Figure out how many rebar bars your slab needs, plus total length, weight, and material cost. Enter dimensions, spacing, and bar size.
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Concrete is strong in compression and weak in tension. Rebar fixes that. A 20 ft by 10 ft driveway slab at 4 inches thick, reinforced with #4 bars at 12 inches on center both ways, needs roughly 650 linear feet of steel. Miss the spacing calculation and you either over-order (money wasted) or under-order (a second trip to the supplier and a cold joint while you wait).
This tool handles the grid math. It counts bars in each direction based on your spacing, adds the lap splice allowance your code or spec requires, applies a waste factor for cuts, and multiplies through to weight and cost. The rebar weight per foot values come from standard ASTM A615 tables: #3 at 0.376 lb/ft, #4 at 0.668 lb/ft, #5 at 1.043 lb/ft, and so on up through #10 at 4.303 lb/ft.
The output is an estimate. It assumes a rectangular slab, uniform spacing, and the standard 20-foot mill length for counting bars needed. Field conditions will vary. Use the number as a planning figure, not a substitute for a bar list from your detailer.
Select your currency from the dropdown at the top if you want the cost estimate in something other than US dollars.
Enter the slab length and pick the unit: feet, inches, meters, centimeters, or millimeters.
Enter the slab width and its unit.
Enter the slab thickness and its unit. Thickness doesn’t affect bar count or length in this calculator, but it’s recorded for the breakdown.
Set the main rebar spacing — the distance between bars running in one direction. Choose inches, centimeters, millimeters, or feet.
Set the cross rebar spacing for the perpendicular direction.
Pick a bar size from #3 (3/8 inch) to #10 (1-1/4 inch).
Enter the price per unit length. Switch between per-foot and per-meter pricing with the dropdown.
Add a lap splice percentage if your bars will be overlapped. Ten percent is common for slab work where bars are spliced.
Set the waste factor. Ten percent is the default and covers cuts and offcuts.
Enter any accessories cost. Choose whether that number is a total, a per-square-foot rate, or a per-square-meter rate.
Hit Calculate. The Summary tab shows total rebar, bar counts, lengths, weight, and cost. The Detailed Breakdown tab shows every intermediate value.
The tool converts every dimension to feet internally, then works in feet and pounds before converting weight to kilograms at the end.
Formula: Main Bars = ceil(Slab Width (ft) ÷ Main Spacing (ft)) + 1
Formula: Cross Bars = ceil(Slab Length (ft) ÷ Cross Spacing (ft)) + 1
Formula: Main Bar Length = Slab Length (ft) × (1 + Lap Splice % ÷ 100)
Formula: Total Length = (Main Bars × Main Bar Length) + (Cross Bars × Cross Bar Length)
Formula: Total Length with Waste = Total Length × (1 + Waste % ÷ 100)
Formula: Weight (lbs) = Total Length with Waste × Rebar lb/ft
Formula: Rebar Cost = Total Length with Waste × Price per ft
Formula: Total Cost = Rebar Cost + Accessories Cost
The ceil function rounds up to the nearest whole bar. If your slab width is 10 feet and spacing is 12 inches (1 foot), you get ceil(10 ÷ 1) + 1 = 11 bars. The +1 accounts for the bar at the starting edge. The calculator enforces a minimum of 2 bars in each direction, which prevents nonsensical output on very small slabs.
Bar length starts as the slab dimension. The lap splice percentage inflates that length. A 20-foot slab with a 10% lap allowance gives each main bar a length of 22 feet. That’s conservative, it assumes every bar is spliced, which isn’t always the case but it keeps the estimate on the safe side.
Weight comes from the standard lb/ft figure for the selected bar size. The values match ASTM A615 nominal weights. If you’re working with metric bar designations (10M, 15M, etc.), use the closest imperial size.
Accessories can be entered three ways. A flat total adds straight to the cost. A per-square-foot rate multiplies by the slab area in square feet. A per-square-meter rate multiplies by the slab area converted to square meters.
Say you’re pouring a 24 ft by 14 ft garage slab. You’ll use #5 bars at 16 inches on center in both directions, with a 10% lap splice allowance and a 10% waste factor. Your supplier charges $1.05 per foot for #5 bar. You’re budgeting $180 for chairs and ties.
Step 1: Convert spacing to feet.
