How much topsoil for a raised bed?
Last reviewed 1 July 2026
Short answer
Use the internal dimensions, not the outside of the timber, and add 20–25% for settlement. A 2.4 m × 1.2 m bed filled to 400 mm needs 1.15 m³ of finished soil, so order about 1.44 m³ — two UK bulk bags. For beds deeper than about 450 mm, part-fill the bottom with cheaper material.
Work it out for your project with the topsoil calculator →
Measure inside the timber
A bed built from 150 mm × 47 mm sleepers with an external size of 2.4 m × 1.2 m has an internal size of about 2.31 m × 1.11 m once you take off the board thickness at each end.
That is 2.56 m² instead of 2.88 m² — an 11% difference, which on a deep bed is most of a bulk bag.
Measure the depth from the inside base to where you actually want the soil to finish, which should be 25–50 mm below the top edge so watering does not wash soil over the side.
volume = internal length × internal width × fill depth
Fill depths and quantities
| Bed (internal) | Fill depth | Finished volume | Order | Bulk bags |
|---|---|---|---|---|
| 1.2 × 0.6 m | 300 mm | 0.22 m³ | 0.27 m³ | 1 |
| 1.8 × 0.9 m | 300 mm | 0.49 m³ | 0.61 m³ | 1 |
| 2.4 × 1.2 m | 300 mm | 0.86 m³ | 1.08 m³ | 2 |
| 2.4 × 1.2 m | 400 mm | 1.15 m³ | 1.44 m³ | 2 |
| 3.0 × 1.2 m | 500 mm | 1.80 m³ | 2.25 m³ | 4 |
Bulk bags taken as 0.73 m³.
Do not fill a deep bed with topsoil all the way down
Most vegetables root in the top 300 mm. Filling a 600 mm bed entirely with bought topsoil means paying for 300 mm of soil that nothing will use.
Part-fill the bottom instead. Inverted turf, coarse woody material, rough subsoil from elsewhere in the garden or clean rubble all work, provided drainage is not impeded. Then put quality topsoil in the top 300–400 mm.
This also reduces settlement in the top layer, because the bulk of the depth is material that has already consolidated.
Note: Woody fill settles a lot
Getting the mix right
- A 70:30 topsoil to compost blend suits most vegetable beds. Pure compost is too light and slumps; pure topsoil can be heavy and slow to warm.
- Check the topsoil grade. In the UK, BS 3882 multipurpose grade is the sensible default. Ask for the analysis if you are growing food.
- Do not use soil from a skip or an unknown source in a bed you will eat from without knowing what was on the site.
- Line the timber, not the base. A plastic liner against the boards extends their life. Never line the base — the bed needs to drain into the ground below.
Safety notice: Material planning only