What is the carbon to nitrogen ratio of coffee grounds and leaves together?

NRAES-54 lists coffee grounds at C:N 20:1 but publishes no dry-matter nitrogen percent for them, and a mass balance needs both numbers, because the mixture ratio is a nitrogen-weighted mean of the component ratios. This calculator therefore leaves coffee grounds out rather than filling the gap with a figure from somewhere else. Leaves are listed: 0.9% N at 54:1, 38% moisture. If you have a nitrogen analysis for your grounds, add them as a row and enter it.

Do I measure browns and greens by weight or by volume?

By weight, and specifically by dry weight. Volume ratios only work when both materials happen to have the density and moisture the person writing the ratio had in mind. This tool accepts volume input and converts it using the NRAES-54 published bulk densities, but the arithmetic underneath is always a dry-matter mass balance. Where the appendix publishes no density for a material, volume input is refused for that row rather than guessed.

How much sawdust do I add to a pile of fresh grass clippings?

Enter the grass, add a sawdust row and press Solve on it. Sawdust is 61% dry matter at 442:1, so it carries an enormous amount of carbon per kilogram and the C:N target is reached with a small mass. Check the moisture reading afterwards: that small mass does almost nothing for a wet pile, and the moisture solve will usually ask for several times more. When the two answers disagree and the pile is wetter than 60%, follow the moisture answer.

What C:N ratio should I actually aim for?

Rynk's Table 3.1 gives 20:1 to 40:1 as the reasonable range and 25:1 to 30:1 as preferred, with water content 40-65% reasonable and 50-60% preferred. Below about 20:1 surplus nitrogen leaves as ammonia. Above about 40:1 the pile is carbon-limited and runs cold and slow.

Why does my mixture bulk density say the pile will not breathe?

Because the masses and loose volumes entered add up to more than about 1,000 lb per cubic yard, the working ceiling above which free air space is usually inadequate. Manures, produce waste and sludges all sit between 1,000 and 1,600 lb per cubic yard on their own. The fix is a bulking agent with real structure such as wood chips, shredded prunings or straw. The estimate is optimistic, because real mixtures pack denser than their ingredients suggest.

CarbonBalance | Compost C:N Ratio & Moisture Balance Calculator

A compost pile's carbon-to-nitrogen ratio is a dry-matter mass balance, not a volume ratio: the mixture's C:N is the sum of every ingredient's dry mass times its carbon percent, divided by the same sum weighted by nitrogen percent. That is why "two parts brown to one part green" gives a different answer for wet grass than for dry leaves, and why the same bucket of wood chips counts for far more carbon than a bucket of food scraps. Enter what you actually have, by weight or by volume, and this tool returns the mixture C:N, the mixture moisture, and the exact quantity of any one feedstock needed to land on 30:1.

Your pile

Add at least one feedstock

Enter amounts by

Mass unit

Target C:N ratio 30:1

15:1preferred 25:1–30:150:1

Target moisture 55%

30%preferred 50–60%80%

Solve mode

Press Solve on any feedstock row and its amount is computed instead of entered — the quantity that lands the whole mixture on the target above.

Mixture C:N

Add a feedstock

Mixture moisture

Add a feedstock

Total wet mass

Total dry matter

Loose volume

Mix bulk density

Where the carbon and the nitrogen actually are

The two gauges read the mixture against Rynk's recommended windows. Below them, each ingredient's share of the dry matter, of the carbon pool and of the nitrogen pool — which is where the "it looked like enough greens" mistake becomes visible.

What one amendment would do

Add a feedstock to see what it would take to reach your target.

Amendment	Mass to add	Loose volume	Resulting moisture	Resulting bulk density

Each row is solved independently: it assumes you add that one material to the pile you already have, and nothing else.

Why volume ratios give six different answers

Search for a compost recipe and you will be told 2:1, 3:1, 4:1 or 30:1, sometimes by volume and sometimes by weight, usually without saying which. All of them are attempts to approximate one quantity: the ratio of carbon to nitrogen in the dry matter of the pile. Water carries neither. A wheelbarrow of fresh grass clippings is roughly 82% water by weight, so it contributes less than a fifth of its mass to the balance. A wheelbarrow of dry sawdust is about 39% water and weighs a fraction as much per unit volume in the first place. Two "equal" barrows are not remotely equal inputs.

The governing expression is an accounting identity rather than a model, which is the good news: there is nothing to calibrate and nothing to guess. For ingredients 1 through n, with wet mass W, moisture percent M, dry-matter carbon percent C and dry-matter nitrogen percent N:

Mixture C:N — Richard & Trautmann, Equation 5

R = Σ[ W_n × C_n × (100 − M_n) ] ÷ Σ[ W_n × N_n × (100 − M_n) ]

Mixture moisture — Equation 2

G = Σ( W_n × M_n ) ÷ Σ W_n

Both are linear in each W, so both invert exactly. Fix every ingredient but one, name a target, and the quantity of that last ingredient falls straight out with no iteration and no search. That is the Solve button on each row, and it is the question people actually arrive with: not "what is my ratio" but "how much more of this do I need".

