How to use the raised bed soil calculator
- Enter the inside length and width of the bed and the soil depth.
- Enter how many beds of that size you are filling.
- Keep about 10% extra for settling.
- Adjust the mix so it adds up to 100%, and choose a bag size to see how many bags of each ingredient to buy.
Raised bed formula
Volume = Length × Width × Depth × Number of beds × (1 + settling %) Cubic yards = Cubic feet ÷ 27 · Litres = Cubic feet × 28.32 Each ingredient = Volume × its percentage
Worked example
One 8 × 4 ft bed, 12 in deep, with 10% extra and a 60/30/10 mix:
- Volume = 8 × 4 × 1 = 32 ft³; + 10% = 35.2 ft³ (1.3 yd³)
- Topsoil 60% = 21.1 ft³ → 15 bags of 1.5 cu ft
- Compost 30% = 10.6 ft³ → 8 bags
- Perlite/peat 10% = 3.5 ft³ → 3 bags
Soil needed for common bed sizes (12 in deep)
| Bed size | Cubic feet | Cubic yards | Litres |
|---|---|---|---|
| 2 × 4 ft | 8 | 0.30 | 227 |
| 3 × 6 ft | 18 | 0.67 | 510 |
| 4 × 4 ft | 16 | 0.59 | 453 |
| 4 × 8 ft | 32 | 1.19 | 906 |
| 4 × 12 ft | 48 | 1.78 | 1,359 |
Before settling allowance. For 6 in deep beds halve these numbers; for 18 in, multiply by 1.5.
Choosing the ingredients
Topsoil gives the mix body and holds nutrients; buy screened topsoil or a loam-based garden soil. Compost feeds plants and helps the soil hold water — use well-rotted compost from several sources if you can, such as leaf compost and composted manure. Aeration materials keep the mix light: perlite and vermiculite never break down, while peat and coconut coir hold moisture. In hot, dry climates use more compost and coir; in wet climates use more perlite so beds don’t stay soggy.
Raised bed soil tips
- Loosen the ground underneath before filling so roots and water can move down.
- Mix ingredients well on a tarp or in a wheelbarrow before filling — layers of different materials drain badly.
- Don’t use plain garden soil alone: in a raised bed it compacts and drains poorly.
- Top up each year with 1–2 in (2–5 cm) of compost to replace what plants and settling use up.
- Check bagged soil labels: “raised bed mix” is ready to use; “topsoil” needs compost added.
- Line the bottom with hardware cloth if gophers or moles are a problem, before you add soil.
- Water the new mix thoroughly and let it settle for a day or two before planting, then top up.
Measure the inside of the bed frame, not the outside — thick timber or block walls can take up several inches on each side and reduce the soil you need noticeably.