How to Measure an Arched Door Opening

An arched opening needs more information than a rectangular width and height. The curve must be described in relation to the straight sides, maximum height, frame depth, and actual site condition. This draft guide supports an initial enquiry; the final template or survey method must be confirmed before production.

Identify the arch type

Some openings are semicircular, segmental, elliptical, pointed, or irregular. Do not assign a radius based on appearance. Photograph the complete opening straight on and note whether it is new framing, finished masonry, or an existing frame.

Record the base width

Measure the width between the sides at agreed positions. Take more than one reading and label each location. Note whether the sides are vertical and whether finishes will change the opening.

Find the spring line

The spring line is the level where the curved portion begins. Record its height from the finished floor reference on both sides. If the curve begins at different heights, document the irregularity instead of averaging it without approval.

Record maximum height

Measure from the same floor reference to the highest point of the arch. Show this dimension on a straight-on elevation together with the spring-line height and width.

Depth and construction

Record wall or frame depth and photograph the reveal, head, sill, and surrounding construction. An existing arch may not be symmetrical or consistent through the wall thickness.

When a template may be needed

A full-size template or professional survey may be required for irregular, existing, or complex openings. Agree who creates, verifies, labels, and approves it. The template should be protected from distortion and linked to the correct project and revision.

Photo and drawing checklist

• Straight-on full opening.
• Both sides and the arch curve.
• Finished floor reference.
• Width measurement locations.
• Spring-line heights on both sides.
• Maximum height.
• Depth and surrounding construction.
• Any cracks, uneven finishes, or obstructions.

Common mistakes

• Providing width and maximum height only.
• Assuming the opening is a perfect semicircle.
• Measuring from an unfinished floor without noting the future build-up.
• Using a perspective photograph to calculate the curve.
• Failing to agree who verifies the final shape.