USCC member Steve Churchill of the Urban Worm Company has developed an easy-to-use compost calculator which can quickly calculate C:N ratios for a list of commonly-composted materials. Simply pick from a list of ingredients categorized as greens or browns and the calculator will return a C:N value based on the mass and bulk density of each material. You can choose metric or English units, from as small as ounces and grams to cubic meters and cubic yards, taking into account the bulk density of each ingredient.
The Urban Worm Company is geared more towards beginning vermicomposters and composters, so the calculator doesn't account for advanced factors like particle size, moisture, or the significant variability of general ingredients like food waste. But it can be a handy tool to funnel beginners and small operators into an acceptable C:N range. Beginners might also enjoy the discussion on the page regarding desired C:N ratios for composting vs vermicomposting and how similar volumes of carbon-rich materials will produce massive differences in C:N due to the wide variability in bulk densities.
Use the Urban Worm Company's Compost Calculator.