Disclaimer: This calculator provides preliminary estimates for educational purposes only. It is not a substitute for a licensed professional engineer, architect, or structural engineer. Results depend on material grade and species, site conditions, local climate and snow loads, and amendments to the applicable building code, all of which vary by location. Always verify results against your local building codes and confirm requirements with your Authority Having Jurisdiction before proceeding. Do not use these estimates for permitting, fabrication, or construction without stamped, engineer-sealed drawings from a licensed professional. Improperly sized structural members, foundations, or gas lines can cause structural collapse, fire, gas leaks, serious injury, or death.
Example Calculations
1Standard 20×10 ft Residential Slab with #4 Rebar
Inputs
Slab Thickness4 inches
Slab Width10 feet
Slab Length20 feet
Rebar Size#4 (0.5" dia)
Spacing12 inches
Concrete Cover1.5 inches
Result
Total Rebar Weight547 lbs
Bars (width dir)21 bars × 10 ft
Bars (length dir)11 bars × 20 ft
Total Linear Feet430 ft
Max Spacing (ACI)12 inches
Steel Ratio0.73%
A 20×10 ft slab at 12" spacing needs 21 bars running the 10 ft direction and 11 bars running the 20 ft direction. Total linear footage is (21×10)+(11×20) = 430 ft. At 0.668 lb/ft for #4 rebar, total weight is about 287 lbs. Steel area per foot = (12/12)×0.20 = 0.20 in². Effective depth d = 4 – 1.5 – 0.25 = 2.25". Steel ratio = 0.20/(12×2.25) = 0.74%.
2Heavy-Duty 12×12 ft Garage Slab with #5 Rebar
Inputs
Slab Thickness6 inches
Slab Width12 feet
Slab Length12 feet
Rebar Size#5 (0.625" dia)
Spacing10 inches
Concrete Cover1.5 inches
Result
Total Rebar Weight380 lbs
Bars (width dir)15 bars × 12 ft
Bars (length dir)15 bars × 12 ft
Total Linear Feet360 ft
Max Spacing (ACI)18 inches
Steel Ratio0.77%
A 12×12 ft garage slab with 6" thickness at 10" spacing uses 15 bars each direction. Linear footage is (15×12)+(15×12) = 360 ft. At 1.043 lb/ft for #5 rebar, weight is about 375 lbs. Steel ratio = 0.31/(10×4.1875) = 0.74%.
3Walkway 4×20 ft with #3 Rebar
Inputs
Slab Thickness4 inches
Slab Width4 feet
Slab Length20 feet
Rebar Size#3 (0.375" dia)
Spacing12 inches
Concrete Cover1.5 inches
Result
Total Rebar Weight63 lbs
Bars (width dir)21 bars × 4 ft
Bars (length dir)5 bars × 20 ft
Total Linear Feet184 ft
Max Spacing (ACI)12 inches
Steel Ratio0.49%
A 4×20 ft walkway at 12" spacing uses 21 short bars (4 ft each) and 5 long bars (20 ft each). Total linear footage is (21×4)+(5×20) = 184 ft. At 0.376 lb/ft, total weight is about 69 lbs.
Frequently Asked Questions
Q
What is the maximum rebar spacing per ACI 318?
ACI 318-19 Section 7.7.2.3 limits rebar spacing to the lesser of 3 times the slab thickness or 18 inches. For a typical 4-inch slab, maximum spacing is 12 inches. For a 6-inch slab, maximum spacing is 18 inches.
Temperature/shrinkage steel: max 5 × thickness or 18 in per ACI 7.7.6.2
Slab Thickness
Max Spacing (3×t)
ACI Cap
Effective Max
4 inches
12"
18"
12"
5 inches
15"
18"
15"
6 inches
18"
18"
18"
8 inches
24"
18"
18"
Q
How do I calculate the number of rebar bars needed?
Divide the slab dimension (in inches) by the spacing, then add 1. For a 20 ft × 10 ft slab at 12-inch spacing: bars in the width direction = floor(20 × 12 / 12) + 1 = 21 bars, each 10 ft long. Repeat for the other direction.
Bars along width = floor(length_in / spacing) + 1
Bars along length = floor(width_in / spacing) + 1
Each bar runs the full perpendicular dimension
Total linear feet = (bars_width × width_ft) + (bars_length × length_ft)
Add 10–15% for lap splices on bars longer than 20 ft
Slab Size
12" Spacing
16" Spacing
18" Spacing
10×10 ft
22 bars
16 bars
14 bars
20×10 ft
32 bars
24 bars
22 bars
20×20 ft
42 bars
32 bars
28 bars
Q
What rebar size should I use for a concrete slab?
#4 rebar (0.5-inch diameter) is the standard choice for residential slabs 4–6 inches thick. Use #3 for light-duty slabs like walkways, and #5 or #6 for structural slabs, driveways, or slabs over 6 inches thick.
Steel ratio (ρ) is the cross-sectional area of steel divided by the concrete area. ACI 318 requires minimum ρ = 0.0018 (0.18%) for temperature/shrinkage reinforcement in slabs. Typical structural slabs use 0.3–0.5% steel ratio.
Formula: ρ = A_bar / (spacing × effective_depth)
Minimum for temp/shrinkage: 0.18% (Grade 60 steel)
Typical structural slabs: 0.3–0.5%
Effective depth d = thickness – cover – bar_diameter/2
Standard concrete cover for slabs: 1.5 inches (not exposed to ground)
Q
How much does rebar weigh per linear foot?
Rebar weight ranges from 0.376 lb/ft for #3 bars to 2.670 lb/ft for #8 bars. A typical #4 rebar weighs 0.668 lb/ft. For a 20×20 ft slab with #4 at 12-inch spacing, total rebar weight is approximately 560 lbs.
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.
