calculator and Pond Pump and Filter Turnover Calculator.

What this estimate does and does not do

This calculator estimates a compost pile’s carbon-to-nitrogen ratio from common “green” and “brown” ingredients. It also estimates moisture, green/brown dry-weight share, and how much of a selected green or brown correction material to add to move toward a target ratio.

It does not laboratory-test your compost. Real C:N values vary by season, age, moisture, material source, particle size, and how tightly you pack a bucket. Use the result as a starting point, then adjust from pile behaviour: smell, temperature, texture, visible decomposition, and moisture.

Formula / method used

Volume or weight conversion

In weight mode, the calculator uses the entered weight directly. In volume mode, it estimates wet weight using an approximate bulk density:

wet weight = volume × material density

Then it estimates dry weight from moisture:

dry weight = wet weight × (1 - moisture % ÷ 100)

Carbon and nitrogen estimate

A material with C:N ratio R has roughly R parts carbon for every 1 part nitrogen. The calculator treats the dry material as carbon + nitrogen parts:

nitrogen part = dry weight ÷ (C:N + 1)
carbon part = dry weight - nitrogen part

Blend ratio:

pile C:N = total carbon parts ÷ total nitrogen parts

Moisture estimate

moisture % = total water weight ÷ total wet weight × 100

A common active-compost target is roughly 40–60% moisture.

Correction amount

If the pile is too carbon-rich, the calculator solves for the selected green material. If it is too nitrogen-rich, it solves for the selected brown material:

(current carbon + added carbon) ÷ (current nitrogen + added nitrogen) = target C:N

The result is converted back to your selected weight or volume unit.

Worked example

Suppose you mix by volume:

  • 10 gallons dry leaves
  • 5 gallons grass clippings
  • 2 gallons food scraps
  • target C:N = 30:1

Using approximate densities and moisture values, this is around:

dry leaves: 10 gal × 0.8 lb/gal × 80% dry = 6.4 lb dry
grass: 5 gal × 2.5 lb/gal × 20% dry = 2.5 lb dry
food scraps: 2 gal × 3.5 lb/gal × 20% dry = 1.4 lb dry

After converting each material’s dry weight to carbon and nitrogen parts, the blend is about:

C:N ≈ 30.1:1

That is close to a hot-composting target, although the moisture estimate may still be high enough that the pile needs dry browns, turning, or airflow.

How to use the result

  • Use weight mode when possible for repeatability.
  • Use volume mode for practical bucket/bin estimates.
  • Chop or shred high-carbon materials such as leaves, cardboard, and straw.
  • If the pile smells sour, ammonia-like, or slimy, add dry browns and turn it.
  • If the pile is dry, cold, and unchanged for weeks, add greens and water gradually.
  • Keep food scraps buried to reduce pests.
  • Avoid meat, oils, and pet waste unless you are following a system designed for them.

Assumptions and limitations

  • Material C:N ratios are approximate reference values.
  • Volume mode depends on loose bulk density assumptions.
  • Moisture estimates are rough and can be very different after rain or drying.
  • The calculator does not model pile size, oxygen, turning frequency, ambient temperature, particle size, or microbial activity.
  • The correction amount solves the ratio math, but the pile may still need water or airflow adjustment.

For other garden/outdoor planning tools, see the Pond Pump and Filter Turnover Calculator and Houseplant Repotting and Soil Mix Calculator.

Frequently asked questions

What is the ideal compost C:N ratio?

Active composting is often targeted around 25:1 to 35:1, with 30:1 as a common rule of thumb. Real piles can still work outside that range, but may be slower or smellier.

What are compost greens and browns?

Greens are nitrogen-rich materials such as grass clippings, food scraps, coffee grounds, and fresh manure. Browns are carbon-rich materials such as dry leaves, straw, cardboard, sawdust, and wood chips.

How does this calculator blend C:N ratios?

It estimates wet and dry weights from your amounts, converts each material's C:N ratio into approximate carbon and nitrogen parts, then divides total carbon by total nitrogen.

Can I calculate compost by volume instead of weight?

Yes. Volume mode uses approximate bulk densities for buckets or gallons/litres of material. Weight mode is more consistent if you have a scale.

Why is my pile smelly or slimy?

A smelly or slimy pile is often too nitrogen-rich, too wet, or compacted. Add dry browns, mix/fluff for air, and check moisture.