How to Use This Compost Calculator
Using the dropdown options above, add various compost ingredients to arrive at a recipe which gives you your desired carbon-to-nitrogen ratio, of C:N. Add as many items as you like, and subtract them along the way as needed.
What Is My Desired Carbon-to-Nitrogen Ratio for Composting and Vermicomposting?
Desired Composting C:N
The ideal C:N for composting is approximately 25-30:1. When piled to a point where heat can be trapped, this carbon-to-nitrogen ratio allows for thermophilic – or “heat loving” microbes to thrive, allowing your compost bin or pile to reach temperatures of 150°F or higher. This is often – though not always – achieved with equal volumes of nitrogen-rich green and carbon-rich brown waste.
Higher C:N ratios in compost piles will greatly slow decomposition while lower ratios will result in rotting, release of ammonia, and foul odors.
Desired Vermicomposting C:N
Vermicomposting is a much different process and allows for a much wider acceptable range when it comes to C:N. For larger bins, you want to avoid anything in the 25-30:1 C:N range or lower because we do not want to promote thermophilic composting in worm bins which need to remain between 50°F and 90°F to support and environment where worms can thrive.
So for worm composting, we would like to see C:N at 50:1 or higher. As the microbes consume the carbon material and the nitrogen is converted to the more plant-available forms of ammonium (NH4+) and nitrate (NO3-), the C:N will slowly drop over the course of several weeks, assuming no new additions of brown or green material.
Examples of Greens and Browns for Composting
Common examples of green waste are:
- kitchen scraps
- coffee grounds
- manures, grasses
- vegetable trimmings
Examples of brown waste are:
- wood chips
- corn stalks
What to Watch Out for When Using the Compost Calculator
Like most calculators, this one is garbage in, garbage out in terms of data. The following factors will have a significant impact on your C:N ratio and the performance of your compost pile or vermicompost bin.
Some ingredients have a wide variance in C:N ranges which can of course have an outsized effect on the final C:N you think you're getting.
For instance, fruit waste in particular can have C:N as low as 20:1 and as high as 50:1. This is why fruit wastes themselves are not included in the calculator. So, for example, if your food waste includes high amounts of fruit waste, this can effect your calculated C:N.
Particle Size and Composting Efficiency
Composting and vermicomposting are aerobic processes. Diverse particle sizes allow for greater pore spaces to allow oxygen to enter your compost pile. A compost pile with mostly coffee grounds and a low-particle-sized material like sawdust may not compost as effectively as a mix of coffee grounds, sawdust, and wood chips.
Bulk density is the measure of dry weight per unit of volume, often expressed as pounds per cubic foot.
The calculator does take bulk density into account in order to give you accurate C:N ratios. But there is a massive difference in bulk densities, especially with carbon-rich materials. For instance, one cubic yard of horse manure needs 150 gallons of of a loose material like dry leaves to achieve a C:N of 25:1.
But it only takes 3 gallons of sawdust to achieve that same C:N ratio.
Note: This IS a function of sawdust's incredibly high carbon-to-nitrogen ratio, but is also a function of a gallon of sawdust's bulk density relative to dry leaves.
This is a great example of why only considering volume when it comes to your mixture of browns and greens may lead you down the wrong path.
Composting and vermicomposting should both occur at moisture contents between 50-70%.
Below 40-50%, microbial activity slows and composting stops.
Above 70%, the risk of anaerobic conditions increases as moisture displaces air in the pore spaces of the compost, at which point the microbes consume the available oxygen without an avenue to re-oxygenate the pore space.
Got Feedback on the Urban Worm Company's Compost Calculator?
If you see incorrect data, would like to see an ingredient added, or have general questions, please feel free to e-mail me and let me know!