Compost Carbon Nitrogen Ratio Calculator
Calculate the carbon to nitrogen ratio of your compost mix.
Enter materials and amounts to achieve the ideal 25-30:1 C:N ratio for fast decomposition.
Select a green (nitrogen) and brown (carbon) material to calculate the combined C:N ratio of your mix.
Understanding Carbon to Nitrogen Ratio in Composting
The carbon to nitrogen (C:N) ratio is the single most important factor in successful composting. Microorganisms that break down organic matter need both carbon for energy and nitrogen for protein synthesis. The ideal C:N ratio for active composting is between 25:1 and 30:1, meaning 25 to 30 parts carbon for every 1 part nitrogen by weight. Getting this ratio right results in fast decomposition, minimal odor, and nutrient-rich finished compost.
Common Material C:N Ratios:
| Material | C:N Ratio | Category | Notes |
|---|---|---|---|
| Fresh grass clippings | 15:1 - 25:1 | Green (nitrogen) | Best mixed with browns |
| Kitchen vegetable scraps | 15:1 - 20:1 | Green (nitrogen) | Chop small for faster breakdown |
| Coffee grounds | 20:1 | Green (nitrogen) | Include the paper filter |
| Fresh manure (poultry) | 7:1 - 10:1 | Green (nitrogen) | Very high nitrogen, use sparingly |
| Fresh manure (horse) | 25:1 - 30:1 | Balanced | Already near ideal ratio |
| Fresh manure (cow) | 20:1 - 25:1 | Green (nitrogen) | Good activator for brown piles |
| Dry leaves | 40:1 - 80:1 | Brown (carbon) | Shred for faster decomposition |
| Straw | 80:1 - 100:1 | Brown (carbon) | Provides good air circulation |
| Sawdust | 200:1 - 500:1 | Brown (carbon) | Use in very small amounts |
| Cardboard | 300:1 - 500:1 | Brown (carbon) | Shred and wet thoroughly |
| Newspaper | 150:1 - 200:1 | Brown (carbon) | Shred into strips |
| Wood chips | 400:1 - 700:1 | Brown (carbon) | Very slow to decompose |
| Hay | 25:1 - 40:1 | Brown (carbon) | May contain weed seeds |
The Calculation Formula:
Combined C:N = Sum of (Weight x C:N ratio) / Sum of Weight
For a simplified approach, aim for roughly equal volumes of green and brown materials. Since brown materials are usually lighter by weight, equal volumes typically approximate the ideal ratio. In metric terms, a well-balanced compost bin might contain 50 kilograms of green materials mixed with 50 kilograms of dry leaves or straw.
What Happens When the Ratio is Wrong:
- Too much carbon (over 40:1): Decomposition slows dramatically. The pile stays cool and may take a year or more to finish. Add more green materials or nitrogen-rich amendments like blood meal.
- Too much nitrogen (under 15:1): The pile becomes slimy, smells like ammonia, and attracts flies. Excess nitrogen escapes as ammonia gas, wasting valuable nutrients. Add dry leaves, straw, or cardboard to absorb moisture and balance the ratio.
- Just right (25:1 to 30:1): The pile heats up to 130 to 160 degrees Fahrenheit (55 to 71 degrees Celsius) within days, decomposition is rapid, and the compost smells earthy.
Moisture and Aeration:
Even with a perfect C:N ratio, compost needs adequate moisture (40 to 60 percent, like a wrung-out sponge) and oxygen. Turn the pile every 1 to 2 weeks to introduce air. A pile that is too wet or too compacted will go anaerobic, producing foul odors regardless of the C:N ratio. A properly managed hot compost pile can produce finished compost in 4 to 8 weeks.
Practical Tip:
Start with a layer of coarse brown material (sticks, wood chips) at the bottom for drainage. Alternate layers of green and brown materials, each 4 to 6 inches (10 to 15 cm) thick. Water each layer lightly. The pile should be at least 3 feet by 3 feet by 3 feet (1 meter cubed) to generate and retain enough heat for active composting.