Rebar Calculator
A rebar calculator works out the reinforcing steel for a slab or footing from its dimensions and the bar spacing. It counts the bars in each direction of the grid, totals the linear feet, and gives the weight by bar size. Enter your slab dimensions and rebar spacing below to get the grid layout and total rebar.
Enter slab dimensions to see the rebar grid.
Estimate only. Confirm against your local building code: stairs, framing, rebar, roofing and retaining walls are structural and code-dependent. This site does not provide engineering judgment.
How Do You Calculate Rebar for a Slab?
To calculate rebar for a slab, decide the grid spacing (often 12, 16 or 18 inches on center), then count the bars in each direction: slab length divided by spacing plus one, and slab width divided by spacing plus one. A 20 by 12 foot slab at 16 inch spacing needs about 16 bars one way and 10 the other. Add the lengths for total linear feet. The calculator counts both directions and gives the weight.
What Spacing Should Rebar Be?
Rebar spacing depends on the load and slab thickness, commonly 12 to 18 inches on center for residential slabs, with 16 inches typical for a driveway or patio. Closer spacing adds strength and steel. The spacing sets the bar count in each direction directly, so the calculator lets you choose it, and you should follow your engineered plan or local code for structural slabs.
How Do You Calculate Rebar Weight?
Rebar weight comes from the total linear feet times the weight per foot for the bar size. A #4 bar weighs about 0.668 pounds per foot, a #5 about 1.043. So 300 linear feet of #4 rebar weighs about 200 pounds. Weight matters for ordering and handling, since rebar is often priced and sold by weight. The calculator multiplies your total length by the per-foot weight of your chosen bar size.
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Frequently asked questions
- How do I calculate rebar for a slab?
- Count bars each way: length divided by spacing plus one, and width divided by spacing plus one. A 20 by 12 foot slab at 16 inch spacing needs about 16 and 10 bars.
- What spacing should rebar be?
- Commonly 12 to 18 inches on center for residential slabs, with 16 inches typical for driveways and patios. Follow your engineered plan for structural work.
- How much does rebar weigh?
- By bar size: #4 is about 0.668 pounds per foot, #5 about 1.043. So 300 feet of #4 rebar weighs about 200 pounds.