How the topsoil calculator works
Soil and mulch are sold by volume (cubic metres or bulk bags) or by weight. The calculator works out the volume of your bed, adds a settlement allowance (loose soil sinks by 10–20% once it's been rained on and firmed), then converts to tonnes and bulk bags:
How much does a bulk bag cover?
| Depth | 0.75m³ bag covers | 1m³ bag covers |
|---|---|---|
| 50mm (mulch) | 15 m² | 20 m² |
| 100mm (lawn) | 7.5 m² | 10 m² |
| 150mm | 5 m² | 6.7 m² |
| 300mm (border) | 2.5 m² | 3.3 m² |
Figures are before settlement. Ask your supplier for screened topsoil to BS 3882, which is free of stones, roots and debris.
Worked example: a new lawn area
A back garden patch of 5m × 4m needs 150mm of topsoil before turfing. That's 20m² × 0.15m = 3m³. With a 20% settlement allowance it becomes 3.6m³, which is about 4.9 tonnes of screened topsoil at 1.35t/m³. In 0.75m³ bulk bags that's 4.8, so order 5 bags. At £75 a bag the soil costs about £375. The same volume in 40-litre garden-centre bags would be 90 bags, which shows why bulk bags are the sensible choice for anything bigger than a planter.
Once it's spread and firmed, use the turf calculator to order rolls for the same area. Your measurements carry over with the "Next for this job" link under the result.
Bulk bags or a loose load?
- Bulk bags (typically 0.75–1m³) are craned onto your drive or the pavement edge, so they suit most homes. Check the bag size, as some suppliers sell "tonne bags" that hold less than 1m³ of soil.
- Loose tipper loads are usually cheaper per m³ for 6m³ or more, but the lorry tips the soil in one heap. You need a hard standing area close to the garden and somewhere it can sit for a few days.
- Access matters. If everything must go through the house or down a narrow side passage, allow time for barrowing. One person can move roughly 1m³ an hour with a barrow on a short, flat run.
Preparing the ground first
New topsoil works best over ground that drains. Remove weeds, rubble and any old turf, then break up compacted subsoil with a fork, especially on new-build plots where heavy machinery has driven over it. If you're digging out first to keep levels the same, the excavation calculator works out how much soil comes out and whether you need a skip.
Spread topsoil in layers of about 100mm, firming each layer lightly by treading it, and rake it level before the next. Leave it to settle for a week or two, ideally with some rain, before turfing or planting. Then top up any dips.
Common mistakes
- No settlement allowance. A bed filled level with its edging will sink 10–20%. That's why the calculator adds 20% by default.
- Filling raised beds with topsoil only. Deep beds drain and grow better with a mix of topsoil and compost, or a ready-made veg mix. For sleeper beds, the sleeper raised bed calculator works out both the sleepers and the fill.
- Burying the damp-proof course. Keep soil at least 150mm below the DPC of the house walls, so rain can't splash or soak above it.
- Using unscreened soil for a lawn. Stones and roots make a bumpy lawn and blunt mower blades. Screened topsoil costs a little more and saves a lot of raking.
For borders finished with stone instead of mulch, see the gravel calculator. For more garden jobs, browse all our garden and landscaping calculators.