Rebar Development Length & Lap Splice Calculator (ACI 318)

Q: What is the difference between Class A and Class B lap splices?

A Class A splice is 1.0ℓd and may be used only where the steel provided is at least twice the steel required and no more than half the bars are spliced within the lap length. All other tension splices are Class B, at 1.3ℓd.

1 · Bar and concrete

Bar size ASTM A615

Grade (f_y) ψ_g

f'_c psi

psi

Concrete λ

Bar coating ψ_e

Top bar: more than 12 in. of fresh concrete placed below (ψ_t = 1.3)

2 · Cover, spacing and confinement

Clear cover in.

in.

Bar spacing c-c in.

in.

Stirrups/ties across splitting plane

Stirrup spacing in.

in.

Legs per stirrup

Bars developed (n) Eq. 25.4.2.4b

n = number of bars being developed or spliced along the plane of splitting.

3 · Hooks, compression and splices Hook enclosed by ties or stirrups (A_th ≥ 0.4A_hs or s ≤ 6d_b) → ψ_r = 1.0 Hook ends inside a column or wall core with side cover ≥ 2.5 in., or side cover ≥ 6d_b → ψ_o = 1.0 Compression bars enclosed by a spiral or ties per §25.4.9.3 → ψ_r = 0.75

Bars spliced in one place

A_s,prov/A_s,req at splice

Excess reinforcement A_s,req/A_s,prov (development only) §25.4.10

Leave at 1.0 where full f_y must be developed, at noncontinuous supports, and in seismic systems. Never applied to splices.

How the development length and lap splice calculator works

This free rebar development length and lap splice calculator finds the tension development length, the standard hook development length, the compression development length and the lap splice lengths for deformed bars per ACI 318-19 Chapter 25, the edition referenced by the Florida Building Code, 8th Edition (2023). Choose the bar size and grade, the concrete strength, the coating, cover and spacing, and any stirrups crossing the splitting plane. The results update as you type, and a table gives every bar size from #3 to #11 for the same settings.

What it calculates

Tension development length from the detailed 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, at least 12 in.
The simplified tension length from Table 25.4.2.3, shown for comparison
Standard hook development length from the ACI 318-19 Eq. 25.4.3.1a, ℓ_dh = (f_yψ_eψ_rψ_oψ_c/55λ√f'_c)d_b^1.5, at least 8d_b and 6 in., with the bend diameter and hook extensions
Compression development length ℓ_dc, with the 0.75 reduction for spirals or ties
Lap splice lengths: tension Class A (1.0ℓ_d) and Class B (1.3ℓ_d), with the class that applies chosen from the percentage of bars spliced and the ratio of steel provided to steel required, and compression lap splices of 0.0005f_yd_b (Grade 60 and below) or (0.0009f_y − 24)d_b, at least 12 in.
A lap splice schedule for #3 to #11 bars with top bars and other bars side by side, ready for the general notes

Modification factors

ψ_t = 1.3 for top bars with more than 12 in. of fresh concrete placed below them
ψ_e = 1.5 for epoxy-coated bars with cover under 3d_b or clear spacing under 6d_b, 1.2 for other epoxy-coated bars, and ψ_tψ_e ≤ 1.7
ψ_s = 0.8 for #6 and smaller bars, ψ_g = 1.15 for Grade 80 and 1.3 for Grade 100, and λ = 0.75 for lightweight concrete
√f'_c is limited to 100 psi, and the confinement term (c_b + K_tr)/d_b is limited to 2.5

Assumptions and limitations

Deformed bars #3 to #11 in tension or compression. Headed and mechanically anchored bars, bundled bars, welded wire reinforcement and #14 and #18 bars are not covered.
The reduction for excess reinforcement (§25.4.10) is optional and is never applied to lap splices. Do not use it where the full yield strength must be developed or in special seismic systems.
Seismic requirements of ACI 318 Chapter 18, such as development in tension for bars in special moment frames, are not included.

Frequently asked questions

How long is a lap splice for a #5 bar?

For the same Grade 60 #5 bottom bar in 4,000 psi concrete, a Class B tension lap is about 18.5 in., a Class A lap is the 14.5 in. development length, and a compression lap is 19 in. (30 bar diameters). The All bar sizes tab lists every lap length for your settings.

What is the development length of a #5 bar?

For a Grade 60 #5 bottom bar in 4,000 psi normalweight concrete with 1.5 in. cover, 6 in. spacing and no coating, the detailed ACI 318-19 equation gives about 14 in. and the simplified table gives about 24 in. Open the calculator with these settings to see every bar size at once.

What is the difference between Class A and Class B lap splices?

A Class A splice is 1.0ℓ_d and may be used only where the steel provided is at least twice the steel required and no more than half the bars are spliced within the lap length. All other tension splices are Class B, at 1.3ℓ_d.

Why is the detailed equation shorter than the simplified table?

The simplified table assumes a conservative confinement term. The detailed equation credits the actual cover, bar spacing and stirrups through (c_b + K_tr)/d_b, up to 2.5, so typical members get noticeably shorter lengths.

What changed for hooks in ACI 318-19?

ACI 318-19 replaced the hook equation with one proportional to d_b^1.5 and added factors for confinement (ψ_r), location (ψ_o) and concrete strength (ψ_c). Hooks that are not enclosed by ties and do not end inside a column core are noticeably longer than under ACI 318-14.

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For educational and preliminary design purposes only. Always verify results with a licensed PE.