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Concrete calculator
The volume for a slab or trench-fill footing, as a ready-mix order or as bags to mix on site.
You'll need about
- Concrete volume
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- Ready-mix to order
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- If mixing yourself:
- Cement (25kg bags)
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- Sharp sand
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- 20mm gravel
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Includes your wastage allowance. Hand-mix quantities are for a general-purpose 1:2:4 mix (roughly C20). Footing depths and concrete grade should come from your engineer or building control.
Get the full job measuredHow the numbers are worked out
Volume is length x width x depth, plus your wastage allowance. The ready-mix figure is rounded up to the next 0.25 m³. Many suppliers have a minimum load or charge a part-load fee under about 3 m³, so check before you book.
For a 1:2:4 mix, each cubic metre of finished concrete takes roughly 320 kg of cement (13 bags), 0.7 tonnes of sharp sand and 1.3 tonnes of 20mm gravel. You can use about 2 tonnes of all-in ballast instead of the separate sand and gravel.
Typical depths
| Use | Typical depth |
|---|---|
| Garden shed base | 75–100mm |
| Ground-bearing floor slab | 100–150mm |
| Driveway | 100–150mm |
| Trench-fill footing (extension) | Often 1–1.5m deep, 600mm wide |
These are only rules of thumb. Footing depth depends on the ground, nearby trees and drains, and is set by your engineer or building control.
Need the whole job measured?
Send us your drawings and we take off every item, price it at current UK rates and send back an itemised estimate. From £120, in 3–5 working days.