What this calculator does
A bar only develops its strength once it has enough bonded length. This calculator returns the basic tension development length for a bar diameter, grade and concrete strength, plus the Class A and Class B lap lengths used where bars must be spliced.
How to use it
- Enter the bar diameter, the steel yield strength and the concrete characteristic strength.
- Apply the position factor: 1.0 for most bars, 1.3 for top bars with insufficient cover.
- Apply the coating factor: 1.0 bare, 1.2 epoxy-coated in this simplified form.
- Class A applies when no more than half the bars are spliced in one place; Class B allows up to 100 %.
Governing formula
l_d = (f_y · ψ_t · ψ_e · ψ_s / (2.1 · λ · √f′c)) · d_b Where:
-
f_y= Yield strength of reinforcement [MPa] -
ψ_t= Position factor [–] -
ψ_e= Coating factor [–] -
ψ_s= Size factor (0.8 for bars ≤ 19 mm) [–] -
d_b= Bar diameter [mm]
Derived Equations:
l_d,ClassA = l_d l_d,ClassB = 1.3·l_d Worked example — 16 mm bar, B500 in C30/37
The figures below were produced by running this page's own engine with the example inputs (Bar diameter (mm), Steel yield strength, Concrete strength f′c (MPa), Position factor (ψt), Coating factor (ψe)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Basic development length | 445.13 mm |
|---|---|
| Class A splice (≤50 % spliced) | 445.13 mm |
| Class B splice (100 % spliced) | 578.67 mm |
| Minimum embedment | 445.13 mm |
| Assessment | Develop # of a 16 mm bar: l_d ≈ 445 mm basic, 445 mm (Class A, ≤50% spliced), 579 mm (Class B); minimum embedment 300 mm. Verify against the code edition in force. |
- A 16 mm bar takes the ψ_s = 0.8 size factor, so the basic length works out around 585 mm.
- A Class B lap is 1.3 times that, about 760 mm.
- Confirm against the code detailing provisions for the bar position and cover you have.
Assumptions & limitations
Engineering calculation. ACI-style simplified tension development length. It does not include the newer code provisions for confined or confined-unconfined bars, nor any bar-spacing and cover corrections.
- A simplified expression only; the code edition in force, including all detailing provisions, governs.
- Lap splices must satisfy the bar spacing and cover limits as well as the length.
- Compression development, anchorage and hook lengths are not calculated here.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- ACI 318 — building code requirements for structural concrete (development and splice provisions).
- Eurocode 2 — anchorage and lap length, EN 1992-1-1 section 8.