Work out garden bed soil volume instantly with clear inputs, formula shown and shareable results.
Bed volume is length times width times depth, plus a settling allowance because fresh compost and topsoil compact by 10-20% after watering. Buying by the cubic metre in bulk is typically a third the cost of the same volume in 50 litre bags.
Soil volume
volume = length x width x depth x (1 + settling allowance)
20 cm for salads and herbs, 30 cm for most vegetables, and 45 cm or more for root crops and permanent planting.
Bagged compost is aerated and dry. Watering collapses the air voids, and organic matter continues to decompose, so topping up each season is normal.