Rebar Lap Length Calculator

About this calculator

Reinforcing bars can't be manufactured or shipped in infinite lengths, so where two bars must continue as one, they are overlapped by a calculated lap (splice) length so that force transfers from one bar to the other through the surrounding concrete's bond strength rather than relying on a mechanical connector. ACI 318, the governing US concrete design code, specifies tension lap lengths as a function of bar diameter, concrete strength, bar spacing/cover, and splice class (Class A or the more conservative Class B, used when more than half the bars are spliced at one location).

Because the full code calculation involves several factors, field practice commonly uses a simplified rule of thumb — a lap length of roughly 40 times the bar diameter (40d_b) for typical tension splices in normal-strength concrete — as a quick sizing check before a detailed engineering calculation. This calculator applies a bar diameter and a chosen lap factor (40× by default) to size the splice for planning and estimating purposes.