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Compost Calculator

Calculate the perfect C:N ratio for your compost

C:N Ratio

46.7:1

Brown

10 lbs

Green

5 lbs

Unit System

Dry Leaves

C:N ratio 60:1

10 lbs

Grass Clippings

C:N ratio 20:1

5 lbs

Recommendation

Your pile is too carbon-rich (too brown). Add more green materials like grass clippings, food scraps, or coffee grounds.

C:N Ratio Guide

Below 20:1Too much nitrogen - smelly, slimy
25-30:1Ideal - fast decomposition
Above 40:1Too much carbon - slow to decompose

Green vs Brown Ratio

Green (Nitrogen-rich)5 lbs
Brown (Carbon-rich)10 lbs
C:N Ratio: 46.7:1 (ideal: 25-30:1)
Moisture: 40% (ideal: 40-60%)40%

What You'll Need

FCMP Outdoor HOTFROG Roto Tumbling Composter

FCMP Outdoor HOTFROG Roto Tumbling Composter

$148-$1604.6
View on Amazon
REOTEMP 20" Backyard Compost Thermometer

REOTEMP 20" Backyard Compost Thermometer

$20-$304.5
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GARDEN STAR 5 Cu Ft Poly Tray Wheelbarrow

GARDEN STAR 5 Cu Ft Poly Tray Wheelbarrow

$65-$904.4
View on Amazon
VIVOSUN 3-in-1 Soil pH/Moisture/Light Tester

VIVOSUN 3-in-1 Soil pH/Moisture/Light Tester

$10-$154.3
View on Amazon
FCMP Outdoor HOTFROG Roto Tumbling Composter

FCMP Outdoor HOTFROG Roto Tumbling Composter

$148-$1604.6
View on Amazon
REOTEMP 20" Backyard Compost Thermometer

REOTEMP 20" Backyard Compost Thermometer

$20-$304.5
View on Amazon
GARDEN STAR 5 Cu Ft Poly Tray Wheelbarrow

GARDEN STAR 5 Cu Ft Poly Tray Wheelbarrow

$65-$904.4
View on Amazon
VIVOSUN 3-in-1 Soil pH/Moisture/Light Tester

VIVOSUN 3-in-1 Soil pH/Moisture/Light Tester

$10-$154.3
View on Amazon

As an Amazon Associate, we earn from qualifying purchases.

Frequently Asked Questions

Q

What is the ideal C:N ratio for compost?

The ideal carbon to nitrogen (C:N) ratio is 25-30:1. This ratio allows microorganisms to decompose materials efficiently. Too much carbon (above 40:1) slows decomposition; too much nitrogen (below 20:1) causes odors and nitrogen loss.

  • Below 20:1 — Too much nitrogen, pile becomes slimy and smelly
  • 25-30:1 — Ideal range for fast, efficient decomposition
  • 30-40:1 — Acceptable but slower decomposition
  • Above 40:1 — Too much carbon, pile takes months to break down
  • Target 130-160°F internal temperature for hot composting
Q

What are the C:N ratios of common compost materials?

Brown materials are carbon-rich: dry leaves (60:1), straw (80:1), cardboard (350:1), sawdust (400:1). Green materials are nitrogen-rich: grass clippings (20:1), food scraps (15:1), coffee grounds (20:1), fresh manure (15:1).

  • Green materials provide nitrogen for microbial growth
  • Brown materials provide carbon for energy and structure
  • Higher C:N ratio = more carbon-rich material
  • Materials with C:N under 30:1 are typically "greens"
  • Materials with C:N over 30:1 are typically "browns"
MaterialC:N RatioTypeMoisture
Grass clippings20:1Green80%
Food scraps15:1Green80%
Coffee grounds20:1Green65%
Fresh manure15:1Green60%
Dry leaves60:1Brown20%
Straw80:1Brown10%
Cardboard350:1Brown5%
Sawdust400:1Brown20%
Wood chips500:1Brown15%
Q

How do I fix smelly compost?

Smelly compost usually has too much nitrogen (green materials) or is too wet. Add brown materials like dry leaves or shredded cardboard, turn the pile to add oxygen, and avoid adding more food scraps until the smell subsides.

  • Ammonia smell: Too much nitrogen — add browns (leaves, straw)
  • Rotten egg smell: Anaerobic (no oxygen) — turn the pile
  • Sour smell: Too wet — add dry browns, improve drainage
  • Turn pile every 1-2 weeks for oxygen circulation
  • Cover pile during rain to prevent waterlogging
Q

Why is my compost not decomposing?

Slow decomposition often means too much carbon, not enough moisture, or poor aeration. Add more green materials, ensure the pile is as moist as a wrung-out sponge, and turn it regularly to introduce oxygen.

ProblemLikely CauseFix
Not heating upToo much carbon (C:N > 40:1)Add grass clippings or food scraps
Smelly / slimyToo much nitrogen (C:N < 20:1)Add dry leaves, straw, or cardboard
Dry and dustyNot enough moistureWater to "wrung-out sponge" level (40-60%)
Matted / compactedNo oxygenTurn pile, add straw for air pockets
Attracting pestsExposed food scrapsBury food under 4-6" of browns
Q

How much brown to green should I add?

