How to measure a bay window

A useful measurement starts with a clear reference line. Decide what you want to fit before turning window dimensions into an order.

Start with the fixing position

View your bay from above and sketch its shape. Mark the left side A, centre B and right side C. Choose a proposed track line at the intended fixing height. A line along the frame and a line out into the room have different lengths at the corners. If you can only measure the frame, label that basis and ask the supplier to convert it.

Measure each section

Use a metal tape. Measure A from the proposed left end to the first corner, B between the two corners, and C to the proposed right end. Keep the same reference line throughout. Repeat each measurement and write the unit beside it. Our calculator adds these straight sections; it does not turn them into manufacturing dimensions.

Capture what the lengths leave out

Photograph or sketch handles, vents, radiators and opening sashes. Record the largest projection and intended curtain drop. A supplier may request angles, diagonals, bay depth or its own measurement form. Three side lengths alone cannot determine a bay’s corner angles.

Bow bays and blinds

A continuous curve needs the supplier’s template or survey method. For individual blinds, use the headrail depth for the exact product and mounting type. English Blinds describes a paper-template method accounting for corner interference and protruding handles. Raw section lengths must not be submitted as exact blind order widths.

Sources & further reading

Reviewed 2 November 2026. Check the current instructions for your chosen product.