Rebar Lap Length Calculator
Calculate the tension development length ℓd and the Class A or Class B lap splice length of a reinforcing bar to ACI 318-19 from bar size, steel grade, concrete f′c, bar position, coating and spacing.
How it works
ℓd = fy · ψt · ψe · ψg ÷ (K · λ · √f′c) × db (SI, MPa) with K = 2.1 (bars ≤ 20 mm) or 1.7 (larger) when spacing and cover are adequate, 1.4 or 1.1 otherwise · ψt·ψe ≤ 1.7, √f′c ≤ 8.3 MPa, ℓd ≥ 300 mm · lap = 1.3 ℓd (Class B) or 1.0 ℓd (Class A), ≥ 300 mm
ACI 318-19 Table 25.4.2.3 gives the straight tension development length of deformed bars in four simplified cases, and Table 25.5.2.1 sets the lap splice at 1.3 ℓd (Class B) unless at least twice the required steel is provided and no more than half the bars are spliced at the same place (Class A, 1.0 ℓd). The length grows with steel yield strength and bar diameter and falls with the square root of concrete strength. Top bars with more than 300 mm of fresh concrete below them get ψt = 1.3, epoxy coating ψe = 1.2–1.5, Grade 80 steel ψg = 1.15 and lightweight concrete λ = 0.75. The SI constants 2.1, 1.7, 1.4 and 1.1 are the metric versions of 25, 20, 16.7 and 13.3 in the inch-pound formula. Seismic frames, compression laps and bundled bars have further rules.
Worked example
A #4 (12.7 mm, ½″) Grade 60 bar (fy 420 MPa) in 21 MPa (3,000 psi) concrete, bottom bar, uncoated, normal spacing: ℓd = 420 × 1.0 ÷ (2.1 × 1.0 × √20.7) × 12.7 = 420 ÷ 9.55 × 12.7 = 558 mm → 56 cm (about 44 db). Class B lap = 1.3 × 558 = 726 mm → 73 cm (29″), or 57 bar diameters.
Sources: ACI 318-19 Building Code Requirements for Structural Concrete — §25.4.2 and §25.5.2 · CRSI — Development length and lap splice tables
Frequently asked questions
What is the lap length of rebar?
In ACI 318 practice for Grade 60 bars in 3,000–4,000 psi concrete, a Class B tension lap is roughly 50–60 bar diameters for bars up to #6 (20 mm) — about 73 cm for a ½″ bar — and more for top bars, epoxy-coated bars or tight spacing. Many site rules of thumb (40d or 50d) can be unconservative.
What is the difference between Class A and Class B splices?
Class A (1.0 ℓd) is allowed only where the steel provided is at least twice that required and no more than half the bars are spliced within the lap length. Everything else is Class B (1.3 ℓd), so Class B is the normal design case.
Why are top bars longer?
Bleed water and air rise under horizontal bars with a deep pour below them, weakening the bond. ACI applies ψt = 1.3 when more than 300 mm (12 in) of fresh concrete is placed below the bar.
Does this apply to columns in compression?
No. Compression laps are shorter (about 0.071 fy·db in SI, at least 300 mm) and columns in seismic frames have their own location and confinement rules. This calculator covers tension laps of straight deformed bars.
Related calculators
Splice lengths in a real structure must come from the structural drawings or a licensed engineer; this tool is for checking and estimating.