Concrete volume is length × width × depth. The two things that trip people up are that depth is in inches while everything else is in feet, and that concrete is sold by the cubic yard — not the cubic foot.
Get those two conversions right and the rest is straightforward.
The calculation
Depth almost always comes in inches, so convert it first by dividing by 12:
| Thickness | In feet |
|---|---|
| 4 in | 0.333 |
| 5 in | 0.417 |
| 6 in | 0.5 |
| 8 in | 0.667 |
| 12 in | 1.0 |
Worked example: a 10 × 12 patio at 4 inches
Add 10% for waste and you're ordering about 1.6 cubic yards. Ready-mix is typically sold in quarter-yard increments, so that's 1.75 yards.
Worked example: a 20 × 24 driveway at 5 inches
With 10% waste, order about 8.25 cubic yards.
Notice how fast this scales. Going from 4 inches to 5 on that driveway adds roughly 1.5 yards — the thickness decision matters more to your bill than the dimensions do.
Get cubic yards and bag counts for your pour
Open Concrete Calculator →Bags or ready-mix?
Bagged concrete yields roughly:
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 |
| 60 lb | ~0.45 cu ft | ~60 |
| 80 lb | ~0.60 cu ft | ~45 |
That 10 × 12 patio at 40 cubic feet works out to roughly 67 bags of 80 lb mix — about 5,400 lbs to haul, open and mix by hand.
The practical dividing line is around 1 cubic yard:
- Under ~1 yard — bags usually make sense. Post holes, small pads, repairs, a couple of steps.
- Over ~1 yard — call a ready-mix truck. Mixing 45+ bags by hand is punishing work, and more importantly you can't keep up.
Ready-mix suppliers usually have a minimum order (often 1 yard) and charge a short-load fee below some threshold. Worth pricing out: the fee on a small load can still beat two days of hauling bags.
How thick should it be?
| Application | Typical thickness |
|---|---|
| Walkway, garden path | 4 in |
| Patio | 4 in |
| Shed or workshop base | 4–6 in |
| Driveway, passenger cars | 5–6 in |
| Driveway, trucks or RV | 6 in+ |
| Garage floor | 6 in |
These are common practice, not code. Anything structural — footings, foundations, anything bearing a building — is governed by your local building department, and frost depth requirements vary enormously by region. Check before you pour.
Footings and post holes
Footings are just long rectangles: length × width × depth, same formula.
Example: a 40-foot footing, 16 inches wide, 8 inches deep.
Round post holes use the cylinder formula instead:
A 10-inch diameter hole (5-inch radius = 0.417 ft) at 3 feet deep: 3.14159 × 0.174 × 3 = 1.64 cubic feet, so about 3 bags of 80 lb mix per hole. Subtract a little if a post is going in, though most people don't bother — the extra is a useful buffer.
For a 12-post fence that's roughly 36 bags, or about 0.73 cubic yards. Right at the boundary where bags still make sense, especially since post holes get filled one at a time and cold joints aren't a concern.
Where the estimate goes wrong
- Forgetting to subgrade properly. If the gravel base dips an inch in the middle, that dip fills with concrete. On a 20 × 24 driveway an average extra inch is nearly 1.5 cubic yards you didn't order.
- Measuring the forms, not the excavation. Concrete fills whatever space exists, including the slop outside your form boards if they aren't staked tightly.
- Thickening edges without counting it. Many slabs are deepened at the perimeter for strength. That thickened edge is real volume — calculate it as a separate footing and add it.
- Treating bag yield as exact. Yields vary a little by product and by how wet you mix. Use them as planning figures, not guarantees.
- Ordering exactly what you calculated. Short on a ready-mix pour means either a cold joint or an emergency second truck at full short-load pricing. The 10% is insurance, and leftover concrete is easy to dump into a spare form or a post hole.
One last thing about timing
Concrete starts setting as soon as it's mixed, and you generally have a window of around an hour or two before it becomes unworkable — shorter in hot weather, longer in cold. Have your forms built, your base compacted, your reinforcement placed and enough hands on site before the truck arrives.
The calculation is the easy part. The thing that ruins slabs is discovering a problem with the forms while nine cubic yards of concrete is going off in the drum.
Related: How to Calculate Square Footage — for working out slab area before you apply thickness.