How to calculate concrete
Concrete is ordered by volume: cubic yards in the US, cubic meters almost everywhere else. Multiply the length, width and thickness of the pour in the same unit, then convert.
- Slab or footing: length × width × thickness.
- Round column or post hole: π × radius² × depth.
- Steps: add up each step as its own block. The bottom step is one riser tall, the next is two, and so on.
The most common mistake is mixing units. A 4-inch slab is 0.333 ft thick, not 4. Convert inches to feet (divide by 12) before multiplying, or let the calculator do it.
Worked example: a 10 × 10 ft patio, 4 in thick
10 ft × 10 ft × 0.333 ft = 33.3 cubic feet. Divide by 27 to get cubic yards: 33.3 ÷ 27 = 1.23 yd³. Add 10% for spillage and an uneven base and you need about 1.36 yd³, or 62 bags of 80 lb premix.
How many bags of concrete do I need?
Bagged premix is sold by weight, but what matters is how much volume each bag fills once mixed. These are the typical yields printed on US bags:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Metric bag counts in the calculator use the same yield per kilogram, so a 25 kg bag fills about 0.012 m³. Brands differ slightly, so check the yield on the bag you plan to buy.
Bags or ready-mix?
Bags make sense for post holes, small pads and repairs. Past about one cubic yard, mixing by hand gets slow: that is 45 heavy bags, and the first batch starts setting before the last is poured. Ready-mix trucks deliver it in one go. Many suppliers charge a short-load fee for orders under a few yards, so get a quote for both before deciding.
When ordering ready-mix, round up. Running short halfway through a pour leaves a cold joint, a weak seam where fresh concrete meets concrete that has already started to set.
Typical thickness
| Project | Common thickness |
|---|---|
| Walkway, patio, shed floor | 4 in (10 cm) |
| Car driveway | 4–6 in (10–15 cm) |
| Driveway for heavy vehicles, RVs | 6–8 in (15–20 cm) |
| Fence post holes (see the fence calculator) | Hole about 3× the post width, depth ⅓ to ½ of the post’s height above ground |
For a patio made of pavers instead of poured concrete, use the paver calculator. Footing sizes, frost depth and reinforcement are set by local building codes. Check them before you dig.
How much extra to add
Add 5–10% for spillage, slightly deep forms and an uneven base. Add more if the subgrade has not been leveled and compacted, because every low spot fills with concrete.