Rebar in a slab is a grid: bars one way, crossing bars the other, at a set spacing. Count the bars each way, add them up, and you have the linear footage — then the bar size gives the weight. Here is the method, plus the spacing and sizes for common jobs.
Counting the bars
The number of bars each way is the span divided by the spacing, plus one. A 16 × 13 ft slab (5 × 4 m) with #4 bars at 12 in (300 mm) needs about 17 bars one way and 14 the other — roughly 445 linear feet (135 m) of steel before laps.
Rebar Calculator
Enter the slab size and bar spacing for the number of bars, total length, weight and stock lengths.
Spacing and bar size
| Job | Spacing | Bar size |
|---|---|---|
| Patio / light slab | 16–18 in (400–450 mm) | #3–#4 |
| Residential slab / driveway | 12 in (300 mm) | #4 (½ in) |
| Structural / heavy load | 6–8 in (150–200 mm) | #4–#5 |
How much rebar weighs
By bar size: #3 is about 0.38 lb/ft (0.56 kg/m), #4 about 0.67 lb/ft (0.99 kg/m), #5 about 1.04 lb/ft (1.55 kg/m). Multiply by the total length for the weight — that is how rebar is ordered and priced, so the tonnage is what you take to the supplier.
Don’t forget the laps
Where two bars join, they overlap by roughly 40 times the bar diameter — about 20 in (500 mm) for a #4 bar — so the load transfers between them. That is why a lap and waste allowance is added: 10% covers a simple slab, more for a job with many splices. Confirm the lap length for your bar size and local code.