Slab reinforcement quantity
Rebar calculator for a slab
A 10 ft × 10 ft slab with bars at 12 in. spacing and a 3 in. setback from the edges has 10 bars in each direction, 190 ft of bar in total, which is about 127 lb of #4. Enter your slab size, your spacing and setback, and a bar size to count the bars, the total length, the weight and a minimum stock-bar count by total length. The spacing comes from your plans: this tool counts, it does not design. Last updated October 12, 2026.
Rebar grid estimate
Straight bars in a rectangular grid, from the spacing and setback you enter. Quantity only.
Rebar for your slab
Enter the slab, your bar spacing and the setback from the edges.
Bars start one setback from each edge and repeat at the spacing you enter. Diagram not to scale.
Net length plus your allowance.
BARS AND WEIGHT
The stock count is a lower bound from total length, not a cutting plan. Offcuts may require more stock bars; bars longer than your stock need a separately planned splice or longer stock. This tool does not choose bar size, spacing, cover, laps or chairs; those come from your plans and the responsible professional.
The method
Count the rows, then multiply by the bar length
Bars start one setback from each edge and repeat at your spacing. The number of rows across a span is floor((span − 2 × setback) ÷ spacing) + 1. Each bar runs the other dimension less two setbacks. Multiply the bars by their length for the net total, apply your waste and lap allowance, then multiply by the bar’s weight per foot.
The weights per foot come from the published bar table on the rebar weight calculator. Need the concrete volume too? Use the concrete slab calculator.
Worked examples
Five rebar counts, calculated step by step
- A 10 ft × 10 ft slab, 12 in. spacing, 3 in. setback, #4. (120 − 6) ÷ 12 = 9.5, so 9 gaps and 10 bars each way. Each bar is 10 − 0.5 = 9.5 ft. Total 20 bars × 9.5 ft = 190 ft; × 0.668 lb/ft = 126.9 lb.
- The same slab with no setback. 120 ÷ 12 + 1 = 11 bars each way, each 10 ft: 220 ft in total.
- A 20 ft × 12 ft slab, 16 in. spacing, 3 in. setback, 10% allowance. 9 bars along the length (19.5 ft each) and 15 along the width (11.5 ft each) give 348.0 ft net, 382.8 ft with the allowance, 256 lb of #4, and a length-based minimum of 20 stock bars of 20 ft.
- The first slab with bars along the length only. 10 bars × 9.5 ft = 95 ft; with a 10% allowance 104.5 ft, or a length-based minimum of 6 bars of 20 ft.
- A metric slab, 4 m × 3 m, 30 cm spacing, 5 cm setback, #3. 10 bars along the length and 14 along the width: 79.6 m of bar, 98.2 lb (44.5 kg).
The spacings and setbacks above are example inputs to show the arithmetic. They are not recommendations. Stock-bar figures are minimums based on total length only. They do not account for how individual bars fit into stock lengths; cutting waste can increase the number to buy.
Rebar length by slab size and spacing
- 12 in. spacing370 ft · 247 lb
- 16 in. spacing278 ft · 185 lb
- 18 in. spacing251 ft · 167 lb
- 24 in. spacing185 ft · 124 lb
| Slab (ft) | Spacing (in) | Bars each way | Total length (ft) | Weight of #4 (lb) |
|---|---|---|---|---|
| 10 × 10 | 12 | 10 × 10 | 190 | 127 |
| 10 × 10 | 16 | 8 × 8 | 152 | 102 |
| 10 × 10 | 18 | 7 × 7 | 133 | 89 |
| 12 × 12 | 12 | 12 × 12 | 276 | 184 |
| 12 × 12 | 16 | 9 × 9 | 207 | 138 |
| 12 × 12 | 18 | 8 × 8 | 184 | 123 |
| 12 × 16 | 12 | 16 × 12 | 370 | 247 |
| 12 × 16 | 16 | 12 × 9 | 278 | 185 |
| 12 × 16 | 18 | 11 × 8 | 251 | 167 |
| 20 × 20 | 12 | 20 × 20 | 780 | 521 |
| 20 × 20 | 16 | 15 × 15 | 585 | 391 |
| 20 × 20 | 18 | 14 × 14 | 546 | 365 |
What this leaves out
The count is for straight bars in a rectangular grid. It does not include laps, hooks and bends, chairs and supports, tie wire, bar coatings or wire mesh. It does not choose reinforcement, spacing or cover: those depend on the slab’s use and the project plans, and a quantity calculator cannot decide them.
Common questions
Slab rebar questions
How do you calculate rebar for a slab?
Count the bars in each direction, then multiply by the length of each bar. With bars starting one setback from the edge and repeating at the spacing, a row count is floor((span − 2 × setback) ÷ spacing) + 1. A 10 ft × 10 ft slab at 12 in. spacing and a 3 in. setback has 10 bars each way, each 9.5 ft long, which is 190 ft in total.
How much does the rebar for a slab weigh?
Multiply the total length by the bar’s weight per foot. The 190 ft of #4 in the 10 ft × 10 ft example weighs 126.9 lb at 0.668 lb/ft. The rebar weight calculator lists every bar size.
What spacing and bar size should I use?
Use the spacing, bar size and cover from your plans or the responsible professional. This calculator counts bars from the layout you enter; it does not choose reinforcement, and the spacings in the tables are comparison inputs only.
Does the count include laps, bends and chairs?
No. It counts straight bars in a rectangular grid and shows a waste and lap allowance as a percentage you choose. Overlaps, hooks, bends, supports and tie wire depend on your details and are extra.
How many stock bars do I need?
Dividing total length, including your allowance, by stock length and rounding up gives a lower bound, not a cutting plan. The 20 ft × 12 ft example has 382.8 ft with allowance, equivalent to a minimum of 20 stock bars of 20 ft by total length. Offcuts can require more bars. If a required bar is longer than the stock, use longer stock or a splice detail from your plans. Confirm the cutting list before ordering.
What if the bars run in one direction only?
Choose “along the length only”. For the 10 ft × 10 ft example that is 10 bars of 9.5 ft, or 95 ft, instead of 190 ft for the grid.
What is a setback?
It is the distance from the slab edge to the first bar. You choose it from your plans. With no setback, bars start at the edge: the same 10 ft × 10 ft slab at 12 in. spacing has 11 bars each way instead of 10.
Does this include wire mesh?
No. Welded wire mesh is sold in sheets or rolls with its own spacing and wire size. This tool counts individual bars.
Sources and limits
Bar weights: San Antonio Water System, Standard Specifications for Construction, Item 301, Table 1 (March 2008), read on October 12, 2026; the same values appear in ASTM A615 bar tables. The layout method is geometry from the dimensions you enter.
Quantities are planning estimates. Ask your supplier about stock lengths, and follow your plans for bar size, spacing, cover and laps. For concrete quantity see the concrete slab calculator.
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