Rebar Lap Length Calculator
Lap length ll / llE by splice percentage, plus the permitted number of splices.
Quick answer
With the default inputs: Bar grade HRB400 · Bar diameter 20 · Concrete class C30 etc.Lap length ll 990 mm
Edit the inputs below to recalculate.Inputs
Result
Lap length ll
990mm
| Anchorage la | 705mm |
| Splice factor ζl | 1.4 |
| Seismic factor | 1.00 |
| Calculated lap | 987mm |
| In bar diameters | 49.3d |
| Permitted splices | 5bars |
| Stagger distance | 1300mm |
Calculated to GB 50010, with a 300 mm minimum. Tied splices in beams should not exceed 25% of the section and in columns 50%. Members in axial or small-eccentricity tension must not use tied laps at all.
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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 |
How to work it out
- Establish the basic anchorage length la (or lab from the tables), then look at the splice percentage.
- Lap length ll = ζl × la, with ζl = 1.2 for 25% or less, 1.4 for 50%, 1.6 for 100%.
- In seismic design multiply by ζaE: 1.15 for grades 1 and 2, 1.05 for grade 3, 1.00 for grade 4.
- Take ll as not less than 300 mm.
- Permitted spliced bars n = ⌊total bars × percentage⌋ within one splice zone.
Where it goes wrong
- Using a round figure like 35d or 40d — lap length varies with concrete grade, bar grade, percentage and seismic level, so a fixed multiple fails somewhere.
- Confusing a lap with an anchorage — a lap transfers force between two bars, an anchorage develops one bar into concrete, and the factors differ; a lap is always longer than a bare anchorage.
- Splicing more than 50% at one section — exceeding the permitted tension percentage is a hidden serious defect; a full row of lapped bottom bars turns the beam into a hinge.
Material and cost
Lapping is the cheapest connection type, costing only extra steel. A mechanical coupler runs 8-20 each and a welded splice 5-15. But laps crowd the section and hinder placing, so bars of 25 mm and above usually go mechanical.
On site
- Tighten stirrups through the splice zone (typically ≤5d and ≤100 mm); this is a mandatory seismic requirement.
- Stagger adjacent laps by 1.3ll; without that offset the percentage calculation is meaningless.
- ll and la are different numbers — if the drawing gives llE, cut to llE rather than converting back.
Basis
- GB 50010-2010 Code for design of concrete structures, Section 8.4 (bar connections)
- 16G101-1 Standard drawings for flat-slab detailing, pages 60-61
- GB 50666-2011 Code for construction of concrete structures, Section 5.4
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Rebar Lap Length Calculator. 2026-10-25. https://jianzhujisuan.com/en/rebar-lap
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.