How much brown material to add to reach 30:1

Rearranging Equation 5 for the mass of one ingredient x, given a target ratio R and the carbon and nitrogen sums of everything already in the pile:

W_x = ( R × ΣN − ΣC ) ÷ [ (100 − M_x) × N_x × (CN_x − R) ]

The denominator sign is the whole story. If the feedstock you picked has a C:N below the target, it can only pull the mixture down — no positive quantity of it will ever raise the ratio to 30:1. The tool says so plainly rather than returning a negative number dressed up as an answer. The target has to lie between what the pile gives you now and what the new material gives you on its own.

Moisture is usually the binding constraint, not carbon

Work the two balances separately for the same pile and they rarely agree. Richard and Trautmann's worked example makes the point exactly: 10 kg of fresh grass clippings needs 3.5 kg of leaves to reach C:N 30:1, but 6.8 kg of the same leaves to reach 60% moisture. Add the smaller amount and the pile sits at 66% water; add the larger and the ratio drifts to about 37:1.

The published guidance for that conflict is direct: above roughly 60% moisture, water is the more critical variable, so err toward the high C:N side. A pile that is slightly carbon-heavy runs slow. A pile that is waterlogged goes anaerobic, and the difference is one you can smell from the far side of the garden. Below 60% moisture, optimise the ratio and add water as needed.

Bulk density and the porosity ceiling

A balanced ratio in a mixture that packs into a wet brick will still fail. Free air space is what keeps the pile aerobic, and bulk density is its usable proxy. A working ceiling of about 1,000 lb/yd³ (593 kg/m³) is the figure commonly given for feedstock mixes; above that, porosity is generally inadequate and a bulking agent is what fixes it. This tool estimates the mix density by adding masses and adding loose volumes.

That estimate is an upper bound on porosity, not a measurement. A real mixture is denser than its ingredients predict, because particles of different sizes intermingle and fill each other's voids. Treat the figure as "this is the best case, and the pile will be denser than this".

Where the numbers come from

Every feedstock figure on this page is transcribed from one table: Appendix A, Table A.1 of the On-Farm Composting Handbook (NRAES-54, Rynk ed., 1992, Northeast Regional Agricultural Engineering Service), "Typical characteristics of selected raw materials". Nothing is averaged with a second source, and no blog or forum figure has been mixed in. Where the appendix prints an Average row it is used; otherwise the Typical row; where that cell is a printed range, its midpoint. The recommended windows are Rynk's Table 3.1, and the mixture equations are Richard and Trautmann's for the Cornell Waste Management Institute.

The appendix itself carries a warning worth repeating: its ranges and averages "should not be considered as the true ranges or averages, just representative values". A sample of your own material will differ. Every moisture and density field on this page is editable for exactly that reason, and moisture is the field most worth measuring yourself — weigh a sample, dry it, weigh it again.

Note 1 — rows where the appendix's own averages are mutually impossible

Five rows carry a marker in the table below. For those, multiplying the published average %N by the published average C:N gives a dry-matter carbon fraction above 60%, which no biomass reaches. Sawdust is the clearest case: 0.24% N at 442:1 implies 106% carbon. The two figures are averages taken over different underlying studies, which the appendix says outright.

This does not break the calculation. Carbon percent never appears on its own in the mixture equation — it enters only as the product of the ratio and the nitrogen percent, so the result is a nitrogen-weighted mean of the components' C:N ratios and stays well defined. It does mean the nitrogen figure for those rows carries more uncertainty than the others, and a pile dominated by one of them is worth checking against a lab analysis. This page shows no carbon percentage anywhere, because for those rows there is no honest one to show.

NRAES-54 Appendix A feedstock table

All 65 rows this calculator can mix, exactly as printed. Ranges are shown as ranges; the calculator uses the midpoint.

Search Group

Material	Type of value	%N (dry weight)	C:N (weight to weight)	Moisture % (wet weight)	Bulk density (lb/yd³)

Rows in the appendix that this tool does not mix

A dry-matter balance needs a nitrogen percent, a C:N ratio and a moisture content. Six appendix rows publish only some of those, so no honest mixture figure can be produced from them and inventing the missing number is exactly the failure this tool exists to avoid. Five further rows are left out deliberately. If you need one of them, add it as a feedstock whose moisture you enter yourself, or use a laboratory analysis of your own material.

Representative values, not your material. The feedstock figures are the NRAES-54 appendix's own averages across published studies, and the handbook states plainly that they are not true ranges or averages for any particular batch. Moisture in particular varies by tens of percentage points with weather and storage — a mixture computed from table moisture can be well outside the range the pile is actually in. Measure and enter your own moisture where the answer matters.

This tool balances a mixture. It says nothing about pile temperature, pathogen reduction, weed seed kill, or whether any particular compost is safe to apply to food crops. Those are separate questions with separate standards, and a correct C:N ratio does not answer any of them.