How to calculate rebar for a concrete slab
Rebar is a counting problem before it is a weighing problem. Get the bar count right and everything after it is multiplication.
Last updated 29 September 2026What only your drawings can tell you
- Bar size. The bar number, from the structural drawings.
- Spacing, centre to centre. There is no safe default. A spacing right for a patio can be wrong for a slab that carries a load.
- Cover, the concrete between a bar and the edge or face, which the plans and the code set.
- Lap length, only if a bar has to be spliced.
Every value used below is an illustrative input chosen to show the method. None is a recommendation.
Step 1: count the bars in each direction
Bars = floor((Dimension − 2 × Cover) ÷ Spacing) + 1
A bar spans the slab less the cover at both ends, and the first and last bars sit one cover in from the edges, so the count is one more than the number of gaps.
Take a slab 20 ft by 12 ft, with bars at 12 in centres and 3 in of cover. Bars that run along the length are spaced across the width, and the reverse.
| Step | Working | Result |
|---|---|---|
| Bar length along the length | 20 ft − 2 × 0.25 ft | 19.50 ft |
| Bars along the length | floor((12 − 0.50) ÷ 1) + 1 | 12 bars |
| Bar length along the width | 12 ft − 2 × 0.25 ft | 11.50 ft |
| Bars along the width | floor((20 − 0.50) ÷ 1) + 1 | 20 bars |
Step 2: total length
| Step | Working | Result |
|---|---|---|
| Along the length | 12 × 19.50 ft | 234.0 ft |
| Along the width | 20 × 11.50 ft | 230.0 ft |
| Total | 234.0 + 230.0 | 464 ft |
Step 3: weight from a published table
A bar’s weight is its length times its weight per foot. The weights below are the nominal figures in the Texas Department of Transportation’s standard specification for reinforcement, which tabulates the ASTM A615 bar dimensions. The bar number is the nominal diameter in eighths of an inch for #3 to #8.
| Bar | Nominal diameter, in | Area, sq in | Weight, lb per ft |
|---|---|---|---|
| #3 | 0.375 | 0.11 | 0.376 |
| #4 | 0.5 | 0.2 | 0.668 |
| #5 | 0.625 | 0.31 | 1.043 |
| #6 | 0.75 | 0.44 | 1.502 |
| #7 | 0.875 | 0.6 | 2.044 |
| #8 | 1 | 0.79 | 2.67 |
| #9 | 1.128 | 1 | 3.4 |
| #10 | 1.27 | 1.27 | 4.303 |
| #11 | 1.41 | 1.56 | 5.313 |
| #14 | 1.693 | 2.25 | 7.65 |
| #18 | 2.257 | 4 | 13.6 |
The full chart, with the soft-metric sizes, metric conversions and where the published tables disagree, is the rebar size chart.
For #4 at 0.668 lb per foot: 464 ft × 0.668 = 310.0 lb for one layer. A second mat doubles both counts: 928 ft and 619.9 lb.
The table is nominal, and the weight of a delivered bar varies within the standard’s tolerance. As a check, each tabulated weight agrees with the bar’s cross-section times the density of steel, about 490 lb per cubic foot, to within the table’s rounding; that check is run in the calculator’s tests. It is not a source, and the table is.
Step 4: sticks to buy, and what a splice costs
Rebar is sold in stock lengths. Where a bar fits in one stick, the sticks needed are the number of bars divided by the bars one stick yields, rounded up. With 20 ft stock, one 19.50 ft bar takes one stick and so does one 11.50 ft bar, because two of them do not fit, so the slab above takes 32 sticks and the offcuts are not reused.
If a bar is longer than the stock, it has to be spliced, and every splice puts a lap length of bar where there would otherwise be none. With 10 ft stock and a 12 in lap (both illustrative), the same slab needs 76 sticks, the total bar length grows from 464 ft to 508 ft, and the weight from 310.0 lb to 339.3 lb. The lap length is set by your drawings or the code, not by this page.
What this leaves out
- Chairs, ties, bolsters and dowels, which are bought separately and are not in these counts.
- Bent bars and hooks, whose extra length comes from the bending schedule.
- Any allowance for waste: the count is the layout, and an overage is added only when you choose one.
- Which bar, spacing, cover or lap to use. Those are engineering decisions and they are yours or the designer’s.
- 2024 Standard Specifications for Construction and Maintenance of Highways, Streets, and Bridges — Item 440, Reinforcement for Concrete, Table 1 "Size, Area, and Weight of Reinforcing Steel Bars" — Texas Department of Transportation
- Bar count, length, weight and sticks are defined by the formulas above — Arithmetic — verifiable by hand; recomputed from the rebar calculator’s own engine
Put it to work
All construction calculators →Enter your own bar size, spacing, cover, stock length and lap, and get the count, length, weight and sticks with the working shown.