By volume, aim for about 2-3 parts brown to 1 part green materials. However, this varies by material density. Use this calculator to check your actual C:N ratio based on the specific materials you're adding.

  • 2:1 by volume (brown:green) is a good starting rule
  • By weight, ratio depends on material density
  • Grass clippings are heavy; leaves are light
  • Shred cardboard and straw for faster decomposition
  • Layer greens and browns like lasagna for even mixing
RecipeBrownsGreensApprox. C:N Ratio
Quick hot pile5 lbs dry leaves5 lbs grass + 3 lbs food scraps~30:1
Balanced garden8 lbs leaves + 2 lbs straw8 lbs grass + 6 lbs food scraps~29:1
Leaf-heavy fall pile15 lbs dry leaves5 lbs grass clippings~50:1 (add greens)
Kitchen scrap heavy3 lbs cardboard10 lbs food scraps~85:1 (add greens)
Q

How long does compost take to finish?

Hot composting (turned regularly, balanced C:N) finishes in 4-8 weeks. Cold composting (passive pile) takes 6-12 months. Vermicomposting (worm bin) takes 3-6 months. Smaller pieces decompose faster.

  • Hot method: 4-8 weeks with regular turning
  • Cold method: 6-12 months, minimal effort
  • Worm composting: 3-6 months for kitchen scraps
  • Chop/shred materials for faster breakdown
  • Finished compost smells earthy, is dark and crumbly

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Example Calculations

1Leaf-Heavy Pile (Too Much Carbon)

Inputs

Dry Leaves10 lbs (C:N 60:1)
Grass Clippings5 lbs (C:N 20:1)

Result

C:N Ratio46.7:1
Brown Materials10 lbs
Green Materials5 lbs
Moisture Level40%
StatusToo Carbon-Rich

Total carbon = (10 × 60) + (5 × 20) = 700. Total nitrogen = 10 + 5 = 15. C:N ratio = 700 / 15 = 46.7:1. This is above 40:1, so more green materials (food scraps, grass) are needed to speed decomposition.

2Three-Material Mix (Near Ideal)

Inputs

Dry Leaves8 lbs (C:N 60:1)
Grass Clippings8 lbs (C:N 20:1)
Food Scraps6 lbs (C:N 15:1)

Result

C:N Ratio33.2:1
Brown Materials8 lbs
Green Materials14 lbs
Moisture Level55%
StatusIdeal Range

Total carbon = (8 × 60) + (8 × 20) + (6 × 15) = 480 + 160 + 90 = 730. Total nitrogen = 8 + 8 + 6 = 22. C:N ratio = 730 / 22 = 33.2:1. This falls in the 20-40:1 ideal range for efficient composting.

3Perfect Balanced Pile (Ideal C:N)

Inputs

Dry Leaves5 lbs (C:N 60:1)
Grass Clippings10 lbs (C:N 20:1)
Food Scraps8 lbs (C:N 15:1)

Result

C:N Ratio27.0:1
Brown Materials5 lbs
Green Materials18 lbs
Moisture Level63%
StatusIdeal Range

Total carbon = (5 × 60) + (10 × 20) + (8 × 15) = 300 + 200 + 120 = 620. Total nitrogen = 5 + 10 + 8 = 23. C:N ratio = 620 / 23 = 27.0:1. This is in the sweet spot of 25-30:1 for fast, efficient decomposition.

Formulas Used

Carbon-to-Nitrogen Ratio

C:N Ratio = Total Carbon / Total Nitrogen

Calculates the overall C:N ratio of the compost pile by summing the carbon and nitrogen contributions of each material.

Where:

Total Carbon= Sum of (weight × C:N ratio) for each material
Total Nitrogen= Sum of weights for all materials

Moisture Level

Moisture % = (Sum of weight × moisture%) / Total Weight × 100

Estimates the overall moisture content of the pile based on each material's moisture percentage.

Where:

weight= Weight of each material in lbs
moisture%= Moisture content percentage of each material

Green vs Brown Weight

Green Weight = Sum of weights where type = "green" | Brown Weight = Sum of weights where type = "brown"

Tracks the total weight of nitrogen-rich (green) vs carbon-rich (brown) materials in the pile.

Where:

Green= Nitrogen-rich materials: grass, food scraps, coffee grounds, manure
Brown= Carbon-rich materials: dry leaves, straw, cardboard, sawdust, wood chips

Mastering Compost Balance: The C:N Ratio Guide

1

The Ideal C:N Ratio: Why 25–30:1 Makes Compost Work

25–30:1 carbon-to-nitrogen ratio is the sweet spot for fast, odor-free composting — this range provides decomposer microorganisms with the exact energy-to-protein balance they need to multiply rapidly and break down organic matter in 4–8 weeks. Below 20:1, excess nitrogen escapes as ammonia gas (the rotten-egg smell). Above 40:1, microbes starve for nitrogen and decomposition stalls for months.

