What you need
- A 25 ft (8 m) or longer tape measure
- A spirit level
- A step ladder
- A pencil and the record sheet below (or your phone’s notes)
- A helper, for wide openings
Step-by-step measurements
- Width of the opening. Measure between the finished sides of the opening (not including stop moulding) at the top, middle and bottom. Record the smallest figure.
- Height of the opening. Measure from the floor to the underside of the header at the left, centre and right. Floors often slope, so record the smallest figure and note the difference.
- Headroom. Measure from the top of the opening to the ceiling, or to the lowest obstruction such as a beam, pipe, duct or light. Clopay publishes 10″ for extension springs and 12″ for torsion springs, plus about 3″ for an opener.
- Side room (left and right). Measure from the edge of the opening to the nearest wall or obstruction on each side. Clopay requires at least 3¾″ per side for its standard track.
- Backroom. Measure from the opening back toward the rear wall. Clopay’s guidance is door height plus 18″, with more needed for an opener.
- Note obstructions. Windows, doors, lights, shelves, electrical panels and garage-door-opener outlets in the path of tracks all affect installation.
Measurement record sheet
| Measurement | Reading 1 | Reading 2 | Reading 3 | Use |
|---|---|---|---|---|
| Opening width | Top: ____ | Middle: ____ | Bottom: ____ | Smallest |
| Opening height | Left: ____ | Centre: ____ | Right: ____ | Smallest |
| Headroom | ____ | Lowest point | ||
| Side room left | ____ | — | ||
| Side room right | ____ | — | ||
| Backroom | ____ | — |
Tip: print this page (Ctrl/Cmd + P) to take the sheet to your garage.
Common measuring mistakes
- Measuring the old door instead of the opening. US doors are sized to the opening.
- Taking one reading. Openings are rarely perfectly square.
- Forgetting the opener. It needs extra headroom and backroom.
- Ignoring obstructions. A single light fixture can block a track.
- Mixing units. Stick to one system and write the unit next to each figure.
Measuring in metric
The process is identical in millimetres. In the UK, also check whether a supplier’s size refers to the door, the internal frame or the overall frame — suppliers such as Garador publish separate figures for each. Convert figures quickly with the size calculator.
Confirm before ordering
Requirements differ between products. Use the manufacturer’s instructions for the specific door, e.g. Clopay’s measuring guide, and have your installer verify on site.
Frequently asked questions
Should I measure the old door or the opening?
Measure the finished opening. US doors are typically sized to the opening, not to the old door leaf.
What if my opening is not square?
Take measurements at the top, middle and bottom (width) and left, centre and right (height). Use the smallest values and note any difference for your installer.
Which tools do I need?
A tape measure, a level, a notepad or the checklist on this page, and a step ladder for headroom measurements.
Ready to replace or install a door?
Have a professional confirm your measurements, clearances and structural requirements before you order.