Rebar Calculator
A Slab's Reinforcing Grid Has Two Directions to Count
Rebar in a slab isn't laid as a single run of bar — it's a grid, with bars spanning one direction spaced at intervals across the other, and a second set running perpendicular to it. Counting only one direction, or forgetting that spacing has to be measured against the dimension it crosses, is the easiest way to underorder reinforcing steel.
The Formula
Bars Other Way = floor(Width ÷ Spacing) + 1
Total Length = (Bars One Way × Width) + (Bars Other Way × Length)
Weight = Total Length × Unit Weight
For each direction, the calculator divides the slab's dimension by the bar spacing (12 in on-center by default) and adds one, since a grid needs a bar at both the starting edge and after each full spacing interval. Each bar in a given direction runs the full length of the perpendicular dimension, so total linear footage is bars-in-one-direction times the crossing dimension, summed for both directions. Multiplying by the bar size's standard unit weight (ASTM A615 nominal weights) gives the total steel weight.
Where This Calculation Matters
- Slab-on-grade reinforcing — driveways, patios, and garage floors commonly use a #3 or #4 grid at 12 to 18 in spacing to control cracking.
- Bar size selection — heavier structural slabs step up to #5 through #8 bar, which increases both the steel's strength contribution and its weight per linear foot substantially.
- Ordering by weight — rebar suppliers frequently price and deliver by the ton, so converting a bar count into total weight is what makes a quote comparable across bar sizes.
Standard Rebar Unit Weights
| Bar Size | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|
| #3 | 0.376 | 0.560 |
| #4 | 0.668 | 0.994 |
| #5 | 1.043 | 1.552 |
| #6 | 1.502 | 2.235 |
| #7 | 2.044 | 3.042 |
| #8 | 2.670 | 3.973 |
Worked Example
A 20 ft × 15 ft slab with #4 rebar at the default 12 in spacing:
| Step | Calculation | Result |
|---|---|---|
| Bars across width | floor(20 ÷ 1) + 1 | 21 |
| Bars across length | floor(15 ÷ 1) + 1 | 16 |
| Total pieces | 21 + 16 | 37 |
| Total length | (21 × 15) + (16 × 20) | 635 ft |
| Total weight | 635 × 0.668 lb/ft | 424.18 lb |
How to Use This Calculator
- Select Imperial (ft, in) or Metric (m, mm) units.
- Enter the Slab Length and Slab Width.
- Optionally adjust the Bar Spacing — defaults to 12 in (300 mm) on-center.
- Select a Bar Size from #3 to #8.
- Select Calculate to see the total bar count and steel weight.
Related Calculations
Pouring the slab itself? The Concrete Calculator sizes the surrounding volume, and the Foundation Calculator covers larger foundation pours that use the same reinforcing approach.