Rebar Calculator
Estimate rebar quantity, total weight, and cost by slab dimensions and bar spacing.
Your Rebar Estimate
- Enter slab size — length and width of the area to be reinforced.
- Choose bar spacing — use presets for common spacing or enter a custom value. Spacing = distance between parallel bars.
- Density is fixed — steel density is standard at 490 lb/ft³.
- Set waste / overlap — 10% covers lap splices and cutting waste. Use 15% for complex layouts.
- Review results — total pieces, linear footage, and weight. Standard rebar comes in 20 ft lengths.
Rebar Sizes and Weight per Foot
Rebar is designated by bar number — each number equals ⅛ inch of diameter. A #4 bar is 4/8" (½") in diameter. Here are the most common sizes used in residential and light commercial work:
| 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 rebar lengths are 20 feet. For runs longer than 20 feet, bars must be overlapped (lapped) — the minimum lap splice is 40 times the bar diameter (e.g., 20 inches for #4 rebar).
Choosing the Right Spacing
Rebar spacing is the center-to-center distance between parallel bars. Tighter spacing means more steel and higher load capacity. For most residential slabs, 18-inch spacing on center in both directions (a grid pattern) with #4 rebar is the standard.
Driveways and garage floors that support vehicle loads typically 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. When in doubt, check your project plans or ask your building inspector.
Formulas Used in This Calculator
The calculator creates a grid of rebar in both directions based on your spacing:
Bars widthwise = (Length ÷ Spacing) + 1
Total pieces = Bars lengthwise + Bars widthwise
Total linear ft = (Bars lengthwise × Length) + (Bars widthwise × Width)
With waste/overlap = Total × (1 + Waste %)
Weight is calculated using the selected bar size's weight per foot multiplied by total linear footage. Standard rebar comes in 20 ft lengths — the calculator assumes full-length bars.
Related Calculators
Rebar grid spacing depends on slab size and load. Browse 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 (patio, sidewalk), #3 or #4 rebar at 18-inch spacing is typical. For driveways and loaded slabs, use #4 at 12 to 16 inch spacing. Alternatively, welded wire mesh (6×6 W1.4/W1.4) can replace rebar in light-duty slabs.
How much overlap do rebar splices need?
The standard lap splice length is 40 times the bar diameter. For #4 rebar (½" diameter), that means a minimum 20-inch overlap. For #5 (⅝"), it's 25 inches. Always wire-tie the splice securely and stagger splices so they don't all occur at the same point in the slab.
Should rebar be placed at the top or bottom of a slab?
For slabs on grade, rebar is typically placed in the lower third of the slab thickness — about 2 inches from the bottom. Use rebar chairs or dobies to hold the bars at the correct height during the pour. The goal is to reinforce the tension zone where cracking starts.
What size rebar do I need for a concrete slab?
For a standard 4-inch residential slab, #3 rebar (3/8 inch) on 18-inch centers is common. For garage floors and driveways, #4 rebar (1/2 inch) on 12–18 inch centers provides better crack resistance. Footings typically use two runs of #4 or #5 rebar. Always check local building codes — seismic zones often require larger rebar and tighter spacing.
How much does rebar weigh per foot?
#3 rebar weighs 0.376 lbs/ft. #4 weighs 0.668 lbs/ft. #5 weighs 1.043 lbs/ft. #6 weighs 1.502 lbs/ft. Weight matters for delivery — a bundle of 40-foot #5 bars (typical bundle of 18) weighs about 750 lbs. The calculator above includes weight output so you can plan delivery logistics accurately.