A 12 × 12 slab covers 144 square feet. The concrete quantity depends mainly on thickness, so it is more useful to compare several thickness scenarios than to memorize one number.
Concrete volume by slab thickness
| Thickness | Theoretical volume | With 5% allowance | With 10% allowance |
|---|---|---|---|
| 3 in | 1.33 yd³ | 1.40 yd³ | 1.47 yd³ |
| 4 in | 1.78 yd³ | 1.87 yd³ | 1.96 yd³ |
| 5 in | 2.22 yd³ | 2.33 yd³ | 2.44 yd³ |
| 6 in | 2.67 yd³ | 2.80 yd³ | 2.93 yd³ |
How the 4-inch calculation works
12 × 12 × (4 ÷ 12) = 48 cubic feet. Divide by 27 to convert cubic feet to cubic yards: 48 ÷ 27 = 1.78 yd³.
Do not choose thickness from this article alone
Patios, sheds, equipment pads and vehicle slabs can have different structural, base and reinforcement requirements. The correct thickness should come from the project design and local conditions.
What the cubic-yard number does not include
- Excavation and subgrade correction
- Compacted aggregate base
- Forms and reinforcement
- Thickened edges or footings
- Pump or buggy placement
- Short-load or delivery charges
- Finishing, joints and curing
Should you automatically add 10%?
No. An allowance should reflect measurement confidence, subgrade variation, placement losses and supplier ordering increments. Start with accurate geometry, then add only a justified project-specific allowance.
Worked budget workflow
- Confirm the finished inside dimensions of the forms.
- Confirm design thickness.
- Calculate theoretical cubic yards.
- Add a reasonable quantity allowance.
- Ask the supplier for delivered pricing, minimums and fees.
- Add site preparation, reinforcement, forms and labor separately.
Use the calculator
Run different thickness and allowance scenarios in the Concrete Calculator. Then use the Concrete Cost Per Yard guide before calling a supplier.
FAQ
How much concrete is needed for a 12 × 12 slab at 4 inches?
About 1.78 cubic yards theoretically, before any project-specific allowance.
Can I round the order to 2 yards?
Possibly, but supplier minimums and order increments vary. Confirm the practical order quantity with the supplier after verifying the design and dimensions.
Does the concrete calculator include labor?
No. Material volume is only one part of installed slab cost.