How much concrete?
Volume from the dimensions, plus a waste allowance — then ready-mix, pre-mixed bags or a site-mix materials list.
Quantities are estimates. Site-mixed concrete by volume is not a certified strength class: where a drawing, engineer or building control asks for C20/C25, GEN or RC concrete, order ready-mix to BS 8500.
Concrete mix ratios and quantities per m³
Typical site-mix quantities derived from published merchant batch figures. Densities used: cement 1,440 kg/m³ loose, sharp sand 1.6 t/m³, 20 mm gravel 1.6 t/m³, all-in ballast 2.0 t/m³.
| Mix | Cement | 25 kg bags | Sharp sand | Gravel / ballast |
|---|---|---|---|---|
| 1 : 2 : 3 (sand : gravel) | 336 kg | 14 | 0.47 m³ / 0.75 t | 0.70 m³ / 1.12 t |
| 1 : 2.5 : 3.5 (sand : gravel) | 288 kg | 12 | 0.50 m³ / 0.80 t | 0.70 m³ / 1.12 t |
| 1 : 1.5 : 2.5 (sand : gravel) | 403 kg | 17 | 0.42 m³ / 0.67 t | 0.70 m³ / 1.12 t |
| 1 : 3 : 6 (sand : gravel) | 168 kg | 7 | 0.35 m³ / 0.56 t | 0.70 m³ / 1.12 t |
| 1 : 5 (ballast) | 288 kg | 12 | — | 1.00 m³ / 2.00 t ballast |
| 1 : 6 (ballast) | 240 kg | 10 | — | 1.00 m³ / 2.00 t ballast |
| 1 : 3.5 (ballast) | 411 kg | 17 | — | 1.00 m³ / 2.00 t ballast |
Damp sand and ballast vary with moisture, so buy aggregates rounded up to the next half or whole cubic metre or bulk bag, and add a little extra cement.
| Designation | Strength N/mm² | Typical use |
|---|---|---|
| GEN1 / ST2 | 10 | Unreinforced strip footings, trench fill, drainage |
| GEN3 / ST4 | 20 | Shed and garage bases, unreinforced floors, light domestic driveways |
| RC20/25 | 25 | Lightly reinforced house or garage floors (ground-bearing) |
| RC25/30 | 30 | Lightly reinforced work, reinforced foundations in non-aggressive ground |
| PAV1 | — | Domestic driveways and patios exposed to frost (air-entrained) |
Designated concretes for housing and general work, as ordered from a ready-mix supplier. The structural design, foundation depth and ground conditions decide what is needed.
How to work out concrete
Volume
Slab or footing: length × width × depth, all in metres. A 3 × 3 m shed base 100 mm thick is 3 × 3 × 0.1 = 0.9 m³. Post holes: π × radius² × depth for each hole, less the part of the hole the post fills.
Waste
Add an allowance for uneven ground, spillage and formwork bulging. 5–10% is a typical starting point; more on rough or hand-dug excavations.
Ready-mix, bags or site mix?
- Ready-mix — best for anything above about a cubic metre or when a strength class is specified. Ask the supplier about minimum loads and part-load charges.
- Pre-mixed bags — easy for small jobs and post holes. A 20 kg bag makes about 0.01 m³, so 1 m³ takes about 100 bags.
- Site mix — cement with sharp sand and gravel or all-in ballast, measured by volume with a bucket or shovel. Keep the water down: too much water weakens the concrete.
Questions groundworkers ask
How do I work out how much concrete I need?
Multiply length × width × depth in metres to get cubic metres, then add 5–10% for waste. A 3 × 3 m slab 100 mm thick is 0.9 m³, or about 1 m³ with 10% added.
How many bags of concrete make 1 m³?
About 100 bags of 20 kg ready-to-use concrete, at roughly 0.01 m³ per bag, or about 80 bags of 25 kg. Check the yield printed on the bag.
What is the mix ratio for concrete?
Common site mixes by volume are 1 : 2 : 3 (cement : sharp sand : gravel) for general work, 1 : 5 cement to all-in ballast for general purpose, and stronger mixes such as 1 : 1.5 : 2.5 for paving. They are guides, not certified strength classes.
Is a 1:2:4 or 1:5 mix the same as C20?
Not reliably. A volume-batched site mix is not tested and cannot be guaranteed to meet a strength class such as C20 or C25. If a strength or designation is specified, order ready-mix concrete to BS 8500.
More free tools for groundworkers
Aggregate calculator — Type 1, sand, gravel, topsoil — tonnes and bags› Excavation calculator — dig volume, bulking, skips and grab loads› All free calculators for the trades›Muck, plant and money — all accounted for
Excavation inspections, plant and materials logged per job, signed dayworks, delay records and live costing.
See Graftum for groundworksEstimates only; for a specified strength class order ready-mix to BS 8500.