How to calculate concrete volume
Concrete is measured by volume: Volume = Length × Width × Thickness. Keep the units consistent — for a metric slab, convert thickness from cm to metres (÷100) so the answer is in cubic metres. For imperial, convert thickness from inches to feet (÷12) for cubic feet, then divide by 27 for cubic yards.
How many bags of concrete?
Each pre-mix bag yields a set volume once mixed — roughly 0.017 m³ (0.6 ft³) for an 80 lb bag, less for smaller bags. Divide your total volume (plus waste) by the bag's yield and round up. For anything above about 1 m³ (≈35 ft³), ready-mix delivery is usually cheaper and easier than mixing dozens of bags.
Concrete and bags for common slab sizes
Volume and pre-mix bags for slabs 10 cm (4 in) thick, including the calculator's default 10% waste allowance. For a thicker slab, scale up — a 15 cm slab needs 1.5 times as much.
| Slab · 10 cm | Concrete | 25 kg bags | 40 kg bags |
|---|---|---|---|
| 2 × 2 m | 0.44 m³ | 81 | 50 |
| 3 × 3 m | 0.99 m³ | 181 | 113 |
| 3 × 4 m | 1.32 m³ | 241 | 151 |
| 4 × 4 m | 1.76 m³ | 321 | 200 |
| 4 × 5 m | 2.2 m³ | 401 | 250 |
| 5 × 5 m | 2.75 m³ | 501 | 313 |
| 6 × 6 m | 3.96 m³ | 721 | 450 |
| Slab · 4 in | Concrete | 60 lb bags | 80 lb bags |
|---|---|---|---|
| 8 × 8 ft | 0.87 yd³ | 53 | 40 |
| 10 × 10 ft | 1.36 yd³ | 82 | 62 |
| 10 × 12 ft | 1.63 yd³ | 99 | 74 |
| 12 × 12 ft | 1.96 yd³ | 118 | 88 |
| 10 × 20 ft | 2.72 yd³ | 164 | 123 |
| 20 × 20 ft | 5.43 yd³ | 328 | 245 |
| 24 × 24 ft | 7.82 yd³ | 471 | 352 |
How many bags make a cubic metre or yard?
| Bag size | Bags per m³ | Bags per yd³ |
|---|---|---|
| 80 lb bag | 58.8 | 45 |
| 60 lb bag | 78.7 | 60.2 |
| 40 kg bag | 113.6 | 86.9 |
| 25 kg bag | 181.8 | 139 |
Tips for ordering
- Always add a waste allowance (5–10%) for spillage, over-dig and uneven subgrade.
- Standard slab thicknesses: ~10 cm (4″) for paths and patios, 15 cm (6″)+ for driveways.
- Yields vary by brand — check your bag, and treat bag counts as a close estimate.
How many 80 lb bags of concrete for a 10 × 10 slab?
About 62 bags for a 4-inch slab, including 10% waste — about 1.36 cubic yards. At that size, ready-mix delivery is often cheaper and far less work.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb, or 60 bags of 60 lb. In metric, a cubic metre takes about 114 bags of 40 kg.
Bags or ready-mix?
Bags suit small jobs (a few footings, a small pad). For larger pours, ready-mix delivered by the cubic metre/yard saves time and often money.
What thickness do I need?
Paths/patios ~10 cm (4″); driveways and load areas 15 cm (6″) or more. Follow local codes for structural work.
Does it work for footings and columns?
Yes — treat length × width × depth as the box volume. For round columns, multiply π × radius² × height separately.