Rebar Calculator
Calculate how much rebar you need for a concrete slab grid from the slab size and bar spacing. Get the bar count, total length, weight and stock pieces to buy.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Rebar pieces
- 40 bars
- 20 bars 19.5 ft long and 20 bars 19.5 ft long on a 12 in grid
- Total length
- 780 ft
- #3 (3/8 in) bars, set 3 in back from each edge
- Weight
- 293 lb
- At 0.376 lb per foot
- Stock to buy (20 ft pieces)
- 42 pieces
- Cut without laps; includes 5% waste
- Ties and chairs
- 400 ties
- One tie per intersection, plus about 45 chairs or dobies at 3 ft on center to hold the grid at mid-depth
Rebar formula
Bars along the length = (Width (in) - 2 x clearance) / spacing, rounded down, + 1
Bars across the width = (Length (in) - 2 x clearance) / spacing, rounded down, + 1
Total length (ft) = Bars along x (Length (in) - 2 x clearance) / 12 + Bars across x (Width (in) - 2 x clearance) / 12
Weight (lb) = Total length x lb per ft (#3 = 0.376, #4 = 0.668, #5 = 1.043)Worked example
A 24 x 12 ft driveway slab with #4 bars at 18 in and 3 in clearance: bars running the 24 ft length are spaced across 12 ft, so (144 - 6) / 18 = 7.67, rounded down to 7, plus 1 = 8 bars, each (288 - 6) / 12 = 23.5 ft long. Bars across are (288 - 6) / 18 = 15.67, so 15 + 1 = 16 bars, each 11.5 ft long.
Total: 24 bars and 8 x 23.5 + 16 x 11.5 = 372 linear feet, weighing 372 x 0.668 = 248 lb. With 20 ft stock the 23.5 ft bars each need two pieces lapped, so 16 + 16 = 32 pieces, or 34 with 5% waste. There are 8 x 16 = 128 intersections to tie.
Quick reference
| Slab | Bars | Total length | #3 weight | #4 weight |
|---|---|---|---|---|
| 10 x 10 ft | 20 | 190 ft | 71 lb | 127 lb |
| 12 x 12 ft | 24 | 276 ft | 104 lb | 184 lb |
| 16 x 16 ft | 32 | 496 ft | 186 lb | 331 lb |
| 20 x 20 ft | 40 | 780 ft | 293 lb | 521 lb |
| 24 x 24 ft | 48 | 1,128 ft | 424 lb | 754 lb |
| 30 x 30 ft | 60 | 1,770 ft | 666 lb | 1,182 lb |
How to calculate rebar for a slab
Rebar in a slab runs both ways in a grid. For the bars running lengthwise, subtract the edge clearance from both sides of the width, divide by the spacing, round down and add one; each of those bars is the slab length minus clearance at both ends. Do the same the other way, then add up the total length and multiply by the weight per foot of the bar size.
For a 4 inch residential slab, #3 bars on a 12 to 18 inch grid or 6 x 6 welded wire mesh is typical. Driveways and garage floors at 5 to 6 inches usually get #4 bars at 12 to 16 inches. Footings take two or three continuous #4 or #5 bars rather than a grid. Follow your plans or local code where they specify more.
Buying, cutting and placing
Rebar comes in 20 foot lengths from masonry and steel suppliers, and 10 foot lengths at home centers. Bars longer than your stock must be spliced by lapping at least 40 bar diameters and tying the overlap. Cut bars with an angle grinder, a portable band saw or a rebar cutter, and add 5 percent for cut-offs.
Tie every intersection, or at least every other one, with 16 gauge tie wire, and support the grid on chairs or dobies every 3 feet so it sits in the middle of the slab rather than on the ground. Rebar lying on the dirt does nothing. Keep 3 inches of cover where concrete meets soil and 1.5 inches from formed edges and the top surface.
Key facts
- Rebar sizes are numbered in eighths of an inch: #3 is 3/8 in, #4 is 1/2 in, #5 is 5/8 in.
- Standard weights are 0.376 lb per foot for #3, 0.668 for #4 and 1.043 for #5.
- A 12 in grid of #3 bars uses about 2 ft of bar per square foot of slab, or roughly 0.75 lb per square foot.
- Lap splices should be at least 40 bar diameters: 15 in for #3, 20 in for #4 and 25 in for #5.
- Rebar is sold in 20 ft lengths at supply yards and 10 ft lengths at home centers; both come in #3, #4 and #5.
Frequently asked questions
How much rebar do I need for a 20 x 20 slab?
With #3 bars on a 12 in grid and 3 in edge clearance, 40 bars totaling 780 linear feet, about 293 lb. Buy 42 pieces of 20 ft stock.
What size rebar for a 4 inch slab?
#3 (3/8 in) bars on a 12 to 18 in grid are standard for 4 in patios and shed slabs. Use #4 for driveways and 5 to 6 in slabs.
How far apart should rebar be in a slab?
12 in on center is the common choice for residential slabs; 16 to 18 in is acceptable for light loads. Follow the engineer's spacing when plans specify it.
How much does rebar weigh?
#3 weighs 0.376 lb per foot (7.5 lb per 20 ft bar), #4 weighs 0.668 lb per foot (13.4 lb) and #5 weighs 1.043 lb per foot (20.9 lb).
Sources and references
Next steps for this project
The usual order for a concrete & masonry project. See all concrete & masonry calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.