The mix ratio is a weighted average, not an average of ratios
Each ingredient carries carbon and nitrogen in a known proportion. Split every material by mass into carbon and nitrogen, add each up across the pile, then divide total carbon by total nitrogen. Because it is weighted by mass, a small amount of a very brown material shifts the result a lot.
Aim for a starting mix of 25:1 to 30:1. Below 20 it turns to ammonia and smell; above 35 it decomposes slowly for want of nitrogen.
C:N ratio of common materials
Browns are carbon-rich (dry, woody); greens are nitrogen-rich (moist, fresh). A pile needs plenty of both, weighted so the total lands near 30:1.
| Material | Type | C:N ratio |
|---|---|---|
| Sawdust | Brown | 400:1 |
| Cardboard | Brown | 350:1 |
| Shredded paper | Brown | 200:1 |
| Straw | Brown | 150:1 |
| Autumn leaves | Brown | 50–60:1 |
| Grass clippings | Green | 20:1 |
| Coffee grounds | Green | 20:1 |
| Vegetable scraps | Green | 15–20:1 |
| Fresh manure | Green | 15–25:1 |
Because browns run 10 to 20 times higher in carbon, a 1:1 mix by weight is far too carbon-heavy. You need much more green mass than the ratios alone suggest.
Reading and fixing the pile
- Too much nitrogen (below 20:1). Wet, slimy, ammonia smell. Add browns — leaves, cardboard, straw.
- Too much carbon (above 35:1). Dry, barely warms, sits unchanged for months. Add greens — grass, coffee grounds, food scraps.
- Heat is the tell. A pile near 25:1 runs hot at 55 to 65 C. If it never warms up, it is usually short on nitrogen.
- The ratio drops as it works. Carbon leaves as CO2, so a mix that starts at 30:1 finishes nearer 10 to 15:1. That fall is normal and marks progress.
Common questions
What is the ideal C:N ratio for compost?
About 25:1 to 30:1 by weight. In that range the microbes work fastest, the pile heats to 55 to 65 C, and finished compost is ready in three to six months.
How is the overall ratio of a mix worked out?
It is a weighted average, not a simple average of the ratios. Each material is split into its carbon and nitrogen mass, those are summed across the whole pile, and the total carbon is divided by the total nitrogen. Heavier ingredients count for more.
What happens if my pile is too high in nitrogen (C:N below 20)?
It overheats and smells of ammonia as the surplus nitrogen escapes as gas. Add carbon-rich browns such as dry leaves, cardboard or straw to bring it back up.
Does one part green to three parts brown work?
As a rough volume guide, yes, but it is only an approximation because materials differ widely in ratio and density. A green like grass is around 20:1 while cardboard is 350:1, so weighing and using the actual ratios is far more accurate.
