1. Decide what you are measuring

Before picking up the tape, confirm whether the supplier wants a brick opening, existing frame size, internal reveal size or finished product size. These are not the same thing, and mixing them up is one of the most expensive ordering mistakes.

• Photograph the opening from inside and outside.
• Note whether the current frame is staying or being replaced.
• Record wall construction, sill type, reveal depth and trim details.
• Ask the supplier exactly which measurement type they want.

2. Measure width and height in three places

Older openings are rarely perfectly square. Measure width at the top, middle and bottom, then height at the left, middle and right. Record all measurements clearly rather than rounding from memory.

• Use millimetres and write down every measurement immediately.
• Check whether the sides are plumb and the head/sill are level.
• Measure diagonals if the opening looks out of square.
• Take the smallest or largest measurement only if the supplier tells you to.

3. Record door details that affect the order

Doors need more than width and height. Threshold height, opening direction, hinge side, handle side, drainage, frame depth and outside ground level can all affect the final product and fitting plan.

• Record whether the door opens inward or outward.
• Mark hinge side and handle side from a consistent viewpoint.
• Check whether the threshold needs low-access or weather-bar details.
• Note vents, alarms, sensors, blinds, pipes, cables and nearby obstructions.

4. Use your measurements wisely

Your measurements are useful for budgeting, comparing products and checking whether an opening is unusual. For expensive external windows and doors, a supplier survey is usually worth it before final ordering.