Which ACI equation does this use?

It uses the detailed ACI 318-19 Eq. 25.4.2.4a, with the (cb+Ktr)/db ≤ 2.5 cap, the √f'c ≤ 100 psi limit, the ψt·ψe ≤ 1.7 cap, and the 12-inch minimum.

cb is the smaller of the distance from the bar center to the nearest concrete surface and one-half the center-to-center spacing of bars being developed.

Rebar Development Length

ACI 318-19 Preliminary estimate

Tension development length ℓ_d for a straight deformed bar by the detailed ACI 318-19 equation — with the confinement cap, the √f'c limit, and every modification factor.

Results update as you type. These are educational estimates — confirm against the governing code and a licensed engineer before design use.

Method & references

Tension development length of deformed bars per ACI 318-19 Eq. 25.4.2.4a:

ℓ_d = (3/40) · (f_y / (λ√f'c)) · (ψ_tψ_eψ_sψ_g) / ((c_b+K_tr)/d_b) · d_b

The confinement term (c_b+K_tr)/d_b is capped at 2.5.
√f'c is limited to 100 psi (f'c credit caps near 10,000 psi).
ψ_t·ψ_e need not exceed 1.7.
ψ_s = 0.8 for #6 and smaller, 1.0 for #7 and larger; ψ_g = 1.0 / 1.15 / 1.3 for Gr 60 / 80 / 100.
Floor: ℓ_d ≥ 12 in (§25.4.2.1).

Reference: ACI 318-19 §25.4.2. This is the tension development length for straight deformed bars; hooked/headed bars, compression development, and splices use separate provisions.