Quick answer
The compost carbon-to-nitrogen ratio is Σ(amount × carbon %) ÷ Σ(amount × nitrogen %). Two ingredients that give 6.5 units of carbon and 0.175 of nitrogen are at 37.1:1, above the usual 25–30:1 target, so add nitrogen-rich material.
Enter each ingredient's amount and its carbon and nitrogen percentages. The calculator returns the C:N ratio and compares it with the 25–30:1 target.
Related tools: Plant Spacing Calculator: Number of plants for a bed at a chosen spacing, in a square or staggered pattern. Lawn Watering Calculator: Gallons of water to apply a depth over a lawn.
How to use the Compost Carbon-to-Nitrogen Ratio Calculator
- Enter ingredient 1 amount (any weight unit, same for all) (0 or more).
- Enter ingredient 1 carbon (% of dry weight) (between 0 and 100).
- Enter ingredient 1 nitrogen (% of dry weight) (between 0 and 100).
- Enter ingredient 2 amount (0 or more). It starts at 0.
- Enter ingredient 2 carbon (%) (between 0 and 100). It starts at 0.
- Enter ingredient 2 nitrogen (%) (between 0 and 100). It starts at 0.
- Enter ingredient 3 amount (0 or more). It starts at 0.
- Enter ingredient 3 carbon (%) (between 0 and 100). It starts at 0.
- Enter ingredient 3 nitrogen (%) (between 0 and 100). It starts at 0.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Ingredient 1 amount (any weight unit, same for all) | A number | ≥ 0 |
| Ingredient 1 carbon (% of dry weight) | A number | ≥ 0, ≤ 100 |
| Ingredient 1 nitrogen (% of dry weight) | A number | ≥ 0, ≤ 100 |
| Ingredient 2 amount | A number | ≥ 0, starts at 0 |
| Ingredient 2 carbon (%) | A number | ≥ 0, ≤ 100, starts at 0 |
| Ingredient 2 nitrogen (%) | A number | ≥ 0, ≤ 100, starts at 0 |
| Ingredient 3 amount | A number | ≥ 0, starts at 0 |
| Ingredient 3 carbon (%) | A number | ≥ 0, ≤ 100, starts at 0 |
| Ingredient 3 nitrogen (%) | A number | ≥ 0, ≤ 100, starts at 0 |
Formula and method
Formula
C:N = Σ(amount × carbon %) ÷ Σ(amount × nitrogen %)
IEEE-754 double precision; shown to at most 6 decimal places.
Combines the carbon and nitrogen in each ingredient, weighted by its amount (dry weight; the amounts must be in the same unit, and the carbon and nitrogen shares are of dry weight). A starting ratio of roughly 25 to 30 parts carbon to 1 part nitrogen is commonly recommended for hot composting; more carbon composts slower, more nitrogen can smell of ammonia. Use lab values or published tables for the carbon and nitrogen share of each ingredient: they vary widely.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Ingredient 1 amount (any weight unit, same for all) = 10; Ingredient 1 carbon (% of dry weight) = 45; Ingredient 1 nitrogen (% of dry weight) = 1.5; Ingredient 2 amount = 5; Ingredient 2 carbon (%) = 40; Ingredient 2 nitrogen (%) = 0.5; Ingredient 3 amount = 0; Ingredient 3 carbon (%) = 0; Ingredient 3 nitrogen (%) = 0.
| Result | Value |
|---|---|
| Carbon-to-nitrogen ratio | 37.1 : 1 |
| Total carbon | 6.5 |
| Total nitrogen | 0.175 |
| Total dry amount | 15 |
| Compared with 25–30 : 1 | Above: add nitrogen-rich material |
Example 2
Inputs: Ingredient 1 amount (any weight unit, same for all) = 100; Ingredient 1 carbon (% of dry weight) = 40; Ingredient 1 nitrogen (% of dry weight) = 1.2; Ingredient 2 amount = 30; Ingredient 2 carbon (%) = 48; Ingredient 2 nitrogen (%) = 2.5; Ingredient 3 amount = 20; Ingredient 3 carbon (%) = 12; Ingredient 3 nitrogen (%) = 3.0.
| Result | Value |
|---|---|
| Carbon-to-nitrogen ratio | 22.3 : 1 |
| Total carbon | 56.8 |
| Total nitrogen | 2.55 |
| Total dry amount | 150 |
| Compared with 25–30 : 1 | Below: add carbon-rich material |
Example 3
Inputs: Ingredient 1 amount (any weight unit, same for all) = 5; Ingredient 1 carbon (% of dry weight) = 50; Ingredient 1 nitrogen (% of dry weight) = 1.0; Ingredient 2 amount = 0; Ingredient 2 carbon (%) = 0; Ingredient 2 nitrogen (%) = 0; Ingredient 3 amount = 0; Ingredient 3 carbon (%) = 0; Ingredient 3 nitrogen (%) = 0.
| Result | Value |
|---|---|
| Carbon-to-nitrogen ratio | 50.0 : 1 |
| Total carbon | 2.5 |
| Total nitrogen | 0.05 |
| Total dry amount | 5 |
| Compared with 25–30 : 1 | Above: add nitrogen-rich material |
The first line is the headline result (carbon-to-nitrogen ratio). Below it the compost Carbon-to-Nitrogen Ratio Calculator lists total carbon, total nitrogen, total dry amount and compared with 25–30 : 1, which describe the same case from other angles. IEEE-754 double precision; shown to at most 6 decimal places.