16 inches ÷ 12 = 1.333 ft.
Step 2: Count main bars (running along the length, spaced across the width).
ceil(14 ÷ 1.333) + 1 = ceil(10.5) + 1 = 11 + 1 = 12 bars.
Step 3: Count cross bars (running along the width, spaced across the length).
ceil(24 ÷ 1.333) + 1 = ceil(18.0) + 1 = 18 + 1 = 19 bars.
Step 4: Calculate bar lengths with lap splice.
Main bar length: 24 ft × 1.10 = 26.4 ft.
Cross bar length: 14 ft × 1.10 = 15.4 ft.
Step 5: Total rebar length before waste.
(12 × 26.4) + (19 × 15.4) = 316.8 + 292.6 = 609.4 ft.
Step 6: Add waste.
609.4 × 1.10 = 670.34 ft.
Step 7: Calculate weight.
#5 bar weighs 1.043 lb/ft. 670.34 × 1.043 = 699.16 lbs.
In kilograms: 699.16 × 0.453592 = 317.15 kg.
Step 8: Count standard 20-foot bars.
ceil(670.34 ÷ 20) = ceil(33.5) = 34 bars.
Step 9: Rebar cost.
670.34 × $1.05 = $703.86.
Step 10: Total cost with accessories.
$703.86 + $180 = $883.86.
The summary tab shows 670 ft of total rebar, 699 lbs, 34 standard bars, and $883.86 total cost. The breakdown tab shows every number, including the slab area (336 ft²) and the per-direction bar counts.
Divide the slab width by your main spacing and the slab length by your cross spacing. Round each result up to a whole number and add one to each count. Multiply the main bar count by the slab length and the cross bar count by the slab width. Add both totals. That’s your total linear feet before lap and waste allowances.
ACI 318 sets the minimum clear spacing between parallel bars at the greater of 1 inch or the bar diameter. For a #4 bar (0.5 inch diameter), that means at least 1 inch of clear space between bars. In practice, slab spacing rarely goes below 6 inches because tighter grids are hard to place and consolidate around. Most residential slabs sit at 12 inches.
A 20 ft by 20 ft slab at 12 inches on center both ways needs 21 bars in each direction, or 42 bars total. At 20 feet per bar, that’s 840 linear feet before lap and waste. With #4 bar at 0.668 lb/ft, the weight comes to about 561 pounds before allowances. Add 10% waste and you’re near 617 pounds.
No. Bar count depends on spacing, not size. A #3 bar at 12 inches on center gives the same bar count as a #6 bar at 12 inches. Bar size changes the weight and the cost, and it changes the structural capacity of the reinforcement, but the grid count stays the same. Choose bar size based on the load the slab needs to carry.
Multiply total rebar length in feet by the lb/ft value for your bar size, then multiply by 0.453592 to convert pounds to kilograms. A #5 bar at 500 feet: 500 × 1.043 = 521.5 lbs. 521.5 × 0.453592 = 236.5 kg. The calculator does this conversion and shows both units in the summary.
Ten percent is a safe default for slab work. Cuts at edges, mistakes, and small offcuts add up. If you’re ordering exact lengths and the supplier cuts to size, you can drop it to 5%. If you’re cutting everything on site with a chop saw, 10 to 15% is realistic. The calculator lets you enter any percentage from zero upward.
Yes. Driveways are rectangular slabs, and the grid math is the same. Measure the driveway length and width, pick your spacing (12 inches is common for driveways), choose a bar size (#4 is typical), and run the numbers. For driveways with curves or irregular shapes, break the area into rectangles and calculate each one separately.
It increases the length of every bar to account for the overlap where two bars are spliced. A 10% lap on a 20-foot bar adds 2 feet, giving a 22-foot effective length. The percentage applies to both main and cross bars. If your bars aren’t spliced, for example, if you’re using 60-foot mill lengths and the slab is shorter than that set the lap splice to zero.
It’s a close estimate, not a bar list. The bar counts are exact for a perfectly rectangular slab with uniform spacing. The length and weight are approximate because real placements have cuts, bends, and splices that the grid model doesn’t capture. Treat the output as a planning number for budgeting and material ordering. For construction, use the bar list from your structural drawings.
Yes. There’s no cost, no sign-up, and no limit on how many times you use it. The calculations run in your browser. You can copy the results or share them using the buttons below the output.
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