Enter the slab dimensions, design spacing, selected bar size and an explicit cutting/overlap allowance. The grid estimates straight lengths and mass; lap length, cover, anchorage and reinforcement design must come from the actual structural requirements. No universal 30–40-diameter lap is assumed.
Structural requirements for reinforcement come from engineering or building code, not from this calculation, and spacing and size vary by load and soil. Cover, the concrete over the bars, must be adequate to prevent corrosion. Welded wire mesh is an alternative for light slabs.
Bends and hooks add length.
Total rebar length (feet) from slab length
| Slab length | 10 |
|---|---|
| Slab width | 10 |
| Bar spacing | 12 |
| Bar size | #4 rebar (0.668 lb/ft) |
| Total rebar length (feet) | 220 |
| Total rebar weight (pounds) | 147 |
Common error: Counting the spaces instead of the bars. A run divided into 12 inch spaces has one more bar than it has spaces.
Rebar: formula
Rebar: changing Slab length
| Slab length | Total rebar length (feet) | Total rebar weight (pounds) |
|---|---|---|
| 8 | 178 | 118.9 |
| 9 | 199 | 132.9 |
| 10 (example) | 220 | 147 |
| 11 | 241 | 161 |
| 12 | 262 | 175 |
Rebar: compare input ranges
| Input | Tested from | To | Total rebar length (feet): low input | Total rebar length (feet): high input | Swing |
|---|---|---|---|---|---|
| Slab length | 9 | 11 | 199 | 241 | 42 (19%) |
| Slab width | 9 | 11 | 199 | 241 | 42 (19%) |
| Bar spacing | 11 | 13 | 220 | 200 | 20 (9.1%) |
Rebar: individual input comparisons
| Input | Value | Total rebar length (feet) | Total rebar weight (pounds) |
|---|---|---|---|
| Slab length | 8 | 178 | 118.9 |
| 9 | 199 | 132.9 | |
| 10 | 220 | 147 | |
| 11 | 241 | 161 | |
| 12 | 262 | 175 | |
| Slab width | 8 | 178 | 118.9 |
| 9 | 199 | 132.9 | |
| 10 | 220 | 147 | |
| 11 | 241 | 161 | |
| 12 | 262 | 175 | |
| Bar spacing | 10 | 260 | 173.7 |
| 11 | 220 | 147 | |
| 12 | 220 | 147 | |
| 13 | 200 | 133.6 | |
| 14 | 180 | 120.2 | |
| Bar size | #3 rebar (0.376 lb/ft) | 220 | 82.7 |
| #4 rebar (0.668 lb/ft) | 220 | 147 | |
| #5 rebar (1.043 lb/ft) | 220 | 229.5 |
Total rebar length (feet): Slab length against Slab width
| Slab length / Slab width | 8 | 10 | 12 |
|---|---|---|---|
| 8 | 144 | 178 | 212 |
| 9 | 161 | 199 | 237 |
| 10 | 178 | 220 | 262 |
| 11 | 195 | 241 | 287 |
| 12 | 212 | 262 | 312 |
Rebar: values and definitions
| Quantity and definition | Value used |
|---|---|
| Slab length — Length of the reinforced area, in feet. | 10 |
| Slab width — Width of the reinforced area, in feet. | 10 |
| Bar spacing — On-center spacing between bars, in inches. | 12 |
| Bar size — Rebar size and its weight per linear foot. | #4 rebar (0.668 lb/ft) |
| Total rebar length (feet) | 220 |
| Total rebar weight (pounds) | 147 |
Slab length: half and double
| Slab length | Total rebar length (feet) |
|---|---|
| 5 | 115 |
| 10 | 220 |
| 20 | 430 |
Rebar in a slab is a grid, and the count comes from the spacing in each direction. A 10 by 10 foot slab with bars every 12 inches uses 11 bars each way, or 220 feet in total. At about 0.668 pounds a foot for a number 4 bar, that weighs roughly 147 pounds.
Closer spacing raises the length fast, since halving the spacing nearly doubles it. Laps at the splices and the bars along the edges add more, and the plans or an engineer set the size and spacing.
Follow the size and spacing on the drawings or from an engineer, and add length for the laps where bars meet. Keep the bars off the ground on chairs so concrete surrounds them.
Estimate the concrete on the concrete volume calculator, footing dimensions on the footing size page and block reinforcement on the CMU block calculator. The material volume and rectangle area calculators give supporting numbers, and the weight converter changes pounds and kilograms.
How far apart should rebar be spaced?
Commonly 12 to 18 inches on center for residential slabs, though the required spacing depends on the design and loads.
Why keep rebar off the ground?
Bars need to sit in the concrete, near the middle or as designed, and be supported on chairs so they are surrounded by concrete and protected from corrosion.