Size a photo for a passport or visa
Crop to an official size, and write the resolution into the file so it prints at that size rather than whatever the printer assumes.
How it works
A passport photo gets rejected for one of two reasons. Either the picture is wrong (eyes closed, a shadow on the wall, a smile) or the geometry is wrong. Only the second kind is a file problem, and it is the kind a validator catches before any human looks at the application. This page handles the geometry: the crop, the proportions, the pixel count, and the physical size the file claims to be.
The measurements, and where they come from
The United States wants 2 x 2 inches, which is 51 x 51 mm, with the head between 1 and 1 3/8 inches (25 to 35 mm) from the bottom of the chin to the top of the head. The digital version must be square and between 600 x 600 and 1200 x 1200 pixels. The United Kingdom, the Schengen area and India all use 35 x 45 mm. Canada is the outlier at 50 x 70 mm, with the head 31 to 36 mm.
Those three 35 x 45 mm specifications are not interchangeable, because the head size differs. The UK asks for 29 to 34 mm chin to crown. Schengen follows ICAO Doc 9303 and asks for 32 to 36 mm, described as the face filling 70 to 80 percent of the frame. India's Passport Seva wants the face to cover roughly 75 to 80 percent. Same rectangle, three different faces inside it, so choose the preset for the country you are applying to rather than the one whose millimetres match.
Pixels become a size only when you name a DPI
A 600 x 600 PNG has no inherent size. It is 600 pixels across, and how wide that is on paper depends on how many pixels the printer is told to lay down per inch. At 300 the print is 2 inches; at 72 it is a little over 8; with no instruction at all the printer, the photo kiosk and the print shop's software each guess, and they do not guess alike.
So the number is written into the file. PNG output carries a pHYs chunk holding pixels per metre: 11811 for 300 DPI, 23622 for 600. JPEG output carries it in the JFIF header's density fields, with the unit byte set to inches. Both are read by print dialogs, by Preview, by Windows Photos and by every layout application. Open the file's properties afterwards and the print size already says 2 x 2 inches.
What this cannot check
Head height is the requirement that fails the most applications, and this tool cannot verify it. Measuring chin to crown needs face detection, and a wrong answer from a face detector is worse than no answer, because it invites you to trust it. What you get instead is a ruler. The crop frame carries a solid line where the top of the head should sit and a shaded band where the chin should land, drawn from the published range for the preset you picked. Line the face up against them yourself.
Everything else about the picture is yours too: the neutral expression, the plain background, the absence of glasses in EU and UK photos, the recency rule (one month for the UK, six elsewhere), and Canada's requirement that a commercial photographer stamp the back of the print. A correctly sized file with a smile in it still comes back.
Background, and the countries that do not want white
The background lift raises pixels that are already near white the rest of the way, which cleans up a photo shot against a white wall that came out grey or faintly blue. It works on luminance with a soft ramp, so it does not cut a hard edge around the subject, and it will lift a white collar or grey hair along with the wall if you push it far. The result note reports what fraction of the image it moved, so you can tell whether it stayed on the wall.
Leave it at zero for the United Kingdom. HM Passport Office asks for a light grey or cream background rather than a white one, and a photo whitened to 255 is further from that specification than the wall you started with. Canada accepts white or light-coloured. The US and Schengen want white or off-white, so a modest lift is usually safe there.
Printing a sheet
A single photo is what an online application wants. A counter application wants prints, and the cheapest way to get them is to send a 6 x 4 inch photo print to any lab or kiosk and cut it up. The sheet option tiles the crop edge to edge on a white 6 x 4 at your chosen DPI: six copies of a 2 x 2, eight of a 35 x 45, four of a 50 x 70. Faint grey lines mark the cuts. An A4 sheet is there for a home printer.
Questions
Which preset for a US visa, a DS-160 or the Diversity Visa lottery?
The US 2 x 2 inch preset. US visa photos use the same square 51 x 51 mm geometry as the passport photo. What differs is the byte limit: the Diversity Visa entry caps the upload at 240 KB, and online passport renewal wants between 54 KB and 10 MB. This tool targets dimensions rather than file size, so if the JPEG lands too large, hand it to the exact-size tool linked below.
My 35 x 45 mm photo came out 413 x 531 pixels and the UK site rejected it.
That is the 300 DPI answer and it is right for a print. The UK's online service is not asking for a print. It wants at least 600 pixels wide by 750 tall, and 35 x 45 mm only reaches that at 600 DPI, which gives 827 x 1063. Switch the DPI to 600. The physical size recorded in the file stays 35 x 45 mm, so the same file still prints at the correct size.
Does the DPI setting change how the photo looks?
Not on screen, and the pixels are identical either way. The DPI is a label saying how densely to lay them down. What changes here is the pixel count, because the tool works backwards from a physical size: 50.8 mm at 300 DPI is 600 pixels, at 600 DPI it is 1200. So 600 does give a finer print, provided the original photo holds enough detail to fill it.
Can I use a phone selfie?
The geometry will be fine. The optics may not be. A selfie held at arm's length puts the lens close enough to widen the nose and pull the ears back, and that distortion is exactly what a biometric check is looking at. Have someone stand two or three metres away and zoom in instead, against a plain wall, with even light and nothing casting a shadow behind the head.