How to Measure a Door Opening Correctly Before Ordering Metal

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To measure a door opening, take the width at the top, middle and bottom, the height at the left, center and right, and the depth of the frame, all in millimeters. Use the smallest figures for the frame opening and note any out-of-square or uneven surfaces, since made-to-measure metal doors are built to these exact dimensions.

Why exact measurements matter for metal doors

Iron and aluminum doors are manufactured to measure, which means every frame is cut and welded to your specific opening. Unlike standard prefabricated doors, there is no room to trim a metal door on site without affecting its finish and strength.

Walls in older buildings are rarely perfectly straight or level. A few millimeters of variation can decide whether a door swings smoothly, seals properly and closes without rubbing. Careful measuring at the start saves time for everyone involved.

Tools you need

You do not need specialized equipment, but accuracy depends on using the right tools:

A steel measuring tape, preferably 5 meters long
A spirit level or a long straight edge
A pencil and a notebook or your phone for photos
A helper to hold the tape at the far end of wide openings

Measuring the width

Measure the width from one side of the opening to the other, from the inside edge of the frame or wall. Take the measurement at three points: near the top, in the middle and near the bottom. Walls often lean, so the smallest of these three numbers is the one to write down for the frame opening.

Repeat the process once more to confirm. Write the figure in millimeters and mark which point it came from.

Measuring the height

Measure from the finished floor to the underside of the lintel or ceiling above the opening. Take readings at the left side, the center and the right side. Again, record the smallest reading. If the floor is not yet finished, note this clearly, because the final floor level will change the usable height.

If there is a threshold or a sill, measure it separately so the door can clear it without dragging.

Measuring depth and the wall thickness

Depth is the distance through the wall, from the inside face to the outside face of the opening. This tells the fabricator how thick the frame needs to be and how the door will sit in the reveal. Measure at the top and bottom of the opening, since thickness can differ.

Also note on which side the door will open, whether it swings into the room or outward, and which side the hinges should be on. Standing inside the room facing the door helps you define this consistently.

Checking for square and level

Use your spirit level to check whether the top of the opening and the floor are level, and whether the vertical sides are plumb. Measure both diagonals of the opening, from corner to opposite corner. If the two diagonal figures differ noticeably, the opening is out of square.

Do not correct the wall yourself. Record what you find, take photos, and let the manufacturer and installer account for it in the drawing.

Preparing your specification for the order

Once you have your figures, write them on a simple sketch of the opening with the width, height, depth and any notes. Include photos from the inside and outside, and mention the swing direction and the finish you have in mind.

Dimensions in millimeters, with the measuring points noted
Diagonals and any levelness findings
Swing direction, handing and whether the door is single or double

A clear written specification lets the factory draw the door correctly before production begins.

Frequently asked questions

Should I measure the width or the height first?

The order does not matter as long as you record every figure carefully. Many people start with the width, then the height, then the depth, and then check the diagonals last.

Do I measure the old door or the opening?

Measure the opening, meaning the space the frame will occupy, not the old door. The old door may have been installed with different clearances or may not fit the opening correctly.

What if my opening is slightly out of square?

This is common and not a problem in itself. Record the diagonal measurements and any level differences so the fabricator can plan the frame accordingly.

Can I send photos instead of measuring myself?

Photos help us understand the site, but precise measurements are still needed. We usually arrange an on-site measurement to confirm the figures before drawing the door.

Common mistakes to avoid

The most common error is measuring only once and using the largest or most convenient figure instead of the smallest reading. Forgetting to note an unfinished floor or an out-of-square wall also leads to doors that do not fit.

Our recommendation

Measure every dimension three times, write down the smallest figure for the opening, and always include a photo and a sketch with your order so the drawing matches the real wall.

Summary

Measure width, height and depth at several points, check the diagonals and levelness, and write everything down in millimeters with photos. Accurate figures are the foundation of a made-to-measure metal door that fits well.

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