What size rebar do I need for a house foundation?

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.

What is the lap length for rebar?

The standard lap length for rebar is 40 times the bar diameter (40D). For example, 12mm bars need a lap of 480mm (12 × 40). Laps should be staggered so they do not all occur at the same point. In some structural specifications, lap lengths of 50D or more may be required.

How much does rebar cost per tonne in the UK?

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.

What is the standard rebar spacing for a slab?

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.

Do I need rebar in a concrete 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.

Rebar Calculator — Reinforcement Bar Quantities

Calculate the number of reinforcement bars, total length, and weight needed for slabs and foundations. Includes lap length allowances.

Slab / Foundation Length

Length of the area to be reinforced

Slab / Foundation Width

Width of the area to be reinforced

Bar Diameter

H12 is most common for domestic foundations

Bar Spacing

200mm centres is standard for most domestic work

Include lap length allowance

Adds 40 × bar diameter at each lap joint (standard bars are 6m)

Include wastage allowance

Adds 10% extra for cutting waste and offcuts

Wastage Percentage

10% standard for rebar cutting waste

Price Per Tonne (optional)

Current UK rebar price for a cost estimate

How We Calculate This

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 × (40 × Bar diameter)
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. Lap lengths are calculated at 40 × bar diameter, which is the standard minimum tension lap per BS 8110 / Eurocode 2.

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.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.