Rebar Calculator — Reinforcement Bar Quantities

This calculator works out the number of reinforcement bars needed in both directions, the total bar length (including lap allowances), and the total weight based on the bar diameter.

The calculation

• Bars in each direction = (Perpendicular dimension ÷ Spacing) + 1
• Total bar length = (Bars × Span) in each direction
• Lap allowance = Number of laps × (Lap multiplier × Bar diameter), default 40D
• Total weight = Total length × Weight per metre

Bar weights used

• H8: 0.395 kg/m
• H10: 0.617 kg/m
• H12: 0.888 kg/m
• H16: 1.579 kg/m
• H20: 2.466 kg/m

These are standard weights for high-yield deformed reinforcement bar (B500B) to BS 4449:2005+A3 (mass per metre = bar diameter² ÷ 162.2). Lap lengths default to 40 × bar diameter — a common best-case default for a tension lap (good bond conditions, a low percentage of bars lapped at one section). It is the minimum, not an over-estimate: the true design lap to Eurocode 2 (BS EN 1992-1-1, §8.7) rises with poorer bond conditions and a higher percentage of bars lapped — commonly 50D–80D and more — so 40D can under-estimate. Always follow your structural engineer’s lap schedule. You can adjust the lap multiplier and standard bar length under Advanced Options.

Standards

Reinforcement should comply with BS 4449:2005+A3 (Steel for the reinforcement of concrete). Reinforcement design and detailing should follow BS 8110 or Eurocode 2 (BS EN 1992). All structural reinforcement must be specified by a qualified structural engineer.