Rebar Calculator — Reinforcement Bar Quantities

For standard domestic strip foundations, H12 (12mm) bars at 200mm centres are commonly specified. For raft foundations, H16 at 200mm centres is typical. The actual specification must come from a structural engineer based on your specific ground conditions and building loads.

A common default tension lap is 40 times the bar diameter (40D) — for example, 12mm bars give a lap of 480mm (12 × 40). This is the best case (good bond conditions, a low percentage of bars lapped at one section). Under Eurocode 2 (BS EN 1992-1-1, §8.7) the required lap rises with poorer bond conditions and a higher percentage of bars lapped, commonly to 50D–80D, so 40D can under-estimate. Laps should also be staggered so they do not all occur at the same point. Always use the lap length on your engineer's bar bending schedule.

Rebar (high-yield deformed bar, H-bar) typically costs between £600-£900 per tonne in the UK, depending on bar size, quantity ordered, and current steel prices. Smaller quantities from builders merchants cost more per kg. Cut and bent rebar to schedule costs more but saves significant labour time on site.

Common spacings for domestic ground-bearing slabs are 200mm centres (most common), 150mm centres (heavier loads or poorer ground), and 300mm centres (light-duty applications). The bars are placed in a grid pattern — both directions — typically on bar chairs 40-50mm from the bottom of the slab.

Not all slabs need rebar. A simple shed base or patio may only need fibre reinforcement or A142 mesh. However, any slab carrying structural loads (garage floors, house slabs) should be reinforced as specified by a structural engineer. Building Regulations require ground-bearing slabs to be designed to suit the ground conditions. Use our Concrete Volume Calculator to work out the concrete needed, and our Concrete Mix Calculator for the mix quantities.