How to Measure a Door Opening Correctly Before Ordering

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To measure a door opening, record the width at the top, middle and bottom, the height at the left, center and right, and the depth of the wall or frame. Use the smallest measurements as your reference and note whether the floor is finished or raw, since both affect the final fit.

Why Accurate Measurements Matter for a Custom Door

A custom iron or aluminum door is made to fit one specific opening, not a standard size from a shelf. Even a few millimeters of error can mean a door that rubs against the frame, leaves a gap for drafts, or cannot be installed at all. Because the work is manufactured in the factory before it reaches the site, the measurements you send become the basis for the entire drawing.

Taking time to measure carefully at the beginning saves the cost and delay of remaking a part. Treat the measuring step as the most important stage of your preparation.

Tools You Need Before You Start

Prepare simple tools that give reliable readings and keep a notebook or a phone to record each figure as you take it.

A steel measuring tape, preferably 5 meters long
A spirit level or a long straight edge
A pencil and paper for a rough sketch
A helper to hold the tape on the far side of the opening
A camera to photograph the opening from the front and the side

A rigid steel tape is more accurate than a flexible fabric one, which can stretch or sag over long spans.

Measuring the Width

Measure the width from one inside edge of the opening to the other. Take three readings: at the top, in the middle and at the bottom. Walls are rarely perfectly straight, so these readings often differ slightly.

Record all three numbers
Use the smallest value as the controlling width for the door leaf
Mark any difference larger than a few millimeters for discussion with the manufacturer

If the opening is out of square, note which side is wider so the door can be adjusted accordingly.

Measuring the Height

Measure the height from the floor to the underside of the lintel or the top of the frame. Take readings at the left side, the center and the right side. As with the width, the smallest reading is the one that matters most for fit.

Check whether the floor is finished or still rough. If tiles, screed or flooring will be added later, the height changes and this must be stated clearly in your order. Measure again after the floor work is complete if possible.

Checking Depth, Plumb and Level

Measure the depth of the wall or the existing frame, since it determines how the door frame sits and which fixing method is suitable. Use the spirit level to check whether the vertical sides are plumb and the top and bottom are level.

An out-of-plumb opening is common in older buildings. Record the deviation rather than trying to correct it yourself, because the installer may need to allow for it in the design.

What to Send With Your Measurements

Good measurements alone are not always enough. Send a clear set of information so the drawing can be prepared correctly.

Written measurements for width, height and depth with all readings
A sketch showing which side is the hinge side and the swing direction
Photos of the opening from the front and from the side
Notes on the floor finish and any obstacles nearby

Clear information speeds up the drawing and approval stage considerably.

Frequently asked questions

Should I measure the width at the top or the bottom?

Measure at the top, middle and bottom and use the smallest reading as your reference. This ensures the door will still fit at the narrowest point of the opening.

Do I measure the opening before or after the floor is finished?

Measure after the floor is finished if possible, because flooring changes the height. If that is not possible, state clearly that the floor is still rough so the allowance can be planned.

Is a flexible tape measure accurate enough?

A rigid steel tape is more reliable for door openings because it does not sag or stretch over long distances. Use a helper to keep the tape straight and taut.

What if my opening is not square?

Record the differences between the sides and the diagonals and send photos with your measurements. The manufacturer can then account for the irregularity in the drawing.

Common mistakes to avoid

The most common errors are measuring only once, using a single reading instead of three, and forgetting to note whether the floor is finished. Each of these can lead to a door that does not fit the opening as expected.

Our recommendation

Measure every dimension at least twice, write the figures down immediately, and send photos with a simple sketch. If the opening is irregular or the floor will change, mention it clearly so the drawing reflects the real site conditions.

Summary

Accurate measurement of width, height and depth, taken at several points and recorded clearly, is the foundation of a well-fitting custom door.

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