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.
Rebar needed
36bars
- Bars each direction12 × 24' lengthwise · 24 × 12' widthwise
- Splice allowance12 ft (12 laps × 12″)
- Total linear feet588 linear ft
- Stock to buy48 × 20 ft bars
Formulas last reviewed on .
How the rebar grid is calculated
The calculator lays out a two-way grid: bars running the length are spaced across the width, and vice versa.
bars widthwise = floor((length − 2 × cover) ÷ spacing) + 1
sticks per run = 1 if run ≤ stock, else 1 + ceil((run − stock) ÷ (stock − lap))
splices per run = sticks per run − 1
pieces to buy = runs × sticks per run, summed over both directions
Subtracting 2 × cover pulls the outermost bars in from the slab edges — 3 inches by default, the standard clearance for concrete cast against the ground, so the steel can’t wick moisture and rust. Dividing the remaining span by the spacing gives the number of gaps between bars; rounding down and adding 1 turns gaps into bars. Rounding down is deliberate: forcing in an extra bar would shrink your cover, while a slightly wider last gap hurts nothing.
Splices happen whenever a run is longer than the sticks you buy. Bars can’t simply butt together — they overlap, 12 inches by default, and every lap is extra steel, so each extra stick only adds its length minus one lap of new reach. Sticks are counted run by run, not by pooling the linear feet: a 10-foot run still consumes a whole 20-foot bar, because the 10-foot offcut is too short to serve as any other full run. Pooled division looks cheaper on paper but leaves you short at the site.
Worked example: a 24 × 12 ft driveway slab
Say you’re pouring a 24 × 12 ft driveway on a 12-inch grid with 3 inches of edge cover, buying 20-foot sticks and lapping splices 12 inches. That’s exactly what the calculator loads by default, so you can follow along in the result box above.
- Bars running lengthwise get spaced across the 12 ft width: 144 in − 6 in of cover = 138. 138 ÷ 12 = 11.5, rounded down to 11 gaps, + 1 = 12 bars, each 24 ft long.
- Bars running widthwise get spaced along the 24 ft length: 288 − 6 = 282. 282 ÷ 12 = 23.5 → 23 gaps, + 1 = 24 bars, each 12 ft long.
- Total bars: 12 + 24 = 36.
- Base steel: 12 × 24 ft + 24 × 12 ft = 576 linear feet.
- Splices: the 24 ft runs outreach a 20 ft stick, so each takes a second, lapped bar — 1 splice per run. 12 runs × 1 lap × 12 in = 12 ft of splice allowance. The 12 ft runs need none. Total: 588 linear feet.
- Sticks to buy: counted run by run. Each 24 ft run needs 1 + ceil((24 − 20) ÷ (20 − 1)) = 2 sticks; each 12 ft run fits in one. 12 × 2 + 24 × 1 = 48 twenty-foot bars.
So the order is 48 sticks of #4, a roll of tie wire, and a bag of chairs. Yes, that’s more steel than the 588 linear feet alone suggests — the second stick in each 24 ft run only gives up 5 feet, and its 15-foot offcut can’t replace a full 24- or 12-foot run. The offcuts become form stakes and dowels, or ask the yard to shear them down.
How much rebar do I need for a 10×10 to 24×24 slab?
The table below covers common square slabs with 3 inches of edge cover. Linear feet are the bare grid — the calculator adds lap-splice allowance whenever a run outruns your stock length.
| Slab size | Total bars @ 12″ grid | Linear ft @ 12″ grid | Linear ft @ 18″ grid |
|---|---|---|---|
| 10 × 10 ft | 20 | 200 | 140 |
| 12 × 12 ft | 24 | 288 | 192 |
| 14 × 14 ft | 28 | 392 | 280 |
| 16 × 16 ft | 32 | 512 | 352 |
| 18 × 18 ft | 36 | 648 | 432 |
| 20 × 20 ft | 40 | 800 | 560 |
| 24 × 24 ft | 48 | 1152 | 768 |
Steel scales fast: doubling a slab’s sides roughly quadruples the linear feet. On a 24 × 24 pour, widening the grid from 12 to 18 inches saves 384 feet of bar — price both and let the load make the call.
Frequently asked questions
Should I use rebar or wire mesh in my slab?
For anything that carries vehicles — driveways, garage slabs, RV pads — use rebar. Welded wire mesh is fine for 4-inch patios and walkways, but only if it actually sits mid-slab; in practice mesh gets walked down to the dirt during the pour, where it does nothing. Rebar on chairs stays put. If you’re already renting a mixer and building forms, the price difference on a small slab is minor.
What rebar spacing should I use for a driveway vs a patio?
Driveways and garage slabs are typically reinforced with #4 bar on a 12-inch grid — that’s the calculator’s default. A patio or shed slab that only sees foot traffic can stretch to an 18- or 24-inch grid, or skip to mesh. Tighten the grid rather than loosen it if your subgrade is soft, poorly compacted, or expansive clay; the steel is what holds a cracked slab together.
How many rebar chairs or supports do I need?
Plan on a chair every 3 to 4 feet along each bar so the grid sits in the middle of the slab, not on the dirt. A quick estimate is your total linear feet divided by 4 — about 150 chairs for the 588-foot driveway example. They’re cheap and sold in bags, so round up to a full bag. Pulling the grid up with a rake hook mid-pour never works as well as people hope.
What’s the difference between #3 and #4 rebar?
The number is the bar diameter in eighths of an inch: #3 is 3/8″, #4 is 1/2″. #3 suits sidewalks, patios, and crack control in light slabs; #4 is the workhorse for driveways, garage floors, and footings. The bar size doesn’t change the grid math — counts and linear feet stay the same — but it changes weight and price, and a thicker bar needs a longer lap at splices.
How much overlap does a rebar splice need?
The common rule for structural work is 40 bar diameters: 15 inches for #3 and 20 inches for #4. This calculator defaults to a 12-inch lap, a widely used minimum for residential flatwork — open the advanced options and raise it if your plans or inspector call for more. Tie every lap in at least two places with wire so the bars don’t drift apart during the pour.
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.