# Rebar Spacing Calculator

> Lay out reinforcing bars across a slab or footing: number of bars for a maximum spacing and end cover, the equal spacing, steel area per metre and total length.

Category: Construction · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/rebar-spacing-calculator

Subtract the end cover from both sides, divide by the maximum spacing and round up to get the number of gaps. Bars are one more than the gaps, set at equal spacing.

Also called: Bar spacing calculator, Number of rebars calculator.

## Inputs
- Units (options: Metric, Imperial; default metric)
- Width to cover (across the bars) (default 3)
- End cover each side (default 50)
- Maximum centre-to-centre spacing (default 150)
- Bar diameter (default 12)
- Length of each bar (default 4; optional)

## Results
- Number of bars
- Actual spacing
- Actual spacing
- Steel area
- Steel area
- Total bar length
- Total bar length

## Formula

```
Gaps = ⌈(W − 2c) ÷ s_max⌉;  Bars = Gaps + 1;  s = (W − 2c) ÷ Gaps;  As per metre = (π d² ÷ 4) × 1000 ÷ s
```

Where:
- W: width the bars are spread across
- c: end cover to the first and last bar
- s_max: maximum spacing
- d: bar diameter

## Worked example
1. Clear width = 3000 − 2 × 50 = 2900 mm; 2900 ÷ 150 = 19.3 → 20 gaps.
2. Bars = 20 + 1 = 21 at 2900 ÷ 20 = 145 mm.
3. One 12 mm bar = 113.1 mm²; × 1000 ÷ 145 = 780 mm²/m. Length = 21 × 4 = 84 m.

**Result:** A 3 m width at no more than 150 mm centres needs 21 bars at 145 mm, giving 780 mm² of steel per metre.

## What the result means

Compare the steel area with the area your design requires. Repeat the calculation in the other direction for a two-way mat.

## Assumptions and limits
- Bars are evenly spaced with the same cover at both ends.
- Does not check code minimum or maximum spacing, cover or lap requirements; those come from the design.

## Questions

### Why add one bar?

Bars sit at both ends of every gap, so there is always one more bar than gaps, like fence posts.

### Why is the spacing smaller than the maximum?

The gaps are rounded up to a whole number and then spread evenly, so the real spacing never exceeds the maximum.

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- [Steel Rebar Weight Calculator](https://calculators.geekmidst.com/calculators/steel-rebar-weight-calculator)
- [Foundation Concrete Calculator](https://calculators.geekmidst.com/calculators/foundation-concrete-calculator)
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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/rebar-spacing-calculator