Rebar Calculator
This rebar calculator tells you how much rebar a slab needs. Enter the length and width, set the bar spacing, edge cover, and bar size, and you'll get the bars each way, the total linear feet, the tie points, and the weight. Everything updates as you type, so it's easy to plan a slab, footing, or driveway. It's free and runs in your browser, so pricing a concrete pour takes seconds.
- Bars for a slab
- Linear feet
- Tie points
- Weight
- Cost estimate
Last updated June 17, 2026 Feet, grid layout Reviewed by the Calcowa team
Enter a slab size and spacing above zero.
Bars run both directions on a grid, set in from each edge by the cover. Spacing is on center, often 12 to 18 inches. Leave the price blank to skip the cost line. Check your plan or local code for the real spec.
How do you calculate how much rebar you need?
Rebar in a slab sits in a grid, so the job is to count the bars running each way, and it's not as fiddly as it sounds. Start with the slab size, then pull the bars in from each edge by the cover, usually about 3 inches, so the concrete fully wraps the steel. The bars running one direction are spaced across the other dimension, so you take the usable width, divide by the spacing, and add one for the bar at the far edge. Do the same the other way, and you've got both counts. Don't forget the laps, since bars come in fixed lengths and longer runs overlap, which is why there's a waste margin. Add up the lengths for the linear feet, multiply by the per-foot weight of your bar size, and you've got the order. This tool runs every step, so you'll get the bars, feet, ties, and weight the moment you type the size.
Estimating a slab, step by step
Here's the quick routine before you order, and it's just three steps:
- 1
Enter the slabType the length and width of the pour in feet.
- 2
Set the gridChoose the spacing, edge cover, and bar size.
- 3
Read the rebarSee the bars, linear feet, ties, and weight you'll need.
Rebar by slab size
Here's a rough guide at 12 inch spacing, 3 inch cover, before laps. It's a starting point, so don't treat it as exact, since your spacing and cover won't match every plan. When you're ready, run your own numbers above.
| Slab | Total bars | Linear feet |
|---|---|---|
| 10 x 10 ft | 20 | 190 |
| 12 x 12 ft | 24 | 276 |
| 20 x 10 ft | 30 | 385 |
| 24 x 24 ft | 48 | 1,128 |
Frequently asked questions
You enter the slab size, the bar spacing, and the bar size, and it lays out a grid of rebar for you. It counts the bars running each way, adds an edge cover so the steel sits inside the concrete, then totals the linear feet, the tie points where bars cross, and the weight. Everything runs in your browser, so you'll see the numbers change as you adjust the spacing, and you can copy the result to take to the yard.
Lay out a grid: count the bars in one direction, count them in the other, and add the lengths. Bars are spaced a set distance on center, often 12 to 18 inches, and they start a few inches in from each edge so the concrete covers them. A 20 by 10 foot slab at 12 inch spacing needs about 30 bars and 385 linear feet of steel, and this tool does that math the moment you type the size.
It depends on the load, but residential slabs and footings commonly run 12 to 18 inches on center, with driveways and patios often at 16 inches. Tighter spacing puts more steel in the slab and handles heavier loads, while wider spacing uses less. Your engineer or local code has the final say, so don't guess on a structural pour. The calculator lets you set any spacing, so you can match the plan exactly.
Weight depends on the bar size. A #4 bar, which is half an inch across, weighs about 0.668 pounds per foot, a #5 weighs 1.043, and a #3 weighs 0.376. The tool multiplies your total linear feet by the weight of the size you pick, so you'll know the load before you order. That matters because steel is often priced and hauled by weight, and a big slab adds up fast.
Yes, because bars only come in set lengths, usually 20 feet, so longer runs have to overlap, and that lap wastes some steel. A common lap is about 40 times the bar diameter, and the waste margin here lets you add a percentage for laps and offcuts. Bump it up for a large slab with many splices, and leave it lower for a small pad where most bars run full length without a single lap.
Yes, it's completely free, with no sign-up, and it runs right in your browser, so nothing you enter leaves your device. Type the slab size, set the spacing and bar size, and read the bars, feet, and weight in a tap. Bookmark it for a slab, a footing, or a driveway, and you'll have a solid rebar estimate whenever you plan a concrete pour.
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Pouring a slab?
Work out the rebar above, then size the concrete with our concrete tool.