Rebar Calculator
This rebar calculator works out the reinforcing grid for a concrete slab: how many bars run each direction, the total linear feet including lap-splice overlap, and how many stock sticks to put on the truck. Enter the slab length and width in feet (or meters), pick a grid spacing of 12, 16, 18, or 24 inches, and the totals update instantly.
Metric options use regional material sizes. Check your selected sizes after switching units.
Grid runs needed
38runs
- Runs each direction13 × 23' 6" lengthwise · 25 × 11' 6" widthwise
- Splice allowance13 linear ft (13 laps × 12 in)
- Total linear feet606 linear ft
- Stock bars (before reusing offcuts)51 × 20 ft bars
Updated .
How the rebar grid and cut lengths are calculated
This tool estimates material for a rectangular grid using the dimensions on your reinforcement plan. It does not select bar diameter, reinforcement spacing, concrete cover, or lap length for structural loads.
usable length = slab length − 2 × edge allowance
lengthwise runs = ceil(usable width ÷ maximum spacing) + 1
widthwise runs = ceil(usable length ÷ maximum spacing) + 1
sticks per run = 1 if run ≤ stock; otherwise 1 + ceil((run − stock) ÷ (stock − lap))
Both directions include a bar at each boundary of the inset rectangle. Rounding the number of bays up keeps the last bay at or below your entered spacing. The bar lengths also subtract the edge allowance at both ends. This geometric allowance is a simplified input: confirm actual cut lengths, clear cover, bar-center positions, hooks, and bends from the placing drawings.
When a run needs more than one stock bar, every additional stick loses some reach to its overlap. The tool adds that lap length to the linear steel total. The default 12-inch lap is an example input, not a recommended minimum. Required laps depend on the reinforcement and concrete design; enter the specified length from your plan.
The stock count allocates whole sticks to each run separately. It is a conservative purchasing estimate before a cutting schedule, not the smallest possible order. Offcuts can often serve other runs, so a fabricator may reduce that quantity by planning cuts and splice locations.
Worked example: material for a 24 × 12 ft grid
This arithmetic example uses the default 24 × 12 ft slab, a 3-inch edge allowance, maximum 12-inch spacing, 20-foot stock, and an illustrative 12-inch lap. These settings are not a driveway reinforcement specification.
- Lengthwise runs: usable width = 144 − 6 = 138 inches. ceil(138 ÷ 12) + 1 = 13 runs, each 23.5 ft long.
- Widthwise runs: usable length = 288 − 6 = 282 inches. ceil(282 ÷ 12) + 1 = 25 runs, each 11.5 ft long.
- Grid total: 13 + 25 = 38 runs.
- Base length: 13 × 23.5 + 25 × 11.5 = 593 linear feet.
- Splices: each long run needs two sticks and one 1-ft lap. Add 13 ft to get 606 linear feet.
- Stock allocation: 13 × 2 + 25 × 1 = 51 twenty-foot bars before reusing offcuts.
A 23.5-foot run can use one 20-foot bar plus a 4.5-foot piece, including the lap. That leaves 15.5 feet from the second stick, enough for an 11.5-foot run if the approved splice layout permits that cutting plan. This is why dividing total footage by stock length and assigning fresh sticks to every run give different orders: neither is an optimized cutting schedule.
Rebar grid quantities for common square slabs
The table uses a 3-inch allowance on every edge and includes bars at both grid boundaries. It compares geometric quantities at two maximum spacings; choose the spacing from your plans, not from the cheaper column.
| Slab size | Runs @ 12″ | Linear ft @ 12″ | Linear ft @ 18″ |
|---|---|---|---|
| 10 × 10 ft | 22 | 209 | 152 |
| 12 × 12 ft | 26 | 299 | 207 |
| 14 × 14 ft | 30 | 405 | 270 |
| 16 × 16 ft | 34 | 527 | 372 |
| 18 × 18 ft | 38 | 665 | 455 |
| 20 × 20 ft | 42 | 819 | 546 |
| 24 × 24 ft | 50 | 1175 | 799 |
Longer slabs can require splices even when the short direction fits a single bar. Enter both dimensions and your available stock length above to see that additional steel separately.
Frequently asked questions
Does this calculator decide the right rebar size and spacing?
No. It counts material for the layout you enter. Bar diameter, grade, grid spacing, cover, and reinforcement location belong to the project design. A driveway, footing, and patio can have different requirements even when their surface areas match. Use the reinforcement schedule or have the designer confirm those inputs before treating this estimate as a buying list.
Why are grid runs different from stock bars to buy?
A grid run is one continuous line of reinforcement across the inset slab area. A long run may need several overlapping pieces cut from stock bars. The primary result counts those lines; the stock result allocates whole bars to make them. Shorter offcuts may reduce the actual purchase once you prepare a cutting schedule that respects the approved splice positions.
What happens when the grid spacing does not divide evenly?
The calculator rounds the number of bays up and includes a bar at each edge of the usable rectangle. With 138 inches available and a maximum 12-inch spacing, twelve bays require thirteen bars. The final bay can be shorter, or the spacing can be distributed evenly. The layout never increases a bay beyond the entered maximum just to reduce the bar count.
How much rebar overlap should I enter?
Use the lap length specified for the project. CRSI explains that lap requirements depend on factors including bar size and grade, concrete strength, spacing, and cover. The 12-inch default here is only for the arithmetic example. If a spliced run has an overlap equal to or longer than its stock bar, the calculator asks you to correct the input instead of ignoring the overlap.
Are rebar chairs, hooks, and bent bars included?
No. This estimate covers straight grid runs and their entered lap overlaps. Chair quantities and heights, hooks, edge thickening reinforcement, dowels, and special bends require the placing drawings and support layout. Add those items separately. Changing the edge allowance adjusts this simplified grid but does not verify the required vertical position or clear cover of the steel.
Sources and assumptions
- CRSI: Lap Splices — Explains why required lap lengths vary with the reinforcement and concrete design.
- CRSI: Placing Bars — Placing drawings, bar spacing, and supports determine the installed layout.
These results are planning estimates only — verify quantities with your supplier and local building code before you buy or build. See our terms of use.