Compost Ratio Calculator — C:N Ratio & Mix Balance

The ideal carbon-to-nitrogen ratio is 25-30:1. This gives microbes enough energy (carbon) and protein (nitrogen) to decompose materials efficiently. Too high and it decomposes slowly; too low and it becomes slimy and smelly.

Green materials (kitchen scraps, grass clippings) are nitrogen-rich, typically 10-25:1. Per the On-Farm Composting Handbook, grass clippings average about 17:1 and food/vegetable scraps about 15:1. The calculator defaults green material to 20:1 to cover a typical mixed kitchen-and-garden bucket; lower it toward 15-17:1 if your greens are mostly fresh grass or food waste.

Brown materials (dry leaves, cardboard, straw) are carbon-rich, typically 40-80:1. Autumn leaves are about 40-80:1, straw averages around 80:1, and corrugated cardboard is roughly 350-560:1. The calculator defaults brown material to 60:1 (mixed dry leaves); raise it for shredded cardboard or wood chips.

Ideal moisture is like a wrung-out sponge (40-60%). Too dry and decomposition stalls because microbes need water. Too wet (dripping or waterlogged) and the heap excludes air, turns anaerobic and smelly, and slows down. Add dry browns to a soggy heap and water to a dry one. The time estimate here is a rough guide, not a cited standard.

Hot composting with good C:N ratio: 8-12 weeks. Cold composting: 6-12 months. Factors include C:N ratio, moisture, aeration (turning), and particle size.