Hot Composting Calculator

The EPA recommends a C:N ratio of 25:1 to 30:1 for active hot composting. Too much carbon (high C:N) slows decomposition because there is insufficient nitrogen for microbial activity. Too much nitrogen (low C:N) causes ammonia loss and odour. A mix of roughly equal volumes of brown (carbon-rich) and green (nitrogen-rich) materials typically gives a ratio in the ideal range.

Brown materials are carbon-rich inputs: dry leaves (C:N approximately 60:1), straw (80:1), cardboard (350:1), wood chips (400:1), and paper (150 to 200:1). They provide the carbon structure for microbial decomposition and help maintain airflow in the pile. A pile with mostly browns will decompose slowly (cold composting).

Green materials are nitrogen-rich inputs: grass clippings (C:N approximately 15:1 to 20:1), vegetable scraps (15:1), coffee grounds (20:1), fresh garden trimmings (15:1 to 30:1), and manure (7:1 to 20:1 depending on animal). They provide the nitrogen that drives microbial metabolism and heat generation in an active pile.

An active hot compost pile should reach 130 to 160 degrees F (55 to 71 degrees C) in the core. These temperatures are required to kill weed seeds and pathogens. The EPA's Process to Further Reduce Pathogens (PFRP) standard requires 55 degrees C or more for 3 consecutive days in aerated static piles or windrows.

The pile should have 50 to 60% moisture content, which is approximately the moisture of a wrung-out sponge. If you grab a handful and squeeze, it should release just a few drops. Too dry and microbial activity slows; too wet and anaerobic conditions produce odours. Water is added when adding dry brown materials or when the pile temperature drops and the pile looks dry.