Concrete Slab Calculator
Calculate cubic yards of ready-mix concrete for a patio, walkway, or shed pad, plus material cost at your price per yard.
How this calculator works
Ready-mix concrete is ordered by the cubic yard, so this calculator starts with the same volume formula as any slab: length times width times depth, converted from cubic feet to cubic yards. The result is what you’d tell a ready-mix supplier when placing an order.
Formula: Cubic yards = (Length ft × Width ft × Depth ft) ÷ 27.
Worked example
A 10 ft × 10 ft patio slab at a 4-inch depth:
- Depth in feet: 4 ÷ 12 = 0.333 ft
- Cubic yards: (10 × 10 × 0.333) ÷ 27 = 1.23 yd³
- At $150/yd³, that’s $185.19 before waste.
Coverage reference
| Slab size | Depth | Cubic yards |
|---|---|---|
| 10 ft × 10 ft | 4 in | ~1.23 yd³ |
| 12 ft × 12 ft | 4 in | ~1.78 yd³ |
| 20 ft × 20 ft | 4 in | ~4.94 yd³ |
How to measure your slab
Measure the finished slab footprint, not the excavated hole, since forms sit inside the excavation with some margin. If the slab steps down in thickness (a thickened edge for a footing, for example), calculate the main field and the thickened section separately and add the volumes together.
Frequently asked questions
How thick should a concrete slab be?
4 inches is the standard depth for patios, walkways, and shed pads carrying foot traffic and light loads. Slabs that will support vehicles — driveways, RV pads — typically go to 5-6 inches.
Why does this calculator return cubic yards instead of bags or tons?
Any pour large enough to justify a ready-mix truck is ordered and priced in cubic yards, not by weight or bag count. If you're mixing your own concrete by hand for a small pour, use the concrete bag calculator instead.
Is it normal to order more concrete than the calculated volume?
Yes. Most ready-mix suppliers and contractors add 5-10% over the calculated volume because sub-grade isn't perfectly flat, forms can bow slightly, and a truck that comes up short mid-pour means a cold joint or a second truck — both worse outcomes than a small leftover.
What's the minimum order for a ready-mix truck?
Most ready-mix suppliers set a minimum order, commonly around 1 cubic yard, and may charge a short-load fee below that. For very small pours under a yard, bagged concrete mixed on-site is often more practical.
Does this include rebar, gravel base, or forms?
No — this calculates concrete volume and material cost only. Rebar/mesh reinforcement, a compacted gravel sub-base, and formwork are separate costs that depend on your specific slab design and local code requirements.
Estimates only. Verify quantities with your supplier before purchasing.