How much concrete for a slab?
Concrete slabs are ordered by volume, not just square footage. Multiply slab length by width, convert thickness from inches to feet, then divide by 27 to get cubic yards. Add a practical waste buffer before ordering.
Common slab sizes
A 10 x 10 ft slab at 4 inches thick is about 1.23 cubic yards before waste. A 12 x 12 ft slab at 4 inches thick is about 1.78 cubic yards. A 20 x 20 ft slab at 4 inches thick is about 4.94 cubic yards.
Slab thickness and waste
Thickness changes concrete volume quickly. Moving from 4 inches to 6 inches increases the volume by 50%. Uneven base, forms, and ordering variance are common reasons to add waste.
Common concrete slab sizes
Cubic yards below are before waste. Add your project waste percentage before ordering.
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 ft x 10 ft | 1.23 cu yd | 1.54 cu yd | 1.85 cu yd |
| 10 ft x 12 ft | 1.48 cu yd | 1.85 cu yd | 2.22 cu yd |
| 12 ft x 12 ft | 1.78 cu yd | 2.22 cu yd | 2.67 cu yd |
| 20 ft x 20 ft | 4.94 cu yd | 6.17 cu yd | 7.41 cu yd |
| 24 ft x 24 ft | 7.11 cu yd | 8.89 cu yd | 10.67 cu yd |
| 30 ft x 40 ft | 14.81 cu yd | 18.52 cu yd | 22.22 cu yd |
Before you calculate
- Measure the actual formed length and width in feet.
- Enter slab thickness in inches, then handle thickened edges or footings as separate estimates.
- Break irregular slabs into rectangles so the volume calculation stays transparent.
Common mistakes
- Using square feet without converting thickness into volume.
- Missing thickened edges, turndowns, steps, or separate pads.
- Ordering exactly the calculated amount with no allowance for site variation.
Formula
cubic yards = (length * width * (thickness / 12) / 27) * (1 + wastePercent / 100)
Assumptions
- The estimate assumes a rectangular slab.
- Thickness is entered in inches.
- Footings and thickened edges are not included.
Example
Estimated concrete needed (cubic yards): 3.26 cubic yards
How to use this calculator
- Enter length in ft.
- Enter width in ft.
- Enter thickness in in.
- Enter waste in %.
- Review the live estimate and compare it with the example result.
- Check the formula, assumptions, product labels, and site conditions before using the Concrete Slab Calculator result to plan materials.
Before you buy materials
- Compare cubic yards, cubic feet, and bag counts before deciding between ready-mix and bags.
- Confirm final slab details with plans, product labels, or a qualified professional when accuracy matters.
FAQ
What is the example concrete slab cubic-yard result?
Use length, width, thickness or depth, and waste, then calculate the planning result. In the default example, the result is 3.26 cubic yards.
How do I calculate concrete for a slab?
Use cubic yards = (length x width x (thickness / 12)) / 27, then add waste if needed. Measure length and width in feet and thickness in inches.
How much concrete is needed for a 10 x 10 slab?
At 4 inches thick, a 10 x 10 ft slab needs about 1.23 cubic yards before waste. With 10% waste, plan around 1.36 cubic yards.
Does slab thickness matter?
Yes. A 4 inch slab and a 6 inch slab with the same footprint require different volumes. This calculator estimates quantity, not the required structural thickness.
Should I order ready-mix or bags for a slab?
Small slabs may be possible with bags, but larger slabs often make more sense as ready-mix. Compare cubic yards, bag yield, labor, delivery, and supplier minimums.
What is the example concrete slab result?
Using the default inputs, the example result is about 3.26 cubic yards. The calculation uses slab length, width, thickness, and waste.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.