Rebar in a slab is laid out as a grid, with one set of bars running the length of the pour and a second set running the width, both spaced an equal distance apart on-center. This calculator figures out how many bars run in each direction based on that spacing, multiplies each set by the length it needs to span, and adds the two totals together to get the overall linear footage of rebar required.

Because rebar is sold in fixed stick lengths — almost always 20 ft for standard #3-#5 bar — the final step divides total linear footage by the stick length and rounds up, since a supplier will not sell a partial stick. This is the number to hand to a supply house when ordering, along with the bar size (diameter) your engineer or local code specifies.