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, roughly $370.76 at the average price.
Concrete slab
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Avg US retail price, checked Aug 2026.
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. The price of $5.98 per 80 lb bag is an average checked in Aug 2026, not a fixed price. 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.
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.