Why 25–30:1
Microbes use roughly 30 parts carbon for every part of nitrogen. Too much carbon (browns such as straw and leaves) slows decomposition; too much nitrogen (greens such as grass clippings and food scraps) can cause odour and ammonia loss.
Finding the percentages
Typical values: dry leaves about 50:1, straw 80:1, grass clippings 20:1, food scraps 15:1. Use values for your own materials where known; the ratio is by dry weight.
How the result changes with ingredient 1 amount
Holding every other input at the first worked example, this table changes only ingredient 1 amount. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Ingredient 1 amount | Carbon-to-nitrogen ratio | Total carbon | Total nitrogen |
|---|---|---|---|
| 5 | 42.5 : 1 | 4.25 | 0.1 |
| 10 | 37.1 : 1 | 6.5 | 0.175 |
| 20 | 33.8 : 1 | 11 | 0.325 |
| 40 | 32.0 : 1 | 20 | 0.625 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Ingredient 1 amount (any weight unit, same for all): left empty shows "Enter ingredient 1 amount (any weight unit, same for all)." beside that field and no result.
- Ingredient 1 nitrogen (% of dry weight): entering "0" shows "At least one ingredient needs a nitrogen share above zero." beside that field and no result.
Choosing between the Compost Carbon-to-Nitrogen Ratio Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Plant Spacing Calculator](/lifestyle/plant-and-flower-calculator/) | bed length, bed width and spacing between plants | Plants needed |
| [Fertilizer Application Calculator](/lifestyle/fertilizer-application-calculator/) | lawn or bed area, nitrogen to apply and nitrogen in the fertilizer | Fertilizer needed |
| [Lawn Watering Calculator](/lifestyle/lawn-watering-calculator/) | lawn area and water depth to apply | Water needed |
| [Plant Spacing Calculator](/lifestyle/plant-spacing-calculator/) | bed length, bed width and spacing | Plants |
| [Ratio Calculator](/math/ratio-calculator/) | first number and second number | Simplified ratio |
Assumptions and limitations
- Moisture is ignored: use dry-weight amounts
- At least one ingredient must contain nitrogen
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for ingredient 1 amount (any weight unit, same for all) and ingredient 1 nitrogen (% of dry weight) to confirm this.
Frequently asked questions
What if the ratio is too low?
Add browns such as shredded paper or straw.
Does moisture matter?
Yes, aim for a damp sponge.
Is the amount in wet or dry weight?
Use consistent weights.
What does the Compost Carbon-to-Nitrogen Ratio Calculator calculate?
Combined carbon-to-nitrogen ratio of up to three compost ingredients. It returns Carbon-to-nitrogen ratio, Total carbon, Total nitrogen, Total dry amount and Compared with 25–30 : 1.
What formula does the Compost Carbon-to-Nitrogen Ratio Calculator use?
C:N = Σ(amount × carbon %) ÷ Σ(amount × nitrogen %). Combines the carbon and nitrogen in each ingredient, weighted by its amount (dry weight; the amounts must be in the same unit, and the carbon and nitrogen shares are of dry weight).
What do I need to enter in the compost Carbon-to-Nitrogen Ratio Calculator?
You enter Ingredient 1 amount (any weight unit, same for all), Ingredient 1 carbon (% of dry weight), Ingredient 1 nitrogen (% of dry weight), Ingredient 2 amount, Ingredient 2 carbon (%), Ingredient 2 nitrogen (%), Ingredient 3 amount, Ingredient 3 carbon (%) and Ingredient 3 nitrogen (%).
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Moisture is ignored: use dry-weight amounts.
What are the limits of this calculator?
Moisture is ignored: use dry-weight amounts. At least one ingredient must contain nitrogen.
What is an example of a compost Carbon-to-Nitrogen Ratio result?
With ingredient 1 amount (any weight unit, same for all) 10, ingredient 1 carbon (% of dry weight) 45, ingredient 1 nitrogen (% of dry weight) 1.5, ingredient 2 amount 5 and ingredient 2 carbon (%) 40, the carbon-to-nitrogen ratio is 37.1 : 1.
Where does the method come from?
The method follows: Weighted-average carbon-to-nitrogen ratio (mass balance); the carbon and nitrogen shares are user inputs.
Related calculators
- Plant Spacing Calculator — Number of plants for a bed at a chosen spacing, in a square or staggered pattern.
- Lawn Watering Calculator — Gallons of water to apply a depth over a lawn.
- Plant Spacing Calculator — Number of plants that fit a bed at a spacing.
- Ratio Calculator — Simplify a ratio of two whole numbers.
- K/D Ratio Calculator — Kill/death ratio.
Sources
- Weighted-average carbon-to-nitrogen ratio (mass balance); the carbon and nitrogen shares are user inputs
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.