Rebar Lap Length Calculator
Lap length ll / llE by splice percentage, plus the permitted number of splices.
Inputs
Result
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 |
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.
Lap length ll for HRB400 (mm, 50% spliced, non-seismic)
| Bar | C25 | C30 | C35 | C40 |
|---|---|---|---|---|
| HRB400 Φ12 | 670 | 595 | 540 | 500 |
| HRB400 Φ14 | 780 | 695 | 630 | 580 |
| HRB400 Φ16 | 890 | 790 | 720 | 665 |
| HRB400 Φ18 | 1005 | 890 | 810 | 745 |
| HRB400 Φ20 | 1115 | 990 | 900 | 830 |
| HRB400 Φ22 | 1225 | 1090 | 990 | 910 |
| HRB400 Φ25 | 1390 | 1235 | 1125 | 1035 |
FAQ
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 calculators
Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.