# Rebar Lap Length Calculator

Lap length ll / llE by splice percentage, plus the permitted number of splices.

- Page: https://jianzhujisuan.com/en/rebar-lap
- Category: Rebar (https://jianzhujisuan.com/en/c/rebar)
- Chinese version: https://jianzhujisuan.com/zh/rebar-lap.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Bar grade HRB400, Bar diameter 20 mm, Concrete class C30, Splice percentage 50%, Seismic detailing Non-seismic, Bars at the section 10 pcs): **Lap length ll 990 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Bar grade | select | HRB400 | — |  |
| Bar diameter | number | 20 | mm |  |
| Concrete class | select | C30 | — |  |
| Splice percentage | select | 50% | — |  |
| Seismic detailing | select | Non-seismic | — |  |
| Bars at the section | number | 10 | pcs |  |

## Formula

- **Anchorage**: la = α × (fy ÷ ft) × d
- **Lap length**: ll = ζl × la × ζaE
- **Splice factor**: 1.2 up to 25%, 1.4 at 50%, 1.6 at 100%
- **Permitted splices**: n = floor(total bars × percentage)
- **Minimum**: ll not less than 300 mm

## Worked example

HRB400, 20 mm, C30, 50% spliced, non-seismic: la ≈ 705 mm, so ll = 1.4 × 705 ≈ 987 mm, rounded to 990 mm; at most 5 of 10 bars may be spliced in one section.

## Questions

### Why does a higher splice percentage mean a longer lap?

The more splices in one section, the more force must transfer through a small region, raising bond stress and the risk of splitting. The code penalises this: 1.2 at 25%, 1.4 at 50%, 1.6 at 100%. That is why splices are staggered — staggering turns a 100% section into 50% and saves steel directly.

### Why are hoops tightened over a lap?

Two bars side by side leave less cover and the concrete tends to split along the bars under load. Closer hoops provide transverse confinement. The code limits hoop spacing over a tension lap to 5d and 100 mm. Missing confining hoops is one of the commonest inspection failures.

### When are tied laps not permitted?

Never in members under axial or small-eccentricity tension — those need welding or mechanical couplers. Tied laps are inadvisable for tension bars over 25 mm and compression bars over 28 mm, and mechanical couplers are preferred for longitudinal bars in seismic frames. Tied laps are the cheapest connection but not universally allowed.

## Related tools

- [Rebar Weight Calculator](https://jianzhujisuan.com/en/rebar-weight): Pick a bar diameter, enter length and quantity to get weight per metre, per bar and total tonnage.
- [Rebar Anchorage Length Calculator](https://jianzhujisuan.com/en/rebar-anchorage): Basic and seismic anchorage length from bar grade, diameter and concrete class.
- [Rebar Cut Length Calculator](https://jianzhujisuan.com/en/rebar-cut-length): Cut length and bar weight with standard hook allowances.
- [Rebar Spacing and Bar Count Calculator](https://jianzhujisuan.com/en/rebar-spacing): Bars and actual spacing from run length and design spacing, with total weight.
- [Stirrup Length and Count Calculator](https://jianzhujisuan.com/en/rebar-stirrup): Stirrup length, count and weight with the 11.9d seismic hook allowance.
- [Rebar Mesh Weight Calculator](https://jianzhujisuan.com/en/rebar-mesh): Weight per square metre and total mesh weight from dimensions and spacings.

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