Everyday
Rebar Size & Quantity Calculator
Enter slab dimensions, bar size, and spacing. The page figures bar count with cover deducted, lap splices for bars over 20 ft, total length and weight, tie points, and cost.
How it works
Behind the numbers
Bar count starts with the pour area minus concrete cover, usually 3 in where concrete sits against soil. The calculator divides the covered width by your spacing and adds one, so a 12 ft wide slab at 12 in spacing with 3 in cover needs 12 bars each way.
Long bars need splices. Steel ships in 20 ft sticks, so the calculator adds one lap per 20 ft of bar length, each lap 40 times the bar diameter. Total length converts to weight with standard unit weights, then to 20 ft sticks to buy and a tie-wire estimate.
The spacing check compares your entry against the ACI 318 slab guideline (the lesser of three times thickness or 18 in) and warns on tight spacing that makes concrete hard to place. Scenario presets fill in common jobs like a 4 in sidewalk at #3 on 18 in centers.
FAQ
Questions, answered
What size rebar do I need for a 4 inch slab?
The common choice is #3 bars at 18 in spacing for sidewalks and patios, or #4 at 12 in both ways for driveways. Spacing should not exceed the lesser of three times the slab thickness or 18 in. The calculator flags spacing that breaks this guideline.
How much does rebar lap?
Plan on 40 times the bar diameter per splice: 15 in for #3, 20 in for #4, 25 in for #5. Bars ship in 20 ft sticks, so any bar longer than 20 ft needs at least one splice. The calculator adds these automatically.
How much does rebar weigh?
#3 weighs 0.376 lb per foot, #4 weighs 0.668 lb per foot, and #5 weighs 1.043 lb per foot. A 20 ft stick of #4 is about 13.4 lb.
How far apart should rebar ties be?
Tie every bar intersection in a mat. For columns, space ties about 12 in apart or per your local code. The calculator counts tie points and estimates tie wire.