A raised bed is a box, so filling it is a volume: the inside length, times the inside width, times the depth you want to fill.
Measure the inside
Use the inside sizes, not the outside. Thick timber or sleeper walls take a surprising amount out of a small bed.
A 2 m by 1 m bed filled 0.4 m deep holds 0.8 m³. That is 800 litres, which is 16 bags of 50 L exactly.
Common sizes
A 2.4 m by 1.2 m bed, the size you get from whole 2.4 m sleepers, holds 1.152 m³ at 0.4 m. That is 1152 litres: 23.04 bags of 50 L, rounded up to 24.
| Inside size | Volume | 50 L bags |
|---|---|---|
| 1.2 × 1.2 m | 0.576 m³ | 12 |
| 2.0 × 1.0 m | 0.800 m³ | 16 |
| 2.4 × 1.2 m | 1.152 m³ | 24 |
Waste and settling
The calculator starts with no waste, because a box is filled, not cut. Soil and compost settle after watering, so add a little if you want the bed full to the brim after the first season.
Bags are counted at the volume printed on them. A bed often takes a mix of topsoil and compost; add the volumes together and the count still works.
The walls
If the walls are garden sleepers, the Sleeper raised bed calculator counts how many to buy, reusing the offcut from one side on another.
Figures are estimates for a shopping list. Check the pack or datasheet before you buy.