Window Fire Escape Calculator
Check whether windows meet Approved Document B fire escape requirements for clear opening dimensions, sill height and egress compliance in UK dwellings.
Width when fully open (min 450mm)
Height when fully open (min 450mm)
Max 1100mm (AD B); no lower Part B limit
If replacing non-compliant windows
How We Calculate This
This calculator checks whether a window meets the fire escape (egress) requirements of Approved Document B of UK Building Regulations.
Requirements (AD B)
Clear opening ≥ 0.33 m², minimum 450mm in any direction
- Minimum clear width: 450mm
- Minimum clear height: 450mm
- Minimum clear area: 0.33 m²
- Sill height: max 1100mm above floor level (AD B, no lower limit)
- Restrictors must be easily overridable without tools
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.