How to calculate concrete for a slab
Multiply length by width by thickness (in feet), then divide by 27 to convert cubic feet to cubic yards. Thickness in inches must be divided by 12 first.
Recommended slab thickness
| Use | Typical thickness |
|---|---|
| Walkways, patios | 4 in |
| Garage floors, driveways | 5 to 6 in |
| Heavy loads, RV pads | 6 in or more (consult an engineer) |
Common slab sizes (4 in thick)
| Size | Cubic yards | With 10% waste |
|---|---|---|
| 10 × 10 ft | 1.23 | 1.36 |
| 12 × 12 ft | 1.78 | 1.96 |
| 20 × 20 ft | 4.94 | 5.43 |
| 30 × 30 ft | 11.11 | 12.22 |
Real example: pouring a 12 × 24 ft backyard patio
Say you are building a patio for foot traffic and furniture. You measure the finished area at 12 ft by 24 ft and plan a standard 4 inch slab. Here is exactly how the numbers work out, the same way the calculator above does it.
| Step | Math | Result |
|---|---|---|
| 1. Area | 12 × 24 | 288 ft² |
| 2. Volume in cubic feet | 288 × (4 ÷ 12) | 96 ft³ |
| 3. Convert to yards | 96 ÷ 27 | 3.56 yd³ |
| 4. Add 10% waste | 3.56 × 1.10 | 3.91 yd³ |
| 5. Order | round up to the next ¼ yard | 4.0 yd³ |
At the placeholder price of $165 per yard, that is about $660 for the concrete itself, before delivery, base gravel, forms and finishing. Many suppliers add a short-load fee on orders this small, so ask about it when you call. Plug your own price into the cost field above, then check the concrete cost calculator to add delivery and tax.
A second example: a 10 × 40 ft driveway at 5 inches
400 ft² × (5 ÷ 12) = 166.7 ft³, which is 6.17 yd³. With 10% waste that is 6.79, so you would order 7 cubic yards. Driveways carry vehicles, so 5 to 6 inches and reinforcement are common choices. Confirm with your local code or a contractor.
Mistakes that make people run short
- Measuring the forms, not the dug area. An uneven subgrade swallows extra concrete, which is why the waste factor exists.
- Forgetting thickness is in inches. Dividing by 12 first is the step most people miss.
- Ordering exactly what the math says. Running 0.2 yards short mid-pour can leave you with a cold joint. Leftover concrete is cheaper than a second truck.
Pouring something small? See how many bags you need instead. Want to double-check your total in cubic yards and truck loads? Use the cubic yard calculator. You can also run the same slab on your phone with the free concrete calculator app.
Frequently asked questions
How many yards of concrete do I need for a 20x20 slab?
A 20x20 ft slab at 4 inches thick needs about 4.94 cubic yards, or roughly 5.4 yards with a 10% waste allowance.
How many yards for a 30x30 slab 4 inches thick?
About 11.11 cubic yards before waste.
Do I need rebar in a 4-inch slab?
Light-use slabs often go without it, but rebar or wire mesh is commonly recommended for driveways and heavy loads. Check local code.
How much extra concrete should I order?
Most pros add 5% to 10% to cover uneven subgrade, spillage and form movement.
Take the calculator to the job site
Free concrete calculator app for iPhone and Android.
Google Play App Store