Length × width × thickness, all in feet, divided by 27 gives you cubic yards of concrete. A 10 ft × 10 ft pad at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards — about 56 eighty-pound bags, or 1.4 yards of ready-mix once you add 10% waste.
The one conversion to remember: 4 inches = 0.333 ft, 5 inches = 0.417 ft, 6 inches = 0.5 ft.
Common pad sizes
At 4 inches thick, before waste.
| Pad size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 | 0.20 | 9 | 12 |
| 8 × 8 | 0.79 | 36 | 48 |
| 10 × 10 | 1.23 | 56 | 74 |
| 10 × 20 | 2.47 | 111 | 148 |
| 12 × 12 | 1.78 | 80 | 107 |
| 20 × 20 | 4.94 | 223 | 297 |
| 24 × 24 | 7.11 | 320 | 427 |
At 6 inches instead of 4, multiply every figure by 1.5.
How thick should a concrete pad be?
| Use | Thickness |
|---|---|
| Walkways, small patios | 4 in |
| Shed and hot tub pads | 4 in |
| Driveways, cars | 5–6 in |
| RVs, trucks, heavy equipment | 6 in + |
Four inches is the residential standard. Go thicker where vehicles park. Anything structural — foundations, footings carrying a building — follows your local code, not a rule of thumb, and usually needs an inspection.
What sits under the slab matters as much as the slab. Plan on 4–6 inches of compacted gravel base for drainage and a stable bed; our gravel calculator sizes that layer.
Bags or ready-mix?
The crossover is around 2 cubic yards — roughly 90 eighty-pound bags.
Bags make sense for post holes, small pads, and repairs. They are available at any hardware store and you mix what you need when you need it.
Ready-mix wins past a couple of yards on three counts: it is usually cheaper per yard, it arrives mixed to a consistent strength, and it gives you one continuous pour. Ninety bags mixed by hand means ninety chances for a cold joint — the weak seam you get where a fresh batch meets concrete that has already begun to set. Most suppliers have a short-load minimum (often 1 yard) and charge for it whether you use it or not.
Bag yields
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Footings and post holes
Footings use the same math as a slab: length × width × depth.
Post holes are cylinders — π × (diameter ÷ 2)² × depth. A 10-inch hole 3 feet deep is about 1.6 cubic feet, so a 12-post fence needs roughly 20 cubic feet, or 33 sixty-pound bags. Set posts at least a third of their above-ground height into the ground, and always below the local frost line.
Always add waste
Ten percent is the working default. Forms bow outward under the weight of wet concrete, subgrades are never perfectly level, and some always ends up on the ground. Ordering an extra wheelbarrow costs a few dollars; running short mid-pour costs you a cold joint across the middle of the slab, and there is no fixing that after the fact.
Worked example: a 12 × 16 patio
At 4 inches thick:
- Volume: 12 × 16 × 0.333 = 64 cubic feet
- Cubic yards: 64 ÷ 27 = 2.37 cubic yards
- With 10% waste: 2.6 cubic yards — order 3 from a ready-mix supplier
- As bags: 107 eighty-pound bags, which is why you order ready-mix
- Gravel base at 4 in: another 2.4 cubic yards, about 3.2 tons
Run your own dimensions through the Concrete Calculator — it covers slabs, footings, and post holes, gives bag counts for all three bag sizes, and flags when ready-mix is the better call.