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Slab Concrete Calculator
Work out exactly how much concrete your slab needs. Enter length, width and thickness: get cubic yards, bags, trucks and cost instantly.
Estimates only. Bag yields per manufacturer specs (80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 40 lb ≈ 0.30 ft³). Ready-mix trucks carry ≈ 10 yd³. Always round up and confirm with your supplier.
A 10 by 10 ft slab at 4 inches thick needs 1.23 cubic yards of concrete. Use the free slab concrete calculator below for your exact measurements. It converts length, width and thickness into cubic yards, bag counts, truck loads and material cost in one step.
How to use the slab concrete calculator
- Measure the length and width of your slab area in feet.
- Enter the slab thickness in inches (4 inches suits most patios and paths).
- Add a waste margin. Ten percent covers spillage and uneven ground.
- Read your results: cubic yards, cubic metres, bag counts and truck loads appear instantly.
Formula
Volume = Length × Width × (Thickness ÷ 12) ÷ 27. The result is cubic yards.
Practical tips
- Order 10 percent extra. Running short mid-pour leaves a cold joint that weakens the slab.
- For driveways that carry cars, pour 5 to 6 inches thick instead of 4.
- One cubic yard covers about 81 square feet at 4 inches thick.
- Ready-mix trucks carry around 10 cubic yards. Below 1 yard, bags are usually cheaper.
Frequently asked questions
How thick should a concrete slab be?
Four inches suits patios, paths and shed bases. Driveways and garage floors need 5 to 6 inches to carry vehicle weight.
How many 80 lb bags for a 10×10 slab at 4 inches?
A 10 by 10 ft slab at 4 inches is 1.23 cubic yards, which needs 62 bags of 80 lb concrete (with 10 percent waste).
Should I order extra concrete?
Yes. Add 10 percent. It is far cheaper than a second delivery, and a short load can ruin the pour.
Planning more than a slab? Try the full concrete calculator with 12 project types, or work out material costs next.
Worked examples
Example 1: 10 by 10 ft patio, 4 inches thick. Volume = 10 x 10 x (4 / 12) / 27 = 1.23 cubic yards. With 10% waste: 1.36 cubic yards, which is 62 x 80 lb bags.
Example 2: 20 by 20 ft driveway, 6 inches thick. Volume = 20 x 20 x (6 / 12) / 27 = 7.41 cubic yards. With 10% waste: 8.15 cubic yards, which is 367 x 80 lb bags (about one truck load).
Example 3: 12 by 12 ft shed base, 4 inches thick. Volume = 12 x 12 x (4 / 12) / 27 = 1.78 cubic yards. With 10% waste: 1.96 cubic yards, which is 88 x 80 lb bags.
Slab concrete reference table
Cubic yards are before waste. Bag counts are 80 lb bags with 10% waste included.
| Slab size | 4 in thick | 6 in thick |
|---|---|---|
| 4 x 4 ft | 0.20 yd3 (10 bags) | 0.30 yd3 (15 bags) |
| 6 x 6 ft | 0.44 yd3 (22 bags) | 0.67 yd3 (33 bags) |
| 8 x 8 ft | 0.79 yd3 (40 bags) | 1.19 yd3 (59 bags) |
| 10 x 10 ft | 1.23 yd3 (62 bags) | 1.85 yd3 (92 bags) |
| 12 x 12 ft | 1.78 yd3 (88 bags) | 2.67 yd3 (132 bags) |
| 20 x 20 ft | 4.94 yd3 (245 bags) | 7.41 yd3 (367 bags) |
More slab questions
How many cubic yards is a 12×12 slab at 4 inches?
A 12 by 12 ft slab at 4 inches thick is 1.78 cubic yards, or 1.96 cubic yards with 10% waste.
Is it cheaper to buy bags or ready-mix for a slab?
Below about 1 cubic yard, bags are usually cheaper. Above that, ready-mix costs less per yard and saves hours of mixing.
What area does 1 cubic yard of concrete cover?
One cubic yard covers about 81 square feet at 4 inches thick, or 54 square feet at 6 inches thick.