Rebar Calculator
Rebar in a slab goes down in a grid, and the count depends on your spacing and edge clearance, not just the slab size. Enter your slab's length and width and your grid spacing below. The calculator returns how many bars run each direction, the total linear feet to order, and a cost estimate, so you can buy the right number of sticks the first time.
How much rebar you need for a slab
Lay out a slab grid wrong and every bar is off, which means a second trip or a short order at the worst time. This tool takes your slab size and grid spacing and gives the bar count each way, the total linear feet, how many sticks to buy, and the lap to add when the slab runs longer than a single bar. Get the layout right before the bolt cutters come out.
The grid math
Rebar in a slab runs as a grid. The number of bars one direction is the slab length minus your edge clearance on both sides, divided by the spacing, plus one for the last bar. Run the same the other way, multiply each count by the length it spans, and add the two for total linear feet. Edge clearance is usually about 3 inches so the steel stays inside the concrete.
Bar size by job
| Bar | Diameter | Typical use |
|---|---|---|
| #3 | 3/8 in | Patios, sidewalks, light slabs |
| #4 | 1/2 in | Driveways, walls, footings |
| #5 | 5/8 in | Foundations, heavy slabs, columns |
Most flatwork runs the bars 12 to 18 inches on center. Tighter spacing and a bigger bar carry more load, but follow your plan or local code on anything structural rather than a rule of thumb.
Laps and waste
Rebar comes in sticks, commonly 20 feet, sometimes 10, 30, or 40. When your slab is longer than a stick, you overlap two bars and tie them, and that lap is roughly 40 times the bar diameter, about 20 inches for a #4 and 25 for a #5. Every splice eats that much extra steel, so add the laps plus about 10 percent for waste and the odd bad cut.
Weight and sticks
Yards often price rebar by weight, so it helps to know it. A #3 bar weighs about 0.376 pound per foot, a #4 about 0.668, and a #5 about 1.043. Multiply by your total linear feet for the order weight. The tool also rounds your footage up to whole sticks so you know how many to load.
A real slab example
A 12 by 12 foot slab with #4 bar at 16 inches on center and 3 inches of edge clearance: each direction is 144 minus 6 inches, divided by 16, plus one, which is about 10 bars. Ten bars at 12 feet is 120 linear feet each way, 240 total. Add laps and 10 percent waste and you're near 265 feet, about 14 of the 20-foot sticks, roughly 160 pounds of steel.
Do you even need rebar
Not every slab does. A simple sidewalk or small patio often does fine with welded wire mesh or fiber-reinforced concrete. Driveways, footings, and anything carrying real load want rebar on a grid. When the slab is structural, size and place the steel to your engineer or local code, not a calculator estimate.
Rebar sizes and spacing, decoded
Bar numbers are eighths of an inch: #3 is 3/8, #4 is 1/2, #5 is 5/8. Driveways and patios typically run #3 or #4 on 12-to-18-inch centers; footings and walls specify heavier bars per the plan. Grade 60 is the standard strength. Chairs or dobies every few feet keep the grid in the middle third of the slab, which is the whole point; rebar lying on the ground does nothing.
| Bar | Diameter | Weight per 20-ft stick | Common use |
|---|---|---|---|
| #3 | 3/8 in | 7.5 lb | Patios, walks |
| #4 | 1/2 in | 13.4 lb | Driveways, slabs |
| #5 | 5/8 in | 20.9 lb | Footings, walls |
Quick answers
Rebar or wire mesh for a driveway? Rebar for anything carrying vehicles; mesh handles shrinkage cracks in light-duty slabs. The cost gap on a typical driveway is a few hundred dollars for a much stronger slab.
How much does rebar cost? Roughly $8 to $15 per 20-foot stick of #4 at retail, less by the ton. Figure the stick count above and price both suppliers.
How much overlap where bars meet? Forty bar diameters is the common rule: 20 inches for #4. Tie with wire; welding is a no unless the bar is spec'd weldable.
Common questions
It depends on slab size and grid spacing. Enter both above and the calculator gives bar counts each way and total linear feet. Tighter spacing means more steel; 12 to 18 inches is common for residential slabs.
Twelve to eighteen inches on center covers most residential slabs and footings, but follow your engineer or local code for anything structural. The tool lets you set whatever spacing your plan calls for.
Commonly 20 feet, though 10 and 40 foot lengths exist. The calculator converts your total linear feet into stick count so ordering is simple.
For driveways, footings, and anything bearing load, yes, it controls cracking and adds tensile strength. Thin decorative slabs sometimes use wire mesh instead. Check your local requirements.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.