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Rebar Spacing Calculator — Bar Count, Weight & Steel Ratio

Get accurate rebar counts, spacing limits, and steel weight for concrete slabs per ACI 318-19

Total Rebar Weight

287 lbs

Total Bars

32

Linear Feet

430 ft

1.5" for interior slabs, 2" for exposed, 3" for ground contact

Rebar Summary

287 lbs
total rebar weight
Total Bars
32
21 + 11
Linear Feet
430 ft
#4 @ 12" o.c.

Bar Count Breakdown

Width direction (20 ft spans)
21 bars× 10 ft each
Length direction (10 ft spans)
11 bars× 20 ft each
Total32 bars (430 linear ft)

ACI 318-19 Compliance

Max Spacing (ACI)12" o.c.
Your Spacing12" o.c. ✔
Effective Depth (d)2.25"
Steel Area per ft0.200 in²/ft
Steel Ratio (ρ)0.74%

Steel Ratio by Bar Size (at 12" spacing)

#30.40%
#40.74%
#51.18%
#61.73%
#72.42%
#83.29%

ACI minimum for temp/shrinkage: 0.18%. Your #4 at 12" spacing = 0.74%

Rebar Properties Reference

BarDia (in)Area (in²)Wt (lb/ft)
#30.3750.110.376
#40.50.20.668
#50.6250.311.043
#60.750.441.502
#70.8750.62.044
#810.792.67

Pro Tips

Placement: Position rebar at 1/3 depth from the top for slabs on grade. Use rebar chairs or dobies to maintain consistent cover.
Lap Splices: Overlap bars by 40 diameters minimum (20" for #4, 25" for #5). Add 10–15% to total footage for splices on bars over 20 ft.
Tying: Tie every intersection or every other intersection with 16-gauge tie wire. Alternate ties in a checkerboard pattern for speed.
Ordering: Rebar is sold in 20 ft lengths. Order 5–10% extra for waste, bends, and lap splices.

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.

  • Formula: max spacing = min(3 × thickness, 18 in)
  • 4" slab: max spacing = min(12, 18) = 12 inches
  • 6" slab: max spacing = min(18, 18) = 18 inches
  • 8" slab: max spacing = min(24, 18) = 18 inches (capped)
  • Temperature/shrinkage steel: max 5 × thickness or 18 in per ACI 7.7.6.2
Slab ThicknessMax Spacing (3×t)ACI CapEffective Max
4 inches12"18"12"
5 inches15"18"15"
6 inches18"18"18"
8 inches24"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 Size12" Spacing16" Spacing18" Spacing
10×10 ft22 bars16 bars14 bars
20×10 ft32 bars24 bars22 bars
20×20 ft42 bars32 bars28 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.

  • #3 (0.375"): walkways, patios, thin slabs — 0.376 lb/ft
  • #4 (0.5"): standard residential slabs, driveways — 0.668 lb/ft
  • #5 (0.625"): heavy-duty slabs, garage floors — 1.043 lb/ft
  • #6 (0.75"): structural slabs, footings — 1.502 lb/ft
  • #7/#8: grade beams, foundations, commercial work
Bar SizeDiameterArea (in²)Weight (lb/ft)
#30.375"0.110.376
#40.500"0.200.668
#50.625"0.311.043
#60.750"0.441.502
Q

What is the steel ratio for concrete slabs?

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.

  • #3: 0.376 lb/ft — lightest standard bar
  • #4: 0.668 lb/ft — most common for residential
  • #5: 1.043 lb/ft — roughly 1 lb per foot
  • #6: 1.502 lb/ft — 50% heavier than #5
  • #7: 2.044 lb/ft, #8: 2.670 lb/ft — commercial grade

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Last Updated: Mar 9, 2026

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.

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