650 mm rule calculator
Approved Document O (2021) para 3.9a and MHCLG FAQ 15; equivalent area from Appendix D. The same engine as O-Monkey.
What the rule says
Section 3 of Approved Document O deals with the things that stop windows being opened in practice. Paragraph 3.9 sets two conditions for an opening to count beyond 100 mm, and the first, 3.9a, is about reach: on a window that opens outwards, the handle should be no more than 650 mm from the inside face of the wall at the window's maximum openable angle. MHCLG's FAQ 15 turns that into a calculation — the 650 mm standard gives the safe opening angle, and that angle is the one used for the equivalent area.
It is not a stop on the window. The window may open further; it simply counts as open only that far. And because the distance is measured from the inside face of the wall, where the frame sits in the wall matters: the same window counts for less in a deep reveal than set forward.
A worked example
- A 600 × 1200 mm side-hung casement, frame 150 mm back: the handle has 500 mm to travel, so the window counts up to arcsin(500 ÷ 600) = 56.4° and gives 0.56 m² of equivalent area.
- Fully open at 90° it would give 0.62 m² — the rule costs 9%.
- As two 300 mm casements in the same frame, each leaf can open to 90°: 0.62 m² between them.
- As a tilt-and-turn, opening inward, the rule does not apply: 0.62 m².
- A wider leaf in a deeper wall bites harder: a 750 mm casement with its frame 200 mm back counts at 36.9° and gives 0.63 m² instead of 0.80 m² — 20% less.
The angle for common reveals and leaf widths
The largest angle, in degrees, at which an outward-opening leaf counts — hinge-to-handle distance across, reveal depth down:
| Reveal | 400 mm | 500 mm | 600 mm | 700 mm | 800 mm | 900 mm | 1000 mm |
|---|---|---|---|---|---|---|---|
| 50 mm | 90° | 90° | 90° | 59.0° | 48.6° | 41.8° | 36.9° |
| 75 mm | 90° | 90° | 73.4° | 55.2° | 46.0° | 39.7° | 35.1° |
| 100 mm | 90° | 90° | 66.4° | 51.8° | 43.4° | 37.7° | 33.4° |
| 125 mm | 90° | 90° | 61.0° | 48.6° | 41.0° | 35.7° | 31.7° |
| 150 mm | 90° | 90° | 56.4° | 45.6° | 38.7° | 33.7° | 30.0° |
| 175 mm | 90° | 71.8° | 52.3° | 42.7° | 36.4° | 31.9° | 28.4° |
| 200 mm | 90° | 64.2° | 48.6° | 40.0° | 34.2° | 30.0° | 26.7° |
| 250 mm | 90° | 53.1° | 41.8° | 34.8° | 30.0° | 26.4° | 23.6° |
| 300 mm | 61.0° | 44.4° | 35.7° | 30.0° | 25.9° | 22.9° | 20.5° |
For a side-hung casement the hinge-to-handle distance is its width; for a top-hung window, its height. 90° means the leaf counts fully.
Which windows it applies to
| Applies | Does not apply |
|---|---|
| Side-hung casements that open out | Tilt-and-turn and other inward-opening casements |
| Top-hung windows that open out | Bottom-hung windows that open in |
| On every floor, the ground floor included | Sliding sashes, sliders and bifolds, counted on their clear opening |
| Centre-pivot windows, and top-hung roof windows counted in their pivot position | |
| French and patio doors — the paragraph is about windows |
O-Monkey — free in your browser
Enter the reveal once — every window gets the rule
O-Monkey applies the 650 mm rule to every outward-opening window from one reveal depth (with a per-window override), shows the angle each one counts at, and suggests the swaps — narrower leaves, a tilt-and-turn — that would add the most free area.
Free to check. Building Control-ready PDF with the Appendix B checklist £25 +VAT (£30 inc VAT) per house type, every plot included.
Frequently asked questions
What is the 650 mm rule in Part O?
Paragraph 3.9a of Approved Document O says the handle of a window that opens outwards should be no more than 650 mm from the inside face of the wall when the window is at its maximum openable angle. MHCLG’s FAQ 15 adds that the 650 mm standard sets the safe opening angle, and that this angle is the one to use in the equivalent area calculation. So an outward-opening window counts towards the Part O free area only up to the angle where its handle reaches 650 mm.
How is the angle worked out?
The handle is at the free edge of the opening part, a distance L from the hinge, and the frame sits back from the inside face of the wall by the reveal depth. Opened to an angle α, the handle is reveal + L × sin α from the inside face, so the largest angle that counts is arcsin((650 − reveal) ÷ L). A 600 mm casement with its frame 150 mm back counts at 56.4°; any leaf of 500 mm or less in that wall counts fully, at 90°.
Does the rule apply on the ground floor?
Approved Document O gives no exemption for it. Paragraph 3.9 sits in the section on protection from falling, but its handle condition has no floor-level limit — only the guarding condition depends on the drop outside — so O-Monkey and this calculator apply it on every floor, ground floor included.
Does the window need a physical stop at that angle?
No. The rule decides how far the window is counted as open for the equivalent area, not how far it can physically open. The Future Homes Hub’s Part O guidance makes the same point, and notes that the position of the frame in the depth of the wall matters.
Which windows does it apply to?
Windows that open outwards: side-hung and top-hung casements. O-Monkey does not apply it to inward-opening windows such as tilt-and-turns and bottom-hung hoppers, to sliding sashes, sliders and bifolds, which are counted on their clear opening, to centre-pivot windows, or to French and patio doors — the paragraph is about windows. Top-hung roof windows are counted in their centre-pivot position at 90°, so the rule does not limit those either.
How do I get more free area out of the same opening?
Bring the frame forward in the wall, so the reveal is shallower; split a wide casement into narrower leaves, which can open further; or use an inward-opening tilt-and-turn, which the rule does not limit. In a wall with a 150 mm reveal, a single 600 × 1200 mm casement counts for 0.56 m²; the same opening as two 300 mm casements counts for 0.62 m²; as a tilt-and-turn, 0.62 m².
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Sources: Approved Document O (2021 edition), paras 3.8–3.9 and Appendix D · Approved Document O frequently asked questions (MHCLG), FAQ 15.