Mulch is one of the easiest materials to work out, because it is sold by the litre and the maths lines up neatly with millimetres.
One millimetre, one square metre, one litre
A layer 1 mm thick over 1 m² is 0.001 m³, which is exactly one litre. So the depth in millimetres is also the number of litres per square metre.
A 50 mm layer is 50 litres a square metre. One 50 L bag covers one square metre at that depth.
| Depth | Litres per m² | 50 L bags | 70 L bags | 100 L bags |
|---|---|---|---|---|
| 25 mm | 25 | 0.5 | 0.36 | 0.25 |
| 50 mm | 50 | 1 | 0.71 | 0.5 |
| 75 mm | 75 | 1.5 | 1.07 | 0.75 |
| 100 mm | 100 | 2 | 1.43 | 1 |
A worked border
A 4 m by 3 m border at 50 mm is 0.6 m³, or 600 litres. With 5% waste it is 630 litres. In 50 L bags that is 12.6, so 13. In 70 L bags it is 9 exactly.
B&Q lists Verve bark and mulch at 50 L and 100 L. Bunnings lists 50 L pine bark and a 70 L bag. Check the litres on the bag you pick up.
Bulk
Bulk deliveries are sold by the cubic metre or by the bag, and the volume of a bulk bag varies by supplier. The calculator gives cubic metres and litres for bulk, and does not guess a bag size.
One cubic metre is 1000 litres, which is 20 bags of 50 L.
Before you spread
Mulch settles and breaks down, so a fresh layer looks deeper than it will in a few months. Keep it clear of stems and the base of trees, and follow the depth suggested on the bag.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.