Free Compost Rate calculator with clear step-by-step results.
Compost is sold by volume but spread by depth, so the starting point is area times depth. Multiplying by bulk density gives the mass you actually have to shift, and restating the same figure as tonnes per hectare lets you compare a garden dressing against agronomic guidance.
Volume required
Volume m3 = area m2 x depth mm / 1000
Field rate
Tonnes per hectare = (mass kg / area m2) x 10,000 / 1000
Ten to fifteen millimetres as an annual mulch on established beds, and 25-50 mm when improving a new bed before planting. Deeper than that risks smothering shallow roots.
Moisture content and screening. A fine, wet green-waste compost can exceed 800 kg/m3 while a dry, woody one may be under 400.