Rebar Development Length Calculator — ACI 318-19M §25.4
Development length is the embedment a bar needs to reach yield without slipping. Get ℓ_d wrong and the section's flexural capacity is compromised. This page explains §25.4 and gives you a calculator that handles all four modification factors.
Formulas
| Quantity | Formula |
|---|---|
| Tension development length (general) | ℓ_d = (f_y · ψ_t · ψ_e · ψ_s · ψ_g) / (1.1 · λ · √f'_c · ((c_b + K_tr)/d_b)) · d_b |
| Casting position factor ψ_t | 1.3 if more than 300 mm of fresh concrete is below the bar; 1.0 otherwise |
| Epoxy coating factor ψ_e | 1.5 if epoxy with cover < 3d_b or clear spacing < 6d_b; 1.2 otherwise (epoxy); 1.0 (uncoated) |
| Bar size factor ψ_s | 1.0 for #19 (M19) and larger; 0.8 for smaller bars |
| Grade factor ψ_g | 1.0 for f_y ≤ 420; 1.15 for f_y = 550; 1.3 for f_y = 690 |
| Lightweight factor λ | 0.75 lightweight; 1.0 normal-weight |
How to use it
- Enter the bar diameter, grade f_y, and concrete f'_c.
- Specify casting position (top or bottom bar), coating, and weight type.
- Enter c_b (smaller of cover or half clear spacing) and K_tr (transverse reinforcement index).
- The calculator gives you ℓ_d in mm.
- If you're checking a hooked bar, switch to ℓ_dh mode (§25.4.3).
Frequently Asked Questions
What's K_tr?
K_tr captures the confinement provided by stirrups crossing the splitting plane. K_tr = (40·A_tr) / (s · n), where A_tr is the area of transverse reinforcement, s is its spacing, and n is the number of bars being developed. ACI lets you take K_tr = 0 conservatively.
Can I cap (c_b + K_tr)/d_b?
Yes — §25.4.2.4 limits it to 2.5 to prevent very short development lengths.
What's the minimum ℓ_d?
ACI 318-19 §25.4.2.1 requires ℓ_d ≥ 300 mm regardless of calculation.
Is this the same as splice length?
Class A splice = 1.0·ℓ_d, Class B splice = 1.3·ℓ_d (§25.5.2). Class A only when not more than half the bars are spliced within ℓ_d AND As provided ≥ 2·As required.