Loft Head Height Calculator — Is My Loft Tall Enough to Convert?

This calculator uses trigonometry to determine the head height at any point across the loft width, based on the roof pitch angle, ridge height, and eaves height.

The formula

Height at distance x from centre = Ridge height - (x × tan(pitch angle))

Where x is the horizontal distance from the centre ridge to any point across the loft floor.

Usable width calculations

• Distance for 2.1m height: (Ridge height - 2.1) ÷ tan(pitch)
• Distance for 1.5m height: (Ridge height - 1.5) ÷ tan(pitch)
• Usable width: 2 × distance from centre (both sides of ridge)

Feasibility check (industry rule-of-thumb)

There is no statutory minimum ceiling height for a habitable room (the requirement was removed from the Building Regulations in 1985). Builders and building-control surveyors commonly use a practical rule-of-thumb of 2.1m head height over at least 50% of the floor width to judge whether a loft is worth converting — this is a convention, not a figure set by Approved Document K. The calculator applies that rule-of-thumb by comparing the width above 2.1m to the total floor width; if more than 50% of the width has 2.1m+ clearance the conversion is likely feasible (subject to structural checks). The percentage is computed against the loft width you enter, so an over-stated width will deflate the result.

What Approved Document K actually requires: AD K (2013) only governs stair headroom — clause 1.11 / Diagram 1.3 sets 2.0m over stairs and landings, reduced for loft conversions (clause 1.13 / Diagram 1.4) to 1.9m at the centre of the stair and 1.8m at the edge. It says nothing about habitable-room ceiling height or floor-area percentages. The only modern numeric ceiling-height standard is the optional Nationally Described Space Standard (2.3m over 75% of gross internal area), which is a planning standard, not a Building Regulation.

Space zones

• Above 2.1m: Habitable space — can be counted as living area
• 1.5m to 2.1m: Useful for furniture, desks, beds, and circulation
• Below 1.5m: Eaves storage only (wardrobes, cupboards)