Rebar Calculator
Calculate rebar quantity, weight, and tie wire for concrete work.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Rebar Calculator
This calculator estimates the number of rebar bars, total linear footage, 20-foot pieces, weight, and tie wire needed to reinforce a concrete slab or footing with a standard grid pattern. Here is how to get an accurate estimate:
- Enter your slab dimensions. Type the length and width of your slab or footing in feet. For a rectangular driveway, patio, or foundation footing, these are the outside dimensions.
- Choose bar spacing. The most common residential spacing is 12 inches on center for patios and driveways. Use 6 or 8 inches for heavier loads. Local codes may specify the spacing for structural footings.
- Select bar size. #4 rebar (1/2 inch diameter) is the standard for residential slabs. #3 works for light-duty flatwork. #5 and larger are used for structural footings and heavier construction.
- Lap splice overlap. When a slab is larger than 20 feet in either direction, bars must be overlapped at joints. The standard overlap is 30 bar diameters. Leave this checked for an accurate piece count when buying from the lumber yard.
- Read your results. The primary result is total linear feet. The breakdown shows bars in each direction, 20-foot pieces to purchase, total weight (useful for delivery estimates), and tie wire in pounds.
Results update as you type. Use Share to send your inputs to a supplier or contractor, or Copy to paste the summary into a quote or materials list.
About the Rebar Calculator
Rebar reinforces concrete against tension and bending forces. In a grid pattern, bars run in both directions at a set spacing. The number of bars in each direction is determined by dividing the perpendicular dimension (in inches) by the spacing and adding one bar to close the last panel. For example, a 20-foot slab with 12-inch spacing needs 21 bars in each direction.
Standard rebar is sold in 20-foot lengths at most suppliers. When a run exceeds 20 feet, bars must be spliced with a lap overlap equal to at least 30 times the bar diameter. For #4 rebar (0.5 inch diameter), that is a 15-inch overlap per splice. This calculator adds that extra footage automatically when the checkbox is on.
Tie wire secures bar intersections before the pour. The rule of thumb is one tie per intersection, and a pound of tie wire handles approximately 50 ties. Weight figures use the nominal linear weight published by the Concrete Reinforcing Steel Institute (CRSI): #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, and so on up to #8 at 2.670 lb/ft.
Frequently Asked Questions
How much rebar do I need for a 20x20 concrete slab?
For a 20x20 foot slab with #4 rebar at 12-inch spacing, you need 21 bars running in each direction, for 42 total bars. That is 21 bars times 20 feet plus 21 bars times 20 feet, giving 840 linear feet before any overlap. Since the slab is exactly 20 feet, no lap splices are needed and you would buy 42 standard 20-foot sticks. Total weight is roughly 561 pounds. Tie wire for the 441 intersections comes to about 9 pounds.
What rebar spacing should I use for a residential slab?
For most residential patios and driveways, 12-inch spacing with #4 rebar is the standard minimum. Garage floors and areas subject to heavier vehicle loads often use 12 inches with #4 or #5. Footings and structural elements typically require engineer-specified spacing, often 6 or 8 inches, with bar sizes from #4 to #6. Always check your local building code, since frost zones, soil conditions, and load requirements vary by region.
What is a lap splice and why does it matter?
A lap splice is the overlap where two rebar pieces join end to end. Since rebar is sold in fixed lengths (typically 20 feet), any run longer than that requires a splice. The minimum overlap is 30 bar diameters to transfer load through the concrete bond. For #4 rebar, that is 30 times 0.5 inch, or 15 inches of overlap per joint. Skipping this extra length means your bars are effectively shorter than needed and the joint may fail under load.
How do I calculate tie wire for rebar?
Count the total number of intersections in your rebar grid: multiply the number of bars running in one direction by the number running in the other direction. Each intersection gets one tie. A standard roll of 16-gauge black annealed tie wire weighs about 3.5 pounds and handles roughly 175 ties. The rule of thumb used in this calculator is 50 ties per pound, which is a practical estimate for hand-tying with standard hook tools. Buying a little extra wire is cheap insurance against running short mid-pour.