Lap Length of 25mm Bar: IS 456 Table & Calculator

Results

Check the working before you order.

Lap length in tension

1,416mm

In bar diameters	56.6 × db
Lap length in compression	1,133 mm
Compression in bar diameters	45.3 × db
Development length (tension)	1,416 mm

Uses the simplified code equations. Your structural drawings and the engineer of record govern: they may specify longer laps, different splice classes, or seismic detailing.

Stagger laps where possible, and do not lap bars larger than #11 / 36 mm in tension (ACI) or 36 mm (IS): use couplers.

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Working

τbd (M20) = 1.20 N/mm² × 1.6 (deformed bars) = 1.92 N/mm²
Ld = φ × 0.87 fy ÷ (4 τbd) = 25.0 × 0.87 × 500 ÷ (4 × 1.92) = 1416 mm (56.6 φ)
Compression: τbd × 1.25 → Ld = 1133 mm (45.3 φ)
Tension lap = max(Ld, 30φ) = 1,416 mm
Compression lap = max(Ld compression, 24φ) = 1,133 mm

Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.

Formula

σs            = 0.87 × fy                      (435 MPa for Fe500)
τbd           = table value × 1.6 for deformed bars   (M20: 1.2 × 1.6 = 1.92 MPa)
Ld (tension)  = φ × σs ÷ (4 × τbd)
Lap, tension  = Ld, at least 30φ   (2 Ld in direct tension)
Lap, compr.   = Ld ÷ 1.25, at least 24φ

Worked Example

A 25 mm bar in M20 concrete with Fe500 steel:

σs = 0.87 × 500 = 435 MPa; τbd = 1.2 × 1.6 = 1.92 MPa
Ld = 25 × 435 ÷ (4 × 1.92) = 1,416 mm (56.6φ); 30φ = 750 mm, so Ld governs
Tension lap = 1,416 mm
Compression: 25 × 435 ÷ (4 × 2.40) = 1,133 mm (45.3φ); 24φ = 600 mm → lap = 1,133 mm

Lap Length of 25mm Bar by Concrete Grade

Deformed (TMT) bars, flexural tension.

Concrete	Fe415 tension	Fe415 compression	Fe500 tension	Fe500 compression
M15	1,410 mm	1,128 mm	1,699 mm	1,359 mm
M20	1,175 mm	940 mm	1,416 mm	1,133 mm
M25	1,007 mm	806 mm	1,214 mm	971 mm
M30	940 mm	752 mm	1,133 mm	906 mm
M35	830 mm	664 mm	1,000 mm	800 mm
M40	750 mm	600 mm	894 mm	715 mm

More Lap Lengths by Bar Size

Frequently Asked Questions

What is the lap length of a 25 mm bar in M20 concrete?

1,416 mm in tension and 1,133 mm in compression with Fe500 steel, or 1,175 mm and 940 mm with Fe415. These are for deformed (TMT) bars in flexural members; laps in direct tension are twice as long.

Is the 50d lap rule correct?

It is close, not exact. By IS 456 the tension lap for Fe500 bars is 56.6d in M20 and 48.5d in M25, so 50d is slightly short in M20. With Fe415 it is 47.0d in M20. The lap shown on the structural drawings always governs.

Where should laps be placed?

Away from the points of highest stress, and staggered so that not all bars are lapped at the same section (IS 456 §26.2.5.1). In beams, bottom bars are usually lapped near the supports and top bars near mid-span.

Why is the compression lap shorter?

IS 456 allows 25% more bond stress in compression, so the compression development length is Ld ÷ 1.25. The lap must still be at least 24 bar diameters.

References

IS 456:2000 §26.2.1: development length Ld = φσs ÷ 4τbd, with σs = 0.87fy

IS 456:2000 §26.2.1.1: design bond stress τbd, increased by 60% for deformed bars and by 25% in compression

IS 456:2000 §26.2.5.1: flexural tension lap = Ld, at least 30φ; compression lap = Ld in compression, at least 24φ