Carbon ("browns") provides energy for microbial metabolism and structural bulk to the pile. Nitrogen ("greens") supplies the protein building blocks microbes need to grow and reproduce. The ratio describes how many parts of carbon exist for every 1 part of nitrogen — so a 30:1 pile has 30 times more carbon than nitrogen by molecular weight.

The calculator computes your actual C:N ratio from the materials you add, using published ratios for 16 common compost ingredients. It instantly shows whether your pile is carbon-heavy (>40:1, add greens), nitrogen-heavy (<20:1, add browns), or in the ideal zone for active decomposition.

C:N Ratio ZonesBelow 20:1Too muchnitrogenSlimy, smellyAdd browns:leaves, strawcardboard25–30:1 IdealPerfect balance130–160°FHot composting4–8 weeksto finishedAbove 40:1Too muchcarbonSlow, cold pileAdd greens:grass clippingsfood scrapsNitrogen-heavyIdeal rangeCarbon-heavy
2

C:N Ratios of Common Compost Materials

Grass clippings at 20:1 and food scraps at 15:1 are the most common "green" (nitrogen-rich) materials in backyard composting. Dry leaves at 60:1 and straw at 80:1 are the workhorse "browns" (carbon-rich). The most extreme materials — sawdust at 400:1 and wood chips at 500:1 — need massive amounts of greens to balance and decompose extremely slowly on their own.

Moisture content varies dramatically by material and affects pile management. Grass clippings arrive at 80% moisture while dry leaves are just 20%. A pile that’s too wet (above 65%) goes anaerobic and smells; too dry (below 40%) halts microbial activity. The calculator estimates overall moisture from the weighted average of your material moisture percentages, targeting the 40–60% "wrung-out sponge" range.

A balanced starter recipe: 5 lbs dry leaves (60:1) + 10 lbs grass clippings (20:1) + 8 lbs food scraps (15:1) = C:N ratio of 27:1. The calculator shows this as "Ideal Range" with a moisture estimate of 63%. This is the gold standard three-ingredient recipe for beginners.

MaterialC:N RatioTypeMoisture %
Food scraps15:1Green80%
Fresh manure15:1Green60%
Grass clippings20:1Green80%
Dry leaves60:1Brown20%
Straw80:1Brown10%
Cardboard350:1Brown5%
3

Troubleshooting: Fixing Smelly, Slow, or Pest-Attracting Piles

Ammonia smell means your C:N ratio is below 20:1 — too much nitrogen causes excess N to escape as gas. The fix is simple: add carbon-rich browns like dry leaves or shredded cardboard. For every 5 lbs of nitrogen-heavy material causing the smell, add 10–15 lbs of dry leaves to push the ratio back above 25:1.

A pile that won’t heat up (staying below 100°F) almost always has too much carbon. The ratio is likely above 40:1, starving microbes of the nitrogen they need to reproduce. Add grass clippings, food scraps, or fresh manure to bring the ratio down. The pile should heat to 130–160°F within 48 hours of correction.

Pest attraction (rats, flies) comes from exposed food scraps on the surface. Always bury food scraps under 4–6 inches of browns. The carbon layer blocks odors that attract pests and maintains aerobic conditions. If flies persist, ensure the pile reaches 130°F+ in the center — this temperature kills fly larvae within minutes.

Tip: Turn your pile every 1–2 weeks to introduce oxygen. Anaerobic (oxygen-starved) conditions cause the worst odors and slowest decomposition, regardless of C:N ratio.

4

Using the Compost Calculator: Add Materials and Check Your Ratio

16 common compost materials are pre-loaded with published C:N ratios, moisture percentages, and green/brown classifications. Add materials by selecting from the dropdown and entering weight in pounds (or kilograms with the unit toggle). The calculator instantly computes your pile’s overall C:N ratio, moisture level, and green vs brown weight balance.

The ratio readout uses color coding: green for ideal (20–40:1), amber for nitrogen-heavy (<20:1), and blue for carbon-heavy (>40:1). Adjust quantities of individual materials and watch the ratio update in real time until you hit the 25–30:1 sweet spot.

  1. 1

    Add your first brown material

    Select dry leaves, straw, or cardboard and enter the weight. Start with 5–10 lbs of leaves as your carbon base.

  2. 2

    Add green materials

    Add grass clippings, food scraps, or coffee grounds. The C:N ratio will drop from the high initial brown value toward the ideal range.

  3. 3

    Check the C:N ratio readout

    Target 25–30:1 for fast hot composting. If above 40:1, add more greens. If below 20:1, add more browns.

  4. 4

    Verify moisture level

    The calculator estimates overall moisture. Target 40–60%. If too wet, add dry browns like shredded cardboard. If too dry, add fresh grass clippings or water the pile.

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Last Updated: Mar 26, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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