Rebar Spacing Calculator
Calculate rebar quantity, spacing, bar count, weight, and ACI 318 maximum spacing compliance for slabs, footings, and column ties.
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What This Calculator Does
The rebar spacing calculator determines how many bars fit in a given slab or footing dimension, the total linear footage and weight, and checks compliance with ACI 318 maximum spacing limits. For slabs: maximum spacing is the lesser of 3 times the slab thickness or 18 inches. For footings: checks minimum and maximum ACI spacing. For column ties: computes tie spacing per ACI 318.
It combines Slab / Footing Length, Slab / Footing Width, Slab Thickness, Bar Size to estimate Total Bars, Total Linear Feet, Total Weight.
Formula & Method
Notation used in the formulas: = Total Bars; = Slab / Footing Length; = Slab / Footing Width; = Slab Thickness; = Bar Size; = Bar Spacing (c/c); = Concrete Cover.
Method summary: inputs are normalized to consistent units, core equations are evaluated, then secondary values are derived and rounded for display.
Use this calculator for quick scenario analysis. Start with baseline values, change one driver at a time, and compare how sensitive the results are to each input shown above.
Worked Examples
20 × 30 ft slab, #5 bar at 12 in spacing
Slab: 20 ft wide × 30 ft long, #5 bar (A = 0.31 in², lb/ft = 1.043), spacing = 12 in = 1 ft. Bars along length: ⌊20/1⌋ + 1 = 21 bars × 30 ft = 630 linear ft. Bars along width: ⌊30/1⌋ + 1 = 31 bars × 20 ft = 620 linear ft. Total linear ft = 630 + 620 = 1,250 ft. Total weight = 1,250 × 1.043 = 1,304 lbs. As per foot = 0.31 / 1.0 = 0.31 in²/ft.
ACI 318 slab maximum spacing check — 8 in slab
For an 8-inch slab: s_max = min(3 × 8, 18) = min(24, 18) = 18 inches. #4 bar at 12 in spacing: 12 ≤ 18 ✓ Compliant. #4 bar at 18 in spacing: 18 ≤ 18 ✓ Compliant (just at limit). #4 bar at 24 in spacing: 24 > 18 ✗ Non-compliant — reduce spacing.
Standard Rebar Sizes Reference Table
| Bar Size | Diameter (in) | Area (in²) | Weight (lb/ft) |
|---|---|---|---|
| #3 | 0.375 | 0.11 | 0.376 |
| #4 | 0.500 | 0.20 | 0.668 |
| #5 | 0.625 | 0.31 | 1.043 |
| #6 | 0.750 | 0.44 | 1.502 |
| #7 | 0.875 | 0.60 | 2.044 |
| #8 | 1.000 | 0.79 | 2.670 |
| #9 | 1.128 | 1.00 | 3.400 |
| #10 | 1.270 | 1.27 | 4.303 |
| #11 | 1.410 | 1.56 | 5.313 |
Inputs Used
- Slab / Footing Length: Used directly in the calculation.
- Slab / Footing Width: Used directly in the calculation.
- Slab Thickness: Used for ACI 318 maximum spacing check (s_max = min(3h, 18 in)).
- Bar Size: Used directly in the calculation.
- Bar Spacing (c/c): Center-to-center spacing between bars.
- Concrete Cover: Clear cover to the rebar — reduces effective spacing from edge.
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