Rebar calculator
How much rebar do you need?
Enter your slab size and grid spacing to get the number of bars each way, total linear feet, how many stock bars to buy, and an estimated cost.
Enter your slab
12″–18″ on center is common for residential slabs and driveways. Follow your plans or local code for bar size and spacing.
Results
- Bars each way
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- Total rebar
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- Stock bars to buy
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- Estimated cost
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Stock bar count includes your lap/waste % and rounds up to whole bars. Add tie wire and chairs (about one per 4 sq ft) to hold the grid mid-slab.
Estimates only — bar size, spacing and splices on structural work must follow your plans or local code.
Quick reference
Rebar for common slab sizes.
A two-way grid with 3″ edge clearance. Bar counts are 20 ft stock bars with 10% added for laps and offcuts.
| Slab | 12″ grid: linear ft | 12″ grid: bars | 18″ grid: linear ft | 18″ grid: bars |
|---|---|---|---|---|
| 10 × 10 ft | 190 | 11 | 133 | 8 |
| 12 × 12 ft | 276 | 16 | 184 | 11 |
| 12 × 20 ft | 464 | 26 | 317 | 18 |
| 20 × 20 ft | 780 | 43 | 546 | 31 |
| 24 × 24 ft | 1128 | 63 | 752 | 42 |
| 20 × 40 ft | 1570 | 87 | 1080 | 60 |
How it's calculated
Bars each way × bar length, plus laps.
Rebar in a slab is laid as a grid. Each bar stops short of the slab edge by the clearance you enter, so it stays fully covered in concrete. The number of bars running lengthwise is the usable width divided by the spacing, plus one; the same goes the other way. Total linear feet is each direction's bar count times its bar length. We add your lap/waste percentage for splices and cut-offs, then divide by the stock bar length and round up to whole bars.
FAQ
Common questions
How do you calculate rebar for a slab?
Subtract the edge clearance from both sides of the slab, divide what's left by the bar spacing and add one to get the number of bars running each way. Multiply each direction's bar count by its bar length, add the two together, then add 10% or so for lapped splices and cut-offs. Divide by your stock bar length (usually 20 ft) and round up.
What rebar spacing should I use?
12 to 18 inches on center, both ways, is common for residential patios, driveways and garage slabs, usually with #3 (3/8") or #4 (1/2") bar. Structural slabs, footings and anything on engineered plans should follow those plans and your local code.
How much should rebar overlap at a splice?
A common rule of thumb is 40 bar diameters — about 20 inches for #4 bar and 15 inches for #3. The calculator's lap/waste percentage covers those overlaps plus offcuts; raise it if your slab needs many splices.
Do I need rebar or is wire mesh enough?
Welded wire mesh controls surface cracking in thin, lightly loaded slabs like walkways. Rebar adds real strength and is the usual choice for driveways, slabs carrying vehicles, and anything on poor or expansive soil.
Pouring the slab too?