Concrete for a 20x32 garage or workshop slab
A 20 ft by 32 ft footprint is 640 sq ft. At 5 inches thick, the slab volume is about 9.88 cubic yards before waste. With 10% waste, the planning number is about 10.87 cubic yards.
Thickness changes the order
The default thickness on this page is 5 inches. Changing thickness from 4 inches to 6 inches increases concrete volume by 50%, so verify the planned thickness before comparing bagged concrete or ready-mix.
Ready-mix, bags, and site planning
For this slab size, ready-mix is usually the practical comparison point. Delivery timing, access, finishing crew, control joints, base prep, and reinforcement should be planned separately from the cubic-yard math.
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
- Defaults represent 20 ft by 32 ft concrete slab.
- Thickness is entered in inches and should match the planned use.
- Base prep, forms, reinforcement, joints, finishing, delivery, and local requirements are separate planning items.
Example
Estimated concrete needed (cubic yards): 10.86 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 20x32 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 20x32 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 10.86 cubic yards.
How many yards of concrete for a 20x32 slab?
At 5 inches thick, a 20x32 slab needs about 9.88 cubic yards before waste and about 10.87 cubic yards with 10% waste.
How many square feet is a 20x32 slab?
A 20 ft by 32 ft slab is 640 square feet. Square footage must still be converted to volume using slab thickness.
Should I use 4 inches or 6 inches for a 20x32 slab?
This calculator estimates quantity only. Required thickness depends on use, load, base, reinforcement, local code, and project plans. Changing thickness has a large effect on cubic yards.
Should I order exactly the calculator result?
No. Use the result as a planning number, keep a waste buffer visible, and confirm final rounding, minimum orders, and delivery rules with the supplier.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.