Rebar Spacing Calculator Formula
Understand the math behind the rebar spacing calculator. Each variable explained with a worked example.
Formulas Used
Number of Bars
num_bars = floor(usable_span / spacing_ft) + 1Actual Spacing
actual_spacing = usable_span / (floor(usable_span / spacing_ft)) * 12First Bar from Edge
first_bar_from_edge = coverVariables
| Variable | Description | Default |
|---|---|---|
span_length | Span Length(feet) | 30 |
desired_spacing | Desired Spacing(inches) | 12 |
cover | Edge Cover(inches) | 3 |
spacing_ft | Derived value= desired_spacing / 12 | calculated |
cover_ft | Derived value= cover / 12 | calculated |
usable_span | Derived value= span_length - 2 * cover_ft | calculated |
How It Works
How to Calculate Rebar Spacing
Formula
Usable Span = Total Span - 2 × Edge Cover
Number of Bars = floor(Usable Span / Spacing) + 1
Actual Spacing = Usable Span / (Number of Spaces) × 12
Edge cover is the minimum distance from the edge of the concrete to the nearest rebar. Typical cover is 3 inches for cast-against-ground concrete and 1.5 inches for formed surfaces.
Worked Example
Rebar across a 30 ft span at 12-inch spacing with 3-inch edge cover.
- 01Cover in feet = 3 / 12 = 0.25 ft
- 02Usable span = 30 - 2 × 0.25 = 29.5 ft
- 03Spacing in feet = 12 / 12 = 1.0 ft
- 04Number of bars = floor(29.5 / 1.0) + 1 = 30 bars
- 05Actual spacing = 29.5 / 29 × 12 = 12.21 inches
Frequently Asked Questions
What is the minimum concrete cover for rebar?
3 inches for concrete cast directly against earth. 2 inches for formed concrete exposed to weather. 1.5 inches for formed concrete not exposed to weather.
What is the maximum rebar spacing?
ACI code limits rebar spacing to 18 inches or 3 times the slab thickness, whichever is less. For temperature/shrinkage steel, maximum spacing is 18 inches or 5 times the thickness.
How do I mark rebar spacing on forms?
Measure and mark the cover distance from each edge, then mark at the calculated spacing intervals. Snap a chalk line to guide placement.
Ready to run the numbers?
Open Rebar Spacing Calculator