Rebar Calculator
Find the reinforcement grid, linear feet, weight, and number of 20-ft bars by slab size, spacing, and bar size.
Use the spacing from the project design. The calculator rounds the number of spaces up so the remaining edge bay never exceeds the selected maximum.
Clear distance from the concrete edge to the outside surface of the bar. The calculator adds half the selected bar diameter to locate each outer bar centerline.
How to Use
- 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 clear cover - enter the clear concrete cover required by the project detail. Cover is measured to the bar surface; the calculator accounts for bar radius when laying out the centerlines.
- 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 slab-grid takeoff, use the bar size, clear concrete cover, and maximum on-center spacing from the project design. Divide each clear grid dimension by the maximum spacing, round the number of spaces up, then add one bar for the opposite edge. A 10 x 10 ft slab with 3-inch clear cover and #4 bar at an 18-inch maximum spacing produces an 8 x 8 grid; with 10% laps/waste it is about 166 linear feet, or 9 twenty-foot 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. Size the pour itself with the concrete slab calculator.
Rebar Spacing by Slab Thickness
Rebar spacing by slab thickness: a 4-inch slab uses a 16 to 18-inch grid of #3 or #4 bar, a 6-inch slab a 12 to 18-inch grid of #4 or #5, and an 8-inch slab a 12 to 16-inch grid of #5. The 18-inch figure is the practical maximum for crack-control steel regardless of thickness; tighter spacing 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 |
Tighter spacing adds load capacity. The 18-inch ceiling comes from the ACI maximum for crack-control steel (the lesser of 5 times the slab thickness or 18 inches). Structural and footing slabs are engineer-specified.
Rebar by Slab Size (#4 bar, 18-in grid, 10% waste)
| Slab | Grid | Linear Feet | 20-ft Bars | Weight |
|---|---|---|---|---|
| 10 x 10 ft | 8 x 8 | ~166 ft | 9 | ~111 lb |
| 20 x 20 ft | 14 x 14 | ~599 ft | 30 | ~400 lb |
| 24 x 24 ft | 17 x 17 | ~877 ft | 44 | ~586 lb |
| 30 x 40 ft | 28 x 21 | ~1,819 ft | 91 | ~1,215 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 = Round grid width ÷ maximum spacing up, then add 1
Bars widthwise = Round grid length ÷ maximum spacing up, then add 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.
Frequently Asked Questions
What size rebar do I need for a 4-inch slab?
For a standard 4-inch residential slab like a patio or sidewalk, #3 or #4 rebar on an 18-inch grid is typical. For driveways and loaded slabs, use #4 at 12 to 16-inch spacing. Welded wire mesh (6×6 W1.4/W1.4) is an alternative for light-duty slabs.
How much overlap do rebar splices need?
The field rule of thumb is 40 times the bar diameter - about 20 inches for #4 (½ inch) and 25 inches for #5 (⅝ inch). The ACI 318 code value for common conditions runs a bit longer: roughly 24 inches for #4 and 30 inches for #5. Use the larger number when in doubt, wire-tie each splice, and stagger them so they do not all land at the same line.
Do I need rebar in a 4-inch slab?
For a small, lightly loaded 4-inch slab like a shed floor or sidewalk, welded wire mesh is often enough to control cracking. For anything that carries vehicle or structural load, or any slab you want to last, a #3 or #4 rebar grid is the safer choice - rebar holds the slab together if it cracks, which mesh does less well. Many local codes require rebar for driveways and garage slabs.
How much rebar do I need for a 20x20 slab?
A 20 x 20 ft slab with 3-inch clear cover, #4 bar, an 18-inch maximum grid spacing, and 10% laps/waste works out to a 14 x 14 grid - about 599 linear feet, or 30 standard 20-foot bars, weighing about 400 lb. Use the spacing required by the project design and enter your exact size above to confirm.
Should rebar go at the top or bottom of a slab?
For slabs on grade, place rebar in the lower third - about 2 inches from the bottom - on chairs or dobies to hold it at height during the pour. The goal is to reinforce the tension zone where cracking starts.
How many bars does my grid need?
The grid layout result shows it directly - for example a 20 × 12 ft slab at 18-inch spacing produces about a 9 × 14 grid. The calculator counts bars each direction, totals the linear feet, and rounds up to whole 20-foot bars so you know exactly how many to buy.
What is the minimum spacing of rebar in a slab?
There is no fixed minimum in typical slab-on-grade work - bars can be placed as close as clearance and aggregate size allow, and ACI 318 requires a clear spacing of at least the bar diameter, 1 inch, or 1.33 times the maximum aggregate size, whichever is greatest. In practice, residential crack-control grids run 12 to 18 inches on center; tighter than about 12 inches is only used for structural or heavily loaded slabs, which are engineer-specified.
How close should rebar be to the edge of a slab?
Keep the outer bars about 3 inches back from any exposed edge so the concrete fully wraps the steel and it does not rust - this is the clear cover for concrete cast against the ground under ACI 318. For a slab formed against forms rather than earth, 1.5 to 2 inches is typical. The calculator uses 3 inches of clear cover by default and adds half the selected bar diameter when locating the outer bar centerlines.
What is the spacing for rebar in a 100mm slab?
A 100 mm slab is about 4 inches, so the same guidance applies: a 150 to 200 mm (6 to 8 inch) grid of 10 mm (#3) or 13 mm (#4) bar for crack control, closer where loads are higher. The ACI ceiling for temperature and shrinkage steel is the lesser of 5 times the slab thickness or 450 mm (18 inches), which for a 100 mm slab works out to the 450 mm cap. Set the calculator to metric to size a 100 mm slab directly.
Can I use rebar in a footing instead of a slab?
Yes. Footings typically run two or more continuous bars along their length (often #4 or #5) rather than a full grid. Enter the footing length as the length and a small width to approximate the runs, or use the Concrete Footing Calculator for the concrete volume that goes around it.