Mulch calculator for garden beds

How much mulch do I need?

Volume is length × width × depth. A 20 × 4 ft bed with 3 in of mulch takes 20 × 4 × 0.25 = 20 cubic feet, or 0.74 cubic yards. The calculator adds up every bed, adds an allowance (10% unless you change it) because mulch settles and breaks down, then divides by the bag size. Bagged mulch is sold by volume, usually in 2 cubic foot bags in the US.

A worked example

In feet and inches (US). The calculator opens with a 20 × 4 ft bed, 3 in deep, 10% extra and 2 cu ft bags.

• Volume: 20 cu ft, or 0.74 cu yd
• With 10% extra: 22 cu ft
• In 2 cu ft bags: 22 ÷ 2 = 11, so 11 bags of mulch

In metres (UK and elsewhere). A 5 × 1.2 m bed with 7.5 cm of mulch: 0.45 m³, 495 litres with 10% extra, so 10 bags of 50 litres.

How deep should mulch be?

The Royal Horticultural Society recommends a layer at least 2 in (5 cm) thick, ideally 3 in (7.5 cm): thinner than 2 in, weeds come through and the soil still dries out. Lay it over moist soil, after weeding, and never on frozen ground. The calculator starts at 3 in.

How much a bag covers, and when bulk is cheaper

At 3 in deep, a 2 cu ft bag covers 8 square feet and a cubic yard covers 108 square feet. From about 1.3 cubic yards (1 m³, some 18 bags of 2 cu ft) bulk mulch is usually cheaper and comes by truck: the calculator suggests bulk from 1 m³ up, or once you would need more than about forty bags, rounded up to the next half yard.

Common mistakes

• Entering the depth in feet. Depth goes in inches: 3, not 0.25.
• Spreading it too thin. Under 2 in, mulch does not stop weeds.
• Mulching dry soil. Mulch holds the moisture that is already there: water first, then spread.
• Forgetting the top-up. Mulch breaks down, so beds need a fresh inch or so every year.