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Rebar Development Length Calculator

Development length is the embedment a bar needs to reach its yield stress. IS 456: Ld = φ·0.87fy ÷ 4τbd: 47φ for Fe415 in M20 and 48.5φ for Fe500 in M25. ACI 318-19: ld = fy·ψ ÷ (25λ√f'c)·db for No. 6 and smaller with good detailing: 37.9db for Grade 60 in 4,000 psi concrete.

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Development lengthDevelopment length 777 mm777 mmsupport / concretemyconstructioncalculator.comNot to scale
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Development length in tension
777mm
In bar diameters48.5 × db
Development length in compression621 mm
Compression in bar diameters38.8 × db

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.

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Working
  1. τbd (M25) = 1.40 N/mm² × 1.6 (deformed bars) = 2.24 N/mm²
  2. Ld = φ × 0.87 fy ÷ (4 τbd) = 16.0 × 0.87 × 500 ÷ (4 × 2.24) = 777 mm (48.5 φ)
  3. Compression: τbd × 1.25 → Ld = 621 mm (38.8 φ)
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.
Formulas


IS 456:2000


Ld = φ × σs ÷ (4 τbd),   σs = 0.87 fy
τbd (plain, tension): M20 1.2, M25 1.4, M30 1.5, M35 1.7, M40+ 1.9 N/mm²
× 1.6 for deformed bars; × 1.25 in compression


ACI 318-19


ld  = fy ψt ψe ψg ÷ (C λ √f'c) × db,  ≥ 12 in
      C = 25 (No. 6 and smaller) or 20 (No. 7 and larger) with clear spacing ≥ db, cover ≥ db, stirrups
      C = 50/3 or 40/3 for other cases;  ψt ψe ≤ 1.7;  √f'c ≤ 100 psi
ldc = max(fy ÷ (50 λ √f'c), 0.0003 fy) × db,  ≥ 8 in


Worked Example


16 mm Fe500 deformed bar in M25 (IS 456):


    

  1. τbd = 1.4 × 1.6 = 2.24 N/mm²
    2. 

  2. Ld = 16 × 0.87 × 500 ÷ 8.96 = 777 mm = 48.5φ
    3. 

  3. In compression: τbd = 2.80 → 621 mm = 38.8φ
    4. 



IS 456 Development Length in Tension (deformed bars)






Steel
M20
M25
M30
M35






Fe415
47.0φ
40.3φ
37.6φ
33.2φ




Fe500
56.6φ
48.5φ
45.3φ
40.0φ






ACI 318-19 Tension Development, Grade 60, Good Detailing (in)






Bar
4,000 psi
5,000 psi






#3
14.2
12.7




#4
19.0
17.0




#5
23.7
21.2




#6
28.5
25.5




#7
41.5
37.1




#8
47.4
42.4






Worked Examples
Frequently Asked Questions
What is development length?
The length of bar that must be embedded in concrete beyond the point where it is needed, so bond can transfer its full force. It is used at supports, cut-off points and anchorages.
Why are top bars longer under ACI?
Bars with more than 12 in (300 mm) of fresh concrete cast below them bond less well, because water and air collect under them. ACI multiplies their development length by ψt = 1.3.
Is development length the same as lap length?
No. Lap length is based on development length but is often longer (1.3 ld for an ACI Class B splice) and has its own minimums such as 30φ in IS 456.
References
  • ACI 318-19 Table 25.4.2.3 and §25.4.9 (compression)
  • IS 456:2000 cl. 26.2.1 and cl. 26.2.1.1 (design bond stress)
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