Start with volume, then add a little breathing room.
If you’re standing in the yard with a tape measure and trying to work out whether you need three yards of concrete or five, the basic math is refreshingly simple: length × width × thickness. The part people usually trip over is the thickness conversion — four inches is one-third of a foot — and then deciding how much extra to order.
For a simple rectangular patio, calculate the raw volume first, convert it to cubic yards, and then add a small allowance for uneven subgrade, forms that are not perfectly square, and the inevitable little losses during placement. PrepMyBuild defaults to a 10% allowance because it makes the effect easy to see, but you can change it.
A 12 × 20 ft patio poured 4 inches thick is about 2.96 yd³ before allowance. Add 10%, and the planning quantity is about 3.26 yd³.
Concrete for common patio sizes
These examples assume a 4-inch slab and a 10% planning allowance. They’re useful for a quick sense-check, not a substitute for the dimensions of your actual patio.
| Patio size | Raw concrete | With 10% allowance |
|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.36 yd³ |
| 12 × 12 ft | 1.78 yd³ | 1.96 yd³ |
| 12 × 20 ft | 2.96 yd³ | 3.26 yd³ |
| 16 × 20 ft | 3.95 yd³ | 4.35 yd³ |
| 20 × 20 ft | 4.94 yd³ | 5.43 yd³ |
Thickness changes the number faster than you might expect
It’s tempting to focus on the length and width because those are the big visible dimensions. Thickness matters just as much. Moving from a 3-inch slab to a 4-inch slab increases the concrete volume by roughly one-third. Don’t choose thickness from an online example; use the design that fits your project, local conditions and any applicable requirements.
Ready-mix truck or bags?
For a small pad, a handful of bags may be perfectly sensible. Once you get into patio-sized quantities, the bag count becomes an eye-opener. That doesn’t automatically mean ready-mix is the right answer — access, delivery minimums and your ability to place the concrete quickly all matter — but it’s worth comparing before buying.
Our Concrete Bag Calculator is useful when you want to translate the volume into 40, 60 or 80 lb bags. For a larger pour, use the main patio calculator and talk to local suppliers about minimum orders and delivery.
What this number does not include
The concrete itself is only part of a patio. You may also need excavation, compacted base material, forms, reinforcement, joints and drainage planning. Soil, frost conditions, slope and structural requirements can change the project. If something about the site feels unusual, it is worth getting local advice before ordering material.
A simple way to avoid the “almost enough” problem
Measure twice, especially if the patio is not a perfect rectangle. Break an irregular area into simple rectangles and calculate each piece separately. Then keep the raw quantity and the order quantity separate. That way you know exactly how much of the final number is actual geometry and how much is your allowance.