How to use the rebar calculator
- Enter the slab length and width.
- Set the edge clearance, the gap between the ends of the bars and the edge of the slab. 3″ is common for slabs poured against the ground.
- Choose the bar size and grid spacing from your plans. The calculator lays bars both ways at that spacing.
- Set the lap splice. Where a run is longer than one bar, the bars overlap. The calculator adds the overlap and counts the extra bars.
How we calculate rebar
Bars running widthwise = (Length − 2 × clearance) ÷ spacing + 1
Run length = Slab dimension − 2 × clearance
Bars per run = ⌈(Run − lap) ÷ (Bar length − lap)⌉ if the run is longer than one bar
Weight = Total length × weight per foot
Short runs are cut from longer bars where they fit (two 9 ft pieces from one 20 ft bar). Chairs are estimated at one every 3 ft each way, which keeps the mat from sagging when people walk on it during the pour.
Rebar sizes and weights
| Bar | Diameter | Weight per foot | Weight per metre | 40 × diameter lap |
|---|---|---|---|---|
| #3 | 0.375″ (10 mm) | 0.376 lb | 0.560 kg | 15″ |
| #4 | 0.5″ (13 mm) | 0.668 lb | 0.994 kg | 20″ |
| #5 | 0.625″ (16 mm) | 1.043 lb | 1.552 kg | 25″ |
| #6 | 0.75″ (19 mm) | 1.502 lb | 2.235 kg | 30″ |
| #8 | 1″ (25 mm) | 2.67 lb | 3.973 kg | 40″ |
A lap of 40 bar diameters is a common rule of thumb for slabs; structural drawings and ACI 318 may call for more depending on concrete strength and bar position.
Worked examples
#4 bar at 16″ with 3″ clearance: (240 − 6) ÷ 16 = 14.6 → 14 spaces → 15 bars each way, each 19′ 6″ long. One 20 ft bar per run, so 30 bars, 585 linear ft, 391 lb of steel.
#3 at 18″: 8 bars run the 30 ft length, each needing two 20 ft bars lapped 15″ (16 bars). 20 bars run across at 11′ 6″, one per 20 ft bar. Total 36 bars, 476 linear ft, 179 lb.
Frequently asked questions
What rebar spacing should I use for a slab?
Residential slabs on grade commonly use #3 at 18″ or #4 at 16″–24″ each way, but many plain driveways and patios use welded wire mesh or no steel at all. Follow your plans or local code; spacing is a structural decision.
Where should rebar sit in a slab?
Usually in the middle to upper third of the slab thickness, held up on chairs or concrete blocks. Rebar lying on the ground does little to control cracking.
How many sticks of rebar are in a ton?
About 150 sticks of 20 ft #4 bar (2,000 lb ÷ 13.4 lb per stick), or about 96 sticks of 20 ft #5 (20.9 lb each).
How much tie wire do I need?
Tie every other intersection on a slab, using roughly 10–12″ of wire per tie. A 3.5 lb roll of 16-gauge tie wire covers most residential slabs.