Rebar Calculator
Estimate the rebar for a concrete slab or footing: the grid bar count, total linear feet, number of sticks, and weight, from the slab size and spacing.
Reviewed by the ToolNestr Editorial Team — July 2026
How the rebar calculator works
Slab reinforcement is a grid of bars running both directions at a set spacing. The calculator counts the bars in each direction: the slab length divided by the spacing, plus one for the starting bar, gives the number of bars running across the width, and the same math the other way gives the bars running across the length. Each set of bars is as long as the dimension it crosses.
Multiplying the bar counts by their lengths and adding the two directions gives the total linear feet of rebar. Multiplying that by the weight per foot for your bar size gives the total weight. The weights come straight from ASTM A615: a #4 bar is 0.668 pounds per foot, a #5 is 1.043, and so on, and those figures are the same whatever the steel grade.
Real jobs need more bar than the bare grid because bars are spliced with overlaps and trimmed at edges. The lap and waste allowance adds a percentage for that. The calculator also divides the total length by 20 feet, the common stick length, so you can order by the piece.
Bar sizes, weights per foot, and grade designations follow ASTM A615/A615M (Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement); slab reinforcement placement follows ACI 318 (Building Code Requirements for Structural Concrete).
Enter the slab
Length and width in feet, and the grid spacing in inches from your plan.
Pick the bar size
Choose the bar number; the weight per foot is applied for you.
Add laps
Add 10 to 15 percent for splices and offcuts, then order by weight or stick.
The formula explained
Bars each way
A 20 ft × 10 ft slab at 12″ spacing: lengthwise bars = 240⁄12 + 1 = 21, widthwise bars = 120⁄12 + 1 = 11.
Total length
= 21 × 10 + 11 × 20 = 210 + 220 = 430 ft of rebar.
Weight
Using #4 bar (0.668 lb/ft): 430 × 0.668 = 287 lb, or about 22 sticks of 20 ft bar.
Worked example
A 24 ft × 24 ft slab at 16″ spacing with #5 bar and 10% for laps.
Rebar size and weight table
| Bar size | Diameter | Weight (lb/ft) | Typical use |
|---|---|---|---|
| #3 | 3/8" | 0.376 | Patios, light slabs, ties |
| #4 | 1/2" | 0.668 | Driveways, footings, walls |
| #5 | 5/8" | 1.043 | Heavy slabs, foundations |
| #6 | 3/4" | 1.502 | Columns, grade beams |
| #8 | 1" | 2.670 | Heavy structural work |
Real-world context
A detached garage slab is a common job for this calculator. A 24 ft by 24 ft slab reinforced with #3 bar on an 18-inch grid works out to 17 bars in each direction (24 × 12 ÷ 18 + 1 = 17), for a raw grid length of 17 × 24 × 2 = 816 ft. With a 10% lap allowance that comes to about 898 linear feet, weighing roughly 337 lb of #3 bar — about 45 twenty-foot sticks, which is small enough that many suppliers will deliver it bundled rather than by the individual bar.
A backyard patio shows the other end of the scale. A 12 ft by 12 ft slab on a 16-inch grid with #4 bar needs 10 bars each way (12 × 12 ÷ 16 + 1 = 10), for 240 linear feet total. Since every bar in this layout is only 12 ft long — shorter than a standard 20 ft stick — a contractor can cut two bars from each stick with no splice at all, so this is one of the few slab sizes where skipping the lap allowance entirely is reasonable.
Larger foundation slabs push the numbers into a different category of ordering. A 40 ft by 30 ft slab on a tight 12-inch grid with #4 bar needs 41 bars one way and 31 the other, for a raw length of 41 × 30 + 31 × 40 = 2,470 ft. Adding the standard 10% lap allowance brings that to roughly 2,717 linear feet, about 1,815 lb of steel and 136 sticks — enough that ordering by the ton from a rebar supplier, rather than by the individual bar, usually works out cheaper.
Common misconceptions
"Bigger rebar always means a stronger slab." Not necessarily. Going up a bar size increases the steel area, but if the spacing, cover, and concrete strength aren't engineered together, oversized bar in the wrong spot doesn't fix a design that's under-reinforced elsewhere. Bar size, spacing, and slab thickness are meant to be specified as a set by an engineer, not swapped individually.
"Rebar just needs to be somewhere inside the concrete." Position matters as much as quantity. Rebar resists tension, and tension develops on the bottom of a slab under load and the top of a cantilever. Bar placed at the wrong depth — sitting on the ground instead of held up on chairs mid-slab, for instance — can contribute little to nothing to the slab's actual strength even though the total weight of steel matches the calculator's output.
Related calculators
Frequently asked questions
How much rebar do I need for a slab?
Lay out a grid at the required spacing in both directions. The number of bars in one direction is the slab length in inches divided by the spacing, plus one, and each of those bars runs the slab width. Add the two directions together for the total length, then multiply by the weight per foot.
How much does rebar weigh per foot?
By ASTM A615, #3 bar weighs 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, and #8 is 2.670 lb/ft. The weight per foot is the same across Grade 40, 60, and 80 — grade is the steel strength, not the size.
What spacing should rebar be?
Residential slabs commonly use #3 or #4 bar on a 12 to 18 inch grid, but the correct size and spacing come from your engineer and local code based on the load and slab thickness. This tool sizes the material once that spacing is set.
Why add extra for laps?
Bars are spliced by overlapping them, typically 40 bar diameters, wherever one stick ends and the next begins. The waste allowance covers those laps plus offcuts, so add 10 to 15 percent on any slab bigger than a single stick length.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.