Rebar Calculator
Enter your slab size and grid spacing to get the number of rebar bars each way and the total linear feet.
What your result means
Total linear feet is what you order, usually in 20 ft sticks, so the stick count rounds that up for you. The two bar counts are how many run each direction in the grid. This is for a standard residential mat; lap any joints 40 bar diameters (about 20 inches for #4) and set the bars on chairs so they sit mid-slab.
How to use this calculator
- Measure slab dimensions in feet.
- Standard residential rebar spacing is 12 in OC (on center).
- Heavier loads use 8 in OC.
- Read number of bars in each direction plus total linear feet.
- Order standard 20 ft rebar lengths from supplier.
The formula
Each direction gets a bar every spacing interval across the opposite dimension, plus one. Multiply each bar count by the length it spans and add the two for total linear feet.
Worked example
Say you've got a 20 by 20 ft slab on a 12 inch grid. That's 21 bars one way and 21 the other, 42 bars total, for 840 linear feet. At 20 ft a stick, that's 42 sticks.
Rebar size and spacing
| Use | Bar size | Spacing |
|---|---|---|
| Residential slab | #4 (1/2 in) | 12 in OC |
| Driveway | #5 (5/8 in) | 12 in OC |
| Non-load-bearing | #4 | 16 in OC |
Tips & gotchas
- Standard residential bar is #4, a 1/2 inch diameter.
- 12 inch on-center spacing is standard for slabs up to 6 inches thick.
- 16 inch on-center is fine for a non-load-bearing slab.
- Tie the grid at every other intersection, minimum.
- Set the bars up on chairs so they end up in the middle of the slab, not on the dirt.
Frequently asked questions
How much rebar do I need for a slab?
A standard residential mat is a 12 inch grid of #4 bar. Enter your slab and spacing above for the exact count.
What size rebar should I use?
#4 (1/2 inch) for residential, #5 (5/8 inch) for a driveway, and #6 or bigger for structural work.
Do I need rebar in a sidewalk?
It's recommended but not always required. Welded wire mesh is an acceptable alternative.
How do I cut rebar?
A rebar cutter, bolt cutters, or an angle grinder all work.
How much should I overlap rebar at a joint?
At least 40 times the bar diameter, so about 20 inches for #4.
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