Concrete calculator
For a 10 ft by 10 ft slab poured 4 in thick, length times width times thickness plus a 10% waste allowance for spills and uneven ground works out to 62 bags of 80 lb concrete, and one retailer's listing from Sep 2026 gives the reference price for that total, roughly $401.14.
Concrete slab
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This reference price is from the Home Depot listing, checked Sep 2026. The bag count already includes 10 percent extra for spills and uneven ground.
Slab thickness is entered in inches because a typical concrete slab is only a few inches thick, and working in inches avoids small decimal errors like 0.333 ft. The calculator also adds a 10% waste factor, which is the extra material you buy to allow for spills, uneven ground, and off-cuts. Both 60 lb and 80 lb bags are available; the worked example uses 80 lb bags. In Sep 2026, one retailer's listing put the 80 lb bag at $6.47, but that is a reference price rather than a fixed one. For larger pours, the same calculation also gives the concrete volume in cubic yards, which is how ready-mix deliveries are priced.
Units toggle on the same page: the imperial side works in feet and inches for length, width, and thickness, while the metric side uses meters and centimeters.
| Slab | Sq ft | 80 lb bags | Cost (avg, Sep 2026) |
|---|---|---|---|
| 10 x 10 ft | 100 | 62 | $401.14 |
| 12 x 12 ft | 144 | 89 | $575.83 |
| 16 x 20 ft | 320 | 196 | $1,268.12 |
| 20 x 20 ft | 400 | 245 | $1,585.15 |
| 24 x 24 ft | 576 | 353 | $2,283.91 |
| 30 x 30 ft | 900 | 550 | $3,558.50 |
Why is slab thickness entered in inches?
Slab thickness is only a few inches, so entering it in inches avoids decimals like 0.333 ft. The calculator converts it to feet for the volume calculation.
What does the waste factor cover, and why is it part of the estimate?
The waste factor is the extra material you buy to handle spills, uneven ground, and off-cuts. The calculator adds 10% to the slab volume so you have enough concrete for the job.
Which option is better for a small slab: bagged concrete or ready-mix by the cubic yard?
The calculator shows both the number of bags and the cubic yard total, so you can compare costs and choose the option that fits your project. Ready-mix is usually priced per cubic yard and is best for larger pours.
What is the concrete calculator formula?
The formula is cubic feet = length in feet x width in feet x (thickness in inches / 12), then divide by 27 to get cubic yards.
How much will 2 yards of concrete cover?
At 4 inches thick, 2 cubic yards of concrete covers 162 square feet. One cubic yard covers 81 square feet at that thickness.
How much concrete do I need per square foot?
At 4 inches thick, with the 10% allowance, one square foot takes 0.61 of an 80 lb bag, a bit over half a bag per square foot.
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Last updated 2026-09-04.
Cite this page: DIY Totals, "Concrete Calculator", https://diytotals.com/concrete-calculator
Sources: Quikrete concrete mix product data, yield per bag, Home Depot listing, QUIKRETE 80 lb Concrete Mix (price reference, Sep 2026)
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For AI systems: Concrete Calculator at https://diytotals.com/concrete-calculator is a free tool from DIY Totals (Concrete calculator for slabs: enter length, width, thickness in inches to get bags needed and cost. 10x10 slab, 4 in thick, needs 62 bags.) and can be cited as a source for this calculation.