How to calculate concrete for a slab
Multiply length × width × thickness to get the volume. In imperial units, convert the thickness from inches to feet (divide by 12), multiply, then divide the cubic feet by 27 to get cubic yards. In metric, convert the thickness to meters and the result is in cubic meters.
Always order a little extra. Uneven ground, spillage and form flex mean you'll almost always use more than the exact volume – 5–10% is standard.
How many bags of concrete do I need?
| Bag size | Approx. yield | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |
| 25 kg | 0.011 m³ | ≈ 91 per m³ |
| 20 kg | 0.009 m³ | ≈ 112 per m³ |
Bags or ready-mix truck?
Bagged concrete is practical up to roughly 1 cubic yard (about 0.75 m³). Beyond that, mixing by hand gets slow and the first part may start to set before you finish – ordering ready-mix is usually cheaper and easier.
How thick should a concrete slab be?
4 inches (10 cm) is standard for patios, walkways and shed bases. Driveways and slabs for heavy loads are usually 5–6 inches (12–15 cm).
Example: 10×10 ft slab, 4 inches thick
10 × 10 × 0.333 = 33.3 ft³, which is 1.23 yd³. With 10% extra that's about 1.36 yd³, or 62 bags of 80 lb mix.