How Much Concrete Do I Need? Slab, Footing and Post Hole Calculations
Ordering too little concrete is worse than ordering too much: a pour that stops halfway leaves a weak joint. Here's how to work out the volume and the bags.
Volume of a slab or footing
volume = length × width × thickness, all in the same unit.
Example: a patio 10 ft × 10 ft, 4 inches thick. 4 in = 0.333 ft, so 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards (27 ft³ per yd³).
Round holes and columns
volume = π × (diameter ÷ 2)² × depth. A post hole 10 inches wide and 3 ft deep holds about 1.64 ft³ — roughly three 80-lb bags per hole once you account for the post.
From volume to bags
| Bag | Yield |
|---|---|
| 80 lb | about 0.60 ft³ |
| 60 lb | about 0.45 ft³ |
| 40 lb | about 0.30 ft³ |
The patio above plus 10% for waste is about 36.7 ft³ → about 62 bags of 80 lb. The concrete calculator gives the volume, the bag count for each size, and the cement, sand and gravel for mixing on site.
Bags or ready-mix?
Above roughly 1 cubic yard, a ready-mix truck is usually cheaper, faster and more consistent than mixing 45+ bags by hand. Many suppliers have a minimum order or a short-load fee, so ask.
Don't forget
- Add 5–10% for uneven ground and spillage.
- Typical thicknesses: 4 in for patios and walkways, 5–6 in for driveways.
- Structural work — foundations, retaining walls — must follow an engineer's specifications for strength and reinforcement.
Covering the slab afterwards? Work out flooring with the square footage calculator.