Counting bars: divide by spacing, add one
A slab is reinforced with a two-way grid, so you count bars twice — once along the length, once along the width. In each direction, take the run (after subtracting the edge clearance from both sides), divide by the on-center spacing, round down, and add one for the first bar. The "+1" catches the bar that sits at the far edge, and it is the step people forget.
Total bar length and where it comes from
Once you know the bar count each way, the total steel is the count times the span it crosses, for both directions added together.
| Rebar size | Diameter | Weight |
|---|---|---|
| #3 | 3/8 in | 0.376 lb/ft |
| #4 | 1/2 in | 0.668 lb/ft |
| #5 | 5/8 in | 1.043 lb/ft |
| #6 | 3/4 in | 1.502 lb/ft |
Multiply total length by the weight per foot for the order weight. A 20 × 15 ft slab in #4 at 12 inches is about 625 ft of bar, roughly 460 lb after adding laps.
The traps in a rebar takeoff
- Spacing is on center. "12 inch spacing" means 12 inches center to center, not a 12 inch gap between bars.
- Bars start inside the edge. The first bar sits about 6 inches in for concrete cover, not flush with the form, which is why you subtract the edge clearance first.
- Buy in whole sticks. Rebar comes in 20 ft lengths; 625 ft needs 32 sticks, not 31 — round up and add about 10% waste for a real order.
Common questions
How do I count the number of rebar bars for a slab?
For each direction, divide the slab dimension (minus the edge clearance on both sides) by the spacing, drop any fraction, and add one. A 20 ft run with 12 inch spacing after 6 inch edges gives 228 / 12 = 19, plus 1, so 20 bars.
What size rebar do I need for a 4 inch slab?
A #4 bar (1/2 inch) at 12 to 18 inch spacing covers almost every residential slab on grade. #3 is usually too light and #5 is overkill unless the slab is thicker than 6 inches.
Do I add extra length for laps?
Yes. Where two bars meet they must overlap rather than butt end to end, so add roughly 10% to the total bar length, or count each lap as about 1.3 times the development length.
