What to measure
How to use the rebar calculator
Enter the slab’s length and width, the grid spacing (12 in is common) and the edge clearance (how far the bars stop short of the slab edge). Pick the bar size and the bar length your supplier sells.
The formula
- Bars running lengthwise = (width − 2 × clearance) ÷ spacing + 1, each (length − 2 × clearance) long
- Bars running crosswise = (length − 2 × clearance) ÷ spacing + 1, each (width − 2 × clearance) long
- Bars to buy = total length × (1 + extra %) ÷ bar length
Worked example: 20 × 10 ft slab, #4 at 12 in
- Lengthwise: 9.5 ft ÷ 1 ft + 1 = 10 bars × 19.5 ft = 195 ft
- Crosswise: 19.5 ft ÷ 1 ft + 1 = 20 bars × 9.5 ft = 190 ft
- 385 linear ft, plus 10% → 22 bars of 20 ft, about 283 lb
- 10 × 20 = 200 intersections to tie
Use chairs or concrete blocks (“dobies”) to hold the grid at mid-depth of the slab. Structural slabs, footings and walls need an engineer’s or code-specified design.
Rebar for common slab sizes
#4 bar, 12 in on center each way, 3 in edge clearance, 10% extra.
| Slab | Linear ft | 20 ft bars | Weight |
|---|---|---|---|
| 10 × 10 ft | 190 | 11 | 140 lb |
| 12 × 12 ft | 276 | 16 | 203 lb |
| 20 × 10 ft | 385 | 22 | 283 lb |
| 20 × 20 ft | 780 | 43 | 573 lb |
| 24 × 24 ft | 1,128 | 63 | 829 lb |
| 30 × 40 ft | 2,365 | 131 | 1,738 lb |
Common mistakes
- Running bars to the edge of the forms. Bars need concrete around them. Keep the edge clearance so the steel is covered.
- Laying the grid on the ground. Set it on chairs or dobies so it stays at mid-depth while the concrete is placed.
- Confusing spacing with the gap. Spacing is measured center to center of the bars, not the open space between them.
- Forgetting splices. Any run longer than one bar needs an overlap. Keep the extra for laps and offcuts.
Frequently asked questions
How much rebar do I need for a 20x10 slab?
With #4 bar at 12 in on center and 3 in edge clearance, 385 linear feet, or 22 bars of 20 ft with 10% extra for laps. That's about 283 lb and 200 ties.
What spacing should rebar be in a slab?
12 to 18 inches on center each way is common for residential slabs and driveways. Your plans or local code have the final say.
How much does rebar weigh per foot?
#3 weighs 0.376 lb/ft, #4 0.668 lb/ft and #5 1.043 lb/ft (ASTM A615 nominal weights).
How much should rebar overlap?
Lap splices are commonly 40 bar diameters or more (about 20 in for #4 bar), but the required length depends on your design. The 10% extra covers typical laps and offcuts.
How much rebar do I need for a 24x24 garage slab?
With #4 bar at 12 in on center and 3 in edge clearance, 1,128 linear feet, or 63 bars of 20 ft with 10% extra. At 18 in on center it drops to 752 feet and 42 bars.
Should I buy 10 ft or 20 ft bars?
For a 20 × 10 ft slab at 12 in on center, it's 22 bars of 20 ft or 43 bars of 10 ft. Shorter bars are easier to haul, but long runs need more splices.
How much does the rebar for a 20x20 slab weigh?
At 12 in on center with 10% extra, about 323 lb in #3, 573 lb in #4 and 895 lb in #5.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.