Enter the slab size, bar spacing, clearance from the edges and the bar size. The calculator lays out a grid of bars running both ways, adds laps where a run is longer than one stock bar, and tells you how many sticks to buy and what they weigh.
Rebar grid calculator
How it works
- Bars running lengthwise = (width − 2 × edge clearance) ÷ spacing, rounded down, + 1
- Bars running widthwise = (length − 2 × edge clearance) ÷ spacing, rounded down, + 1
- Splices per run = (run − stock length) ÷ (stock length − lap), rounded up, when a run is longer than one bar
- Weight = total feet × nominal weight per foot (#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb/ft, from ASTM A615)
Short runs are nested: if two runs fit in one stock bar, the calculator counts one bar for both.
Worked example
A 20 × 12 ft slab with #4 bar at 18 in on center, kept 3 in from the edges:
- Slab length: 20 ft
- Slab width: 12 ft
- Bar spacing (on center): 18 in
- Clearance from slab edges: 3 in
- Bar size: #4 (1/2 in)
- Stock bar length: 20 ft
- Lap splice length: 24 in
The calculator gives:
- Stock bars to buy: 22 × 20 ft
- Bars running lengthwise: 8 at 19′ 6"
- Bars running widthwise: 14 at 11′ 6"
- Total length: 317.0 ft including laps
- Weight: 212 lb of #4 bar
- Intersections to tie: 112
Across the 138 in of usable width there are 8 bars running lengthwise, each 19 ft 6 in; along the 234 in of usable length there are 14 bars running across, each 11 ft 6 in. That is 317 ft of bar, 212 lb, and 22 sticks of 20 ft bar.
Material and waste notes
- Lap length is not a fixed number. It depends on bar size, concrete strength and the design; the IRC points to Table R608.5.4(1) for concrete walls. The 24 in default is only a placeholder, so enter the lap on your plans.
- Keep bars in the right place with chairs or bricks. Bars lying on the ground do little.
- Concrete cast against earth needs more cover than concrete placed in forms; the edge clearance here is a layout input, not a cover requirement.
- Whether a slab needs rebar, wire mesh or fibers, and at what spacing, is a design decision.
Frequently asked questions
How many rebar do I need for a 20 x 20 slab?
At 18 in each way with 3 in edge clearance, 14 bars each way: 28 bars of 19 ft 6 in, 546 ft in all, or 28 twenty-foot sticks. At 12 in spacing it is 20 bars each way, 40 sticks.
How much does #4 rebar weigh?
0.668 lb per foot, so a 20 ft stick weighs about 13.4 lb (ASTM A615 nominal weight).
What spacing is typical for a slab?
It depends on the slab and its use; 12 to 18 in grids are common in residential flatwork, but follow your plans.
Sources
- ASTM A615/A615M, Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement (nominal bar weights and diameters)
- International Code Council, 2021 International Residential Code, Section R608.5.4 reinforcement installation details (lap splices per Table R608.5.4(1))
- NIST Special Publication 811, Guide for the Use of the International System of Units (unit conversion factors)
Last reviewed: 25 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.