Long, narrow pours like sidewalks need few bars across but long runs with laps.
How it works
Bars each way = (slab size − 2 × cover) ÷ spacing + 1. Each bar runs the slab less the cover; bars longer than the stock length get a lap of 40 bar diameters by default.
Example: a 50×4 ft slab with #3 bars 18 in apart and 3 in edge cover needs 3 + 34 bars, 275 ft of rebar (15 × 20 ft bars) weighing 103 lb.
At a glance
Same settings as the calculator above, changing only the spacing.
| Spacing | Rebar | 20 ft bars | Weight |
|---|---|---|---|
| 12 in | 383 ft | 21 | 144 lb |
| 16 in | 289 ft | 16 | 109 lb |
| 18 in | 275 ft | 15 | 103 lb |
| 24 in | 192 ft | 11 | 72 lb |
Rebar sizes (ASTM A615)
| Bar | Diameter | Weight per ft | 20 ft bar |
|---|---|---|---|
| #3 | 0.375 in | 0.376 lb | 7.52 lb |
| #4 | 0.5 in | 0.668 lb | 13.36 lb |
| #5 | 0.625 in | 1.043 lb | 20.86 lb |
| #6 | 0.75 in | 1.502 lb | 30.04 lb |
| #7 | 0.875 in | 2.044 lb | 40.88 lb |
| #8 | 1 in | 2.67 lb | 53.40 lb |
Common questions
How far apart should rebar be in a slab?
Residential slabs commonly use #3 or #4 bars 12 to 18 inches apart each way, but the spacing should come from your plans or local code.
How much do rebar laps need to be?
Lap length depends on bar size, concrete strength and code. 40 bar diameters is a common rule of thumb: 20 in for #4 bar. Check your plans.
How heavy is rebar?
#3 weighs 0.376 lb/ft, #4 0.668 lb/ft, #5 1.043 lb/ft and #6 1.502 lb/ft under ASTM A615.
Sources
- ASTM A615 bar sizes and weights: https://www.astm.org/a0615_a0615m-22.html