Rebar Calculator
Enter your slab or footing size and the rebar grid spacing. You'll get the number of bars to buy, the grid layout, total linear feet with a lap allowance, and how many intersections you'll need to tie.
Results
How the rebar calculator works
Rebar in a slab is laid as a grid: bars running the full length spaced across the width, and bars running the width spaced across the length. The calculator counts each direction from your grid spacing, adds them up for total linear feet, then adds 10% for the overlaps where bars are spliced together.
Formula: bars one way = floor((span in inches − 2 × cover) ÷ spacing) + 1, done for both directions, then total length × 1.10 ÷ bar length = bars to buy
| Spacing | Typical use |
|---|---|
| 18"–24" o.c. | Patios, sheds, light slabs (#3–#4 bar) |
| 12"–16" o.c. | Driveways, garage & house slabs (#4–#5 bar) |
| Footings | 2–4 continuous bars along the trench (#4–#5) |
The calculator also reports the number of grid intersections — each one gets a wire tie to hold the mat square while the concrete goes in. A roll of tie wire and a twist tool handle a slab's worth of ties easily.
Rebar tips before you order
- Keep steel about 3 inches from every edge and lift it to the middle third of the slab on chairs — rebar lying on the ground does nothing.
- Overlap spliced bars by roughly 40 times the bar diameter — about 20 inches for #4 and 24 inches for #5 — and tie the lap.
- For a plain 4-inch residential slab, welded wire mesh or fibre mesh is often used instead of a bar grid; rebar earns its keep in thicker, load-bearing or structural pours.
- Bar sizes are eighths of an inch: #4 is ½", #5 is ⅝". Heavier loads use bigger bar, not tighter spacing alone.
Frequently asked questions
How much rebar do I need for a 20x12 slab?
At 12-inch grid spacing with 3-inch edge cover, a 20 × 12 ft slab needs about 12 bars running the length and 20 running the width — roughly 480 linear feet, or about 27 twenty-foot bars once you add 10% for laps.
How do I calculate rebar for a slab?
Divide each side (in inches, minus twice the edge cover) by your grid spacing and add one to get the bar count in each direction. Multiply each count by the length of those bars, total the two directions, add about 10% for laps, then divide by the length of bar you're buying.
What spacing should rebar be in a slab?
Twelve to sixteen inches on center is standard for driveways and house slabs; patios and light slabs can go to 18–24 inches. Tighter spacing and larger bar diameter both add strength.
How much should rebar overlap?
Lap spliced bars by about 40 times the bar diameter — roughly 20 inches for #4 (½") bar and 24 inches for #5 (⅝") bar — and tie the overlap. The calculator's 10% allowance covers typical lapping.