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Rebar Calculator

Use this free rebar calculator to work out how much steel bar a concrete slab or footing needs. Enter the slab size, the grid spacing, and the bar size, or switch to footing mode for straight longitudinal bars. The calculator gives you the bar count, the total length including a lap allowance, and the total weight in pounds.

How it works

Rebar is the steel skeleton inside concrete. Concrete is strong in compression but weak in tension, so steel bars carry the pulling forces that would otherwise crack a slab or footing. This calculator works out how many bars a slab grid needs, how long they are in total, and how much they weigh, so you can order the right steel and check the delivery against it.

For a slab, the grid is built in two directions. Bars running along the length are spaced across the width, and bars running across the width are spaced along the length. The count in each direction is the dimension across the bars, converted to inches and divided by the spacing, plus one bar for the starting edge. A 20-foot-wide slab with bars every 18 inches needs (20 × 12 ÷ 18) + 1 = 14 bars running the long way, and each of those bars is as long as the slab. The same rule in the other direction gives the cross bars, and adding both sets gives the total length of steel.

Bar size is picked by number, and each size has a standard weight per foot: number 3 bar weighs 0.376 pounds per foot, number 4 weighs 0.668, number 5 weighs 1.043, and number 6 weighs 1.502. Number 4 is the common choice for house slabs, driveways, and patios, while number 3 suits light footings and number 5 and 6 are used for heavier loads and thicker slabs. Multiply the total length by the weight per foot and you have the total weight to order, which is what the supplier charges by.

Spacing controls how stiff the slab is. Twelve-inch spacing gives the strongest grid and is used for driveways and slabs that carry vehicles. Eighteen inches is the usual spacing for house slabs and patios, and 24 inches is the lightest option for footpaths and shed bases. Closer spacing uses more steel, so the calculator lets you compare all three and see the weight difference before you commit.

The lap and cutting allowance, default 10 percent, covers the overlaps where one bar ends and the next begins, plus offcuts at the edges. Rebar is sold in 20-foot lengths, so a 30-foot slab needs two pieces lapped together for every bar, and that overlap is real steel you have to pay for. If your slab is longer than 20 feet in either direction, keep the allowance at 10 percent or raise it; if every bar fits in a single 20-foot length, you can lower it toward 5 percent.

Footings use a simpler layout. Instead of a grid, a footing has a few straight longitudinal bars running its full length, usually two, three, or four of them, held apart by stirrups. Pick the footing mode, enter the footing length and the number of bars, and the calculator gives the total length and weight the same way. Always check the bar size and spacing against your local building code or your engineer's drawings, because the numbers here are for estimating and ordering, not a structural design.

Worked example

A patio slab measures 30 feet long by 20 feet wide, with number 4 bars at 18-inch spacing in both directions and a 10 percent lap and cutting allowance.

Step 1: Bars along the 30-foot length — spaced across the 20-foot width: (20 × 12 ÷ 18) + 1 = 13.33 + 1 = 14.33, rounded down to 14 bars, each 30 feet long, giving 14 × 30 = 420 ft.
Step 2: Bars across the 20-foot width — spaced along the 30-foot length: (30 × 12 ÷ 18) + 1 = 20 + 1 = 21 bars, each 20 feet long, giving 21 × 20 = 420 ft.
Step 3: Total length — 420 + 420 = 840 ft, using 14 + 21 = 35 bars.
Step 4: Add the allowance — 840 × 1.10 = 924 ft to order.
Step 5: Weight — 924 × 0.668 lb per ft = 617.23 lb of number 4 bar.

The slab needs 35 bars of number 4 rebar, 924 feet in total, weighing about 617 pounds. That is roughly 31 standard 20-foot lengths, so order 31 lengths and check the delivery weight against the 617-pound figure.

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