Rebar Calculator
Calculate rebar quantity, linear feet, and weight for concrete slabs and footings. See also Concrete Calculator, Concrete Block Calculator, and Concrete Column Calculator.
How to Use the Rebar Calculator
Enter the slab dimensions in feet, select the rebar bar size, and specify the spacing in inches on center (OC). The calculator determines how many bars run in each direction for a grid pattern, the total linear feet of rebar needed, the total weight, and the number of lap splices required if bars exceed the standard 20-foot length. Results assume a single layer grid with bars running both directions.
Formula
Bars (lengthwise) = (Width x 12 / Spacing) + 1
Bars (widthwise) = (Length x 12 / Spacing) + 1
Linear Feet = (Bars lengthwise x Length) + (Bars widthwise x Width)
Weight = Linear Feet x Weight per Foot
Lap Splice Length = 40 x Bar Diameter
Example Calculation
Slab: 20 ft x 20 ft, #4 rebar at 12" OC
Bars lengthwise = (20 x 12 / 12) + 1 = 21 bars
Bars widthwise = (20 x 12 / 12) + 1 = 21 bars
Total bars = 42
Linear feet = (21 x 20) + (21 x 20) = 840 ft
Weight = 840 x 0.668 = 561 lbs
Rebar Size Reference Table
| Bar Size | Diameter (in) | Weight (lb/ft) | Area (in²) | Common Use |
|---|---|---|---|---|
| #3 | 0.375 | 0.376 | 0.11 | Stirrups, ties, light slabs |
| #4 | 0.500 | 0.668 | 0.20 | Slabs, driveways, patios |
| #5 | 0.625 | 1.043 | 0.31 | Footings, walls, columns |
| #6 | 0.750 | 1.502 | 0.44 | Beams, heavy footings |
| #7 | 0.875 | 2.044 | 0.60 | Columns, retaining walls |
| #8 | 1.000 | 2.670 | 0.79 | Heavy structural, bridges |
Frequently Asked Questions
What size rebar do I need for a concrete slab?
#4 rebar (1/2 inch) at 12-18 inch spacing is standard for residential slabs 4-6 inches thick. For driveways and garage floors, use #4 at 12 inches OC. For patios and sidewalks, #3 at 18 inches or welded wire mesh (6x6 W1.4) is sufficient.
How much overlap is needed for rebar splices?
The minimum lap splice length is 40 times the bar diameter. For #4 rebar: 40 x 0.5 = 20 inches. For #5: 40 x 0.625 = 25 inches. For #6: 40 x 0.75 = 30 inches. Tie splices with wire at three points minimum.
Where should rebar be placed in a slab?
Rebar should be positioned in the lower third of the slab for ground-supported slabs (about 2 inches from the bottom). Use rebar chairs or dobies to maintain proper cover. For elevated slabs, rebar goes in the bottom for positive moment and top for negative moment areas.
Can I use wire mesh instead of rebar?
Welded wire mesh (WWM) is acceptable for lightly loaded slabs like sidewalks and patios. Standard 6x6 W1.4/W1.4 mesh provides minimal reinforcement. For driveways, structural slabs, or any slab over 5 inches thick, rebar is recommended for superior crack control and structural capacity.
How do I calculate rebar weight for ordering?
Multiply total linear feet by the weight per foot for your bar size. Add 10% for waste (cutting, overlaps, chairs). Standard 20-foot bars: divide total linear feet by 20 to get the number of bars to order. Rebar is sold by the ton for large orders or by the piece at hardware stores.
What is the minimum concrete cover for rebar?
Minimum cover (distance from rebar to concrete surface): 3 inches for concrete cast against earth, 2 inches for exposed exterior surfaces, 1.5 inches for interior slabs, and 0.75 inches for precast elements. Proper cover prevents corrosion and ensures bond strength.