Rebar Calculator
Find the reinforcement grid, linear feet, weight, and number of 20-ft bars by slab size, spacing, and bar size.
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Your Rebar Estimate
- Enter slab size - the length and width of the area to reinforce, in feet.
- Choose spacing - the on-center distance between parallel bars; 18 in is common residential, 12-16 in for driveways.
- Set edge clearance - bars stop 2-3 in short of the slab edge so concrete fully covers the steel; 3 in is the common field value.
- Pick bar size - sets the weight per foot - #4 is the residential default.
- Set waste / overlap - 10% covers lap splices and cutting; use 15% for complex layouts.
- Buy by the 20-ft bar - standard bars are 20 ft; the result rounds linear feet up to whole bars.
For a standard concrete slab, lay #4 rebar in an 18-inch grid both directions, kept about 3 inches back from each edge. Count the bars each way - (width / spacing) + 1, and the same for length, total the linear feet, and round up to whole 20-foot bars. A 10 x 10 ft slab at 18-inch spacing needs about 8 bars (roughly 150 linear feet); a 20 x 20 ft slab needs about 31 bars. Enter your slab size above for the exact grid, weight, and bar count.
How the Rebar Grid Is Laid Out
Rebar Sizes and Weight per Foot
Rebar is designated by bar number - each number is ⅛ inch of diameter, so a #4 bar is ½ inch. The common residential and light-commercial sizes:
| Bar Size | Diameter | Weight per Foot | Common Use |
|---|---|---|---|
| #3 | 3/8" (10mm) | 0.376 lb/ft | Temperature steel, light slabs |
| #4 | 1/2" (13mm) | 0.668 lb/ft | Residential slabs, footings, walls |
| #5 | 5/8" (16mm) | 1.043 lb/ft | Driveways, garage floors, retaining walls |
| #6 | 3/4" (19mm) | 1.502 lb/ft | Structural footings, grade beams |
| #7 | 7/8" (22mm) | 2.044 lb/ft | Heavy structural, commercial |
| #8 | 1" (25mm) | 2.670 lb/ft | Columns, heavy foundations |
Standard bars are 20 feet. For runs over 20 feet, bars are lapped (overlapped) so the force transfers from one bar to the next. The waste factor covers the extra length these laps require - see the lap splice section below for the lengths.
Choosing the Right Spacing
Spacing is measured on center - from the middle of one bar to the middle of the next. Closer bars mean more steel and higher load capacity. For most residential slabs, an 18-inch grid in both directions with #4 rebar is standard.
Driveways and garage floors that carry vehicle loads usually call for 12 to 16-inch spacing with #4 or #5 bars. Structural footings and retaining walls are engineer-specified and may use closer spacing with larger bars. If you are unsure, follow your project plans or ask your building inspector.
Rebar Spacing by Slab Thickness
A common question is how rebar spacing changes with slab thickness. For crack-control (temperature and shrinkage) steel, the practical maximum is 18 inches regardless of thickness - so for everyday slabs from 4 to 8 inches, an 18-inch grid is the wide end and 12 to 16 inches is used where loads are higher:
| Slab Thickness | Typical Spacing | Common Bar | Use |
|---|---|---|---|
| 4 in | 16-18 in | #3 or #4 | Patios, sidewalks, sheds |
| 5 in | 16-18 in | #4 | Standard residential slabs |
| 6 in | 12-18 in | #4 or #5 | Driveways, garage floors |
| 8 in | 12-16 in | #5 | Heavy vehicle, light structural |
Rebar by Slab Size (#4 bar, 18-in grid, 10% waste)
| Slab | Grid | Linear Feet | 20-ft Bars | Weight |
|---|---|---|---|---|
| 10 x 10 ft | 7 x 7 | ~155 ft | 8 | ~103 lb |
| 20 x 20 ft | 14 x 14 | ~615 ft | 31 | ~411 lb |
| 24 x 24 ft | 17 x 17 | ~900 ft | 45 | ~600 lb |
| 30 x 40 ft | 21 x 27 | ~1,815 ft | 91 | ~1,212 lb |
Switch to #5 bar and multiply weight by about 1.56; tighten to 12-inch spacing and the bar count rises by roughly half.
Rebar Formulas
The calculator builds a grid both directions:
Bars lengthwise = (Grid width ÷ spacing) + 1
Bars widthwise = (Grid length ÷ spacing) + 1
Total linear ft = (Bars lengthwise × grid length) + (Bars widthwise × grid width)
With laps/waste = Total × (1 + waste %)
20-ft bars = Total linear ft ÷ 20, rounded up
Weight = Total linear ft × weight per foot for the bar size
Place slab-on-grade rebar in the lower third of the slab on chairs, about 2 inches up. Stagger lap splices so they do not all land at the same point.
Lap Splice Length: How Far Bars Overlap
When a run is longer than a 20-foot bar, two bars are lapped end to end so the load transfers through the concrete. The field rule of thumb is 40 times the bar diameter - about 20 inches for #4, 25 inches for #5. That is the quick number most residential crews use.
The actual code value (ACI 318, Grade 60 bar in 3,000 PSI concrete, standard conditions) runs a little longer: about 16 inches for #3, 24 inches for #4, 30 inches for #5, and 36 inches for #6. When in doubt, use the larger of the two. Stagger splices so they do not all land at the same line, and wire-tie each lap. The waste factor in the calculator covers this extra length.
Finish the Reinforced Pour
Rebar is one layer of the job. The usual order from here:
Code & Source Notes
- Maximum spacing for crack-control (temperature and shrinkage) reinforcement in slabs-on-ground is the lesser of 5 times the slab thickness or 18 inches, per ACI 318. Bar sizes and weights per foot follow ASTM A615.ACI 318 - Building Code Requirements for Structural Concrete
- Lap splice lengths shown are the ACI 318 Class B common-condition values (Grade 60, 3,000 PSI). Actual required length varies with concrete strength, cover, and bar spacing - follow your project drawings or a structural engineer for load-bearing work.ACI 318 - Section 25.5.2.1, Lap Splices
Related Calculators
Rebar grid spacing depends on slab size and load - size the pour itself in the construction calculator collection.