You can hold a photograph at arm's length and say with confidence that the background is grey, the eyes are open and nobody is smiling. You cannot say whether the head is 34 mm tall. Nobody can. That is why head height accounts for so many returned applications.
What is being measured
The measurement runs from the bottom of the chin to the top of the head. The top of the head means the crown, including hair that lies against the skull - not the tip of a loose curl, not a ponytail, not hair that is standing up.
That distinction matters more than it sounds. Someone with volume in their hair can measure four or five millimetres taller than they think, which is more than the entire width of the allowed band in most countries.
The two ways it is written down
In millimetres. The 35 by 45 mm world usually asks for a head between 32 and 36 mm. The UK is stricter and narrower, at 29 to 34 mm. Canada, on its larger 50 by 70 mm print, asks for 31 to 36 mm.
As a proportion. The US specifies 1 to 1 3/8 inches on a 2 inch square, which people usually restate as the head occupying 50 to 69 percent of the image height. India and several Asian authorities describe it as the face filling 70 to 80 percent of the photo - note that this phrasing sometimes means the face rather than chin-to-crown, which is a different measurement again.
Where two documents use different wording, they are usually still measuring chin to crown. Where a rule says "face", read the official page carefully before assuming.
Why the band is narrow
The rules exist because the photo is not decorative. It goes into a chip, gets compared against a live face at a border, and is measured by software that expects the facial landmarks in predictable places. A head that is too small gives the algorithm too few pixels to work with; a head that is too large has usually lost the top of the skull or the bottom of the chin to the crop, and those are landmarks the comparison needs.
Four millimetres of tolerance on a 45 mm print is under ten percent. It is a real engineering constraint, not bureaucratic taste.
Why phone photos miss it
Two reasons, and they pull in opposite directions.
Taken at arm's length, a selfie puts the camera about 50 cm from the face. At that distance a wide phone lens enlarges the nose and narrows the ears - the same effect that makes a fisheye lens comical, in a milder form. Even when the head height is right, the face is subtly the wrong shape, and biometric comparison is exactly the thing that notices.
Taken by someone else from a normal distance, the head is usually far too small in the frame, because people frame a portrait the way portraits look good: head and shoulders, some space around them. Cropping in fixes the proportion but throws away pixels, and if the original was framed loosely enough there may not be enough resolution left.
The fix for both is the same: stand about a metre and a half to two metres away, and use the rear camera rather than the front one.
How to hit the number
- Frame wider than you need. It is always possible to crop in and never possible to invent pixels. Leave headroom.
- Get the head straight first. A tilted head measures taller from chin to crown than the same head upright, so levelling before measuring saves a round trip.
- Flatten the hair, or account for it. Loose volume at the crown is measured as part of the head by some authorities and not by others.
- Measure rather than estimate. Guides that show crown, eye line and chin with live figures turn this from a judgement into a reading.
IDShot places the head inside the allowed band automatically from face and hairline detection, draws the three guides with live measurements beside them, and lets you nudge the crop in half-millimetre steps when a value sits close to the edge of its range.
When the head is too small to fix
If the face occupies only a small part of a photograph you already have, cropping to passport proportions can leave you below the pixel count the portal requires. There is no honest way around that other than taking a new photo - upscaling introduces detail that was never recorded, and the result looks exactly like what it is.