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.
span_length = 30desired_spacing = 12cover = 3
- 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
Ready to run the numbers?
Open Rebar Spacing Calculator