How is bar spacing across a width calculated?

The first and last bars sit one cover distance from each edge, so the usable spread is width minus twice the cover minus one bar diameter. Dividing that spread by the number of gaps (bar count minus one) gives the centre-to-centre spacing.

What is a lap splice and how long must it be?

A lap splice overlaps two bars so force transfers between them. ACI 318 sets the tension lap as a multiple of the development length: Class A is 1.0 times and Class B is 1.3 times the development length, with a 12 inch minimum.

Which development length formula is used?

The simplified ACI 318 expression for tension development length of a deformed bar: ld = (fy x psi factors x db) / (25 x sqrt(f'c)) in inches, with f'c and fy in psi. The tool applies typical factors for uncoated bottom bars and shows the result.

When do I use Class A versus Class B splices?

Class A applies when the area of steel provided is at least twice that required and no more than half the bars are spliced in the lap zone. Otherwise use the longer Class B splice. When in doubt, designers default to Class B.

Is this a substitute for a structural engineer?

No. It gives planning estimates using simplified ACI 318 expressions. Final spacing and splice details must come from the project's structural drawings and a licensed engineer, accounting for the full set of code modification factors.

Rebar Spacing & Lap Splice Calculator

Getting rebar layout right means two things: spacing the bars evenly across the member with correct cover, and lapping them by enough length to transfer load. This calculator handles both, finding the centre-to-centre spacing for a given bar count and computing the ACI 318 tension lap-splice length.

How it works

Spacing distributes the bars between the cover lines:

usable spread = width − 2 × cover − bar diameter
spacing       = usable spread / (bar count − 1)

The lap splice builds on the simplified ACI 318 tension development length:

ld    = (fy × ψt × ψe × db) / (25 × √f'c)   inches
lap A = 1.0 × ld   (min 12 in)
lap B = 1.3 × ld   (min 12 in)

Here f'c and fy are in psi, db is the bar diameter in inches, and the psi factors cover bar position and coating.

Example and notes

Six #5 bars (db = 0.625 in) across a 36 in slab with 2 in cover give a spread of 36 − 4 − 0.625 = 31.4 in over five gaps, about 6.3 in on centre. With f’c = 4000 psi and fy = 60000 psi the development length is roughly 24 in, so a Class B lap is about 31 in. Always confirm spacing and splice class against the structural drawings — the full ACI factor set can change these numbers.