Rebar calculator
Enter the slab size, the bar size and the spacing. You get the bars running each way, the lap splices, the 20 ft sticks to buy, the weight, and the ties and chairs. Or switch to weight to find what a bundle of bars weighs.
Inputs
Result
Rebar sizes and weights
ASTM A615 inch-pound bars. The bar number is the diameter in eighths of an inch, rounded for #9 and up.
| Bar | Diameter, in | lb per ft | 20 ft bar, lb | Area, in² |
|---|
How it is worked out
A 24 x 24 ft garage slab with #4 bars at 18 in each way and 3 in cover: 282 in / 18 in = 15 spaces, so 16 bars each way, each 23 ft 6 in long. That is longer than a 20 ft stick, so each run has one splice of 20 in. 32 runs come to 805 ft, 846 ft with 5% waste: 43 sticks of 20 ft, weighing about 565 lb. The mat has 256 crossings to tie.
Rebar sizes
US rebar sizes are numbered by diameter in eighths of an inch: #3 is 3/8 in, #4 is 1/2 in, #5 is 5/8 in. Slabs on grade commonly use #3 or #4 bars at 12 to 18 in each way; footings and walls #4 or #5. The spacing and size for your job come from the plans, or from the building department for a slab that is not engineered.
Laps and cover
Where one bar ends and the next begins, they overlap and are tied together. 40 bar diameters is a common field rule, 20 in for a #4 bar and 25 in for a #5; engineered plans often call for more. Rebar must also stay covered with concrete so it does not rust: the IRC asks for 3 in where concrete is cast against the earth. Chairs or dobies hold the mat up off the base so it ends up inside the slab, not under it.
For the concrete itself, use the concrete calculator.
Questions
How much rebar do I need for a slab?
Work out the bars each way: slab dimension less twice the edge cover, divided by the spacing, plus one. Multiply each by its run length and add laps where a run is longer than a stick. A 24 x 24 ft slab at 18 in each way takes 32 runs, 846 ft with waste, or 43 sticks of 20 ft.
How much does rebar weigh per foot?
#3 0.376 lb, #4 0.668 lb, #5 1.043 lb, #6 1.502 lb, #8 2.670 lb per foot, by ASTM A615.
How much does a 20 ft stick of #4 rebar weigh?
About 13.4 lb: 20 x 0.668 lb per foot. A 20 ft #5 bar weighs about 20.9 lb.
What is the overlap for rebar?
A common field rule is 40 bar diameters: 20 in for #4 and 25 in for #5. Plans and the engineer govern, and some codes and designs call for more.
What size rebar for a concrete slab?
Residential slabs on grade commonly use #3 or #4 bars at 12 to 18 in each way. Follow your plans or local building department.
How many rebar ties do I need?
One at each crossing if every one is tied, which is the number of bars one way times the number the other. Many crews tie every other crossing on a slab.
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