Lap Length Calculator
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Pick the bar diameter, concrete grade and steel grade, and get the lap (splice) length in mm and as a multiple of φ — for tension or compression, per IS 456.
Lap Length Calculator — IS 456
Tension ≥ Ld or 30φ · compression ≥ Ld or 24φ · zone factor for splices
Lapped splice
Lap = 752 mm ≈ 47φ
IS 456 cl 26.2.5.1 — raise the lap when most bars are spliced in a tension zone
Lap length
752 mm
16 mm, M20, Fe 415, tension · ≈ 47φ
Lap
752 mm
As a multiple
47φ
Ld
752 mm
Floor (30φ)
480 mm
Lap for every diameter · M20, Fe 415, tension (×1)
6 mm
282 mm
47φ
8 mm
376 mm
47φ
10 mm
470 mm
47φ
12 mm
564 mm
47φ
16 mm
752 mm
47φ
20 mm
940 mm
47φ
25 mm
1,175 mm
47φ
32 mm
1,504 mm
47φ
Stagger the splices — don't lap more than half the bars at one section, and keep laps away from points of maximum stress (mid-span for bottom steel, over supports for top). The lap builds on the development length, so grade and bar type move both together.
How the lap length works (IS 456, cl 26.2.5)
A lapped splice must at least develop the bar, so the lap builds on the development length Ld:
- Tension lap = not less than Ld or 30φ, whichever is greater, times a zone factor (1.0 normally; up to 2.0 when most bars are spliced in a tension zone).
- Compression lap = not less than Ld (compression) or 24φ, whichever is greater.
Worked example
A 16 mm Fe415 bar in M20 concrete, in tension:
- Ld = 752 mm (47φ), and 30φ = 480 mm — so the lap = the larger, 752 mm ≈ 47φ.
- In compression the lap is 602 mm ≈ 38φ (min 24φ = 384 mm).
Lap as a multiple of φ (deformed bars, Fe415, factor 1.0)
| Grade | Tension lap | Compression lap |
|---|---|---|
| M20 | 47φ | 38φ |
| M25 | 40φ | 32φ |
| M30 | 38φ | 30φ |
Frequently asked questions
What is lap length in simple terms? The overlap between two bars so the force passes from one to the other through the concrete — used where a single bar isn't long enough to run continuously.
What is the lap length of a 16 mm bar? For Fe415 in M20 tension, about 752 mm (47φ); in compression about 602 mm (38φ). It follows the development length, so it rises with steel grade and falls as concrete grade rises.
What is the 50d thumb rule for lap length? "50d" is a round, safe field value close to the IS 456 tension lap for Fe415/M20 (47φ). Use the exact figure above rather than a blanket 50d, which over-orders steel on higher concrete grades.
Where should laps be placed? Away from points of maximum stress and staggered — no more than 50% of bars lapped at one section. Bottom bars are lapped near supports, top bars near mid-span.
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Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.