How to use this calculator
- Select Room Type.
- Enter Floor Dimensions (mm). The tool converts to m² automatically.
- Enter Window/Door Dimensions (mm). Use visible glass area. If you have multiple openings, add their glass areas and enter the total.
- Select Opening Type (for ventilation). For Sliding, enter the clear opening when fully open (exclude frames/mullions/screens).
- Click Calculate. Review the Ventilation and Light results.
How results are checked
- Ventilation: compares openable area with the target (commonly 5% of floor area).
- Light: compares glazed area with the target (commonly 10% of floor area).
Worked example
Floor: 3600 × 3000 mm → 10.8 m²
Light (glass): 1200 × 1200 mm → 1.44 m² (≥ 10% of 10.8 m² = 1.08 m²) → Light: PASS
Ventilation (Sliding, clear opening): 600 × 1200 mm → 0.72 m² (≥ 5% of 10.8 m² = 0.54 m²) → Ventilation: PASS
Common mistakes
- Mixing units (mm vs m).
- Using frame size instead of glass (daylight) area for light.
- Counting fixed windows for ventilation.
- For sliding windows, using glass size instead of clear opening.
Disclaimer: Guidance only—confirm against the current NCC and referenced documents before relying on results.