If a form asked you for a 51 × 51 mm photo — or a 5.1 × 5.1 cm one — it is asking for the American 2 × 2 inch square under a metric name. Two inches is exactly 50.8 mm, and the official specs round it to 51. Photo labs outside the US quote the metric figure; the documents that use it — the US passport, the printed US visa photo, the India OCI print — publish the inch figure. Same photo either way. This page shows the arithmetic, the full spec behind the square, and what 51 × 51 mm becomes in pixels when the application moves online.
Turn any picture into the 51×51 mm photoThe arithmetic
One inch is defined as exactly 25.4 mm, so 2 inches is 50.8 mm — and 51 mm divided back by 25.4 is 2.008 inches. The rounding the specs apply is two tenths of a millimeter, smaller than the trim tolerance of any photo cutter. Every row below is the same square:
| Inches | 2 × 2 in |
|---|---|
| Millimeters | 50.8 × 50.8 mm exactly (2 × 25.4) — specs round to 51 × 51 mm |
| Centimeters | 5.1 × 5.1 cm |
| Pixels at 300 dpi | 600 × 600 px (2 × 300) |
| Pixels at 600 dpi | 1200 × 1200 px (2 × 600) |
The two pixel rows are not a coincidence of round numbers — they are the print scanned at the two common resolutions, and they matter the moment an application goes digital (more on that below).
One size, two names
The square is an American standard, so its home unit is the inch — the State Department writes 2 × 2 in and adds 51 × 51 mm in parentheses. Outside the US the parenthesis becomes the headline: a photo lab in a metric country prints “51 × 51 mm” on the price list, an application checklist says “5.1 × 5.1 cm,” and nothing on either mentions inches. Searchers on each side of that line often never find the other name.
Everything about the photo itself — what the head must fill, where the eyes sit, what the background must be, where to print one and what the stores charge — lives on this page’s imperial twin: the 2x2 inch photo size page. The two pages describe one photo; this one exists so the metric name finds it too.
The specification
The frame is the easy half. The US passport spec — the standard the 51 × 51 mm figure comes from — also fixes what sits inside it, including a head height of 1–1⅜ in (25–35 mm). These are the same rows our checks measure against:
| Photo size | 2×2 in (51×51 mm) · 600×600 to 1200×1200 pixels, square |
|---|---|
| File size | 54 KB to 10 MB |
| Format | JPEG |
| Head height | 50–69% of the photo's height (1 to 1⅜ in / 25–35 mm) |
| Eye height | 56–69% up from the bottom of the photo |
| Background | Plain white or off-white |
| Recency | Taken within the last 6 months |
The full rules in plain English are on the US passport photo requirements page. Already holding a photo cut to 51 × 51 mm? The free US passport photo checker measures it as-is — head size, eye height, background — and alters nothing.
The documents
What it is not: the 35 × 45 mm portrait the UK, the Schengen states and much of the rest of the world use. Different shape, different head placement — the passport photo size chart maps every country’s frame in one table.
Going digital
A physical size has no pixel count until you pick a resolution — the two rows above are this square at 300 and 600 dpi, and that 600 × 600 to 1200 × 1200 span is the DS-160 upload's own window, detailed on the 600x600 photo page.
Questions
Yes — they are one size with two names. Two inches is exactly 50.8 mm (2 × 25.4), and the official specs round that to 51 mm. The 0.2 mm difference is a rounding convention, about the thickness of two sheets of paper — no photo counter or portal treats the two figures as different sizes.
The ones our sources confirm: the US passport print (2 × 2 in / 51 × 51 mm, head 1–1⅜ in), the printed photo for a US visa application (the DS-160 upload is its digital 600 × 600 counterpart), and the India OCI print. Individual consulates elsewhere sometimes ask for a 2 × 2 photo too — Brazil’s Miami consulate is a documented case — but those rules vary by post, so confirm with yours.
It depends on the resolution. At 300 dpi the square is 600 × 600 pixels; at 600 dpi it is 1200 × 1200. That range is exactly what the DS-160 US visa upload accepts: any square JPEG from 600 × 600 to 1200 × 1200 pixels, 240 KB or smaller.
Yes. 5.1 cm is 51 mm — centimeters and millimeters are the same measurement at a different decimal point. Forms written in cm, mm, or inches (2 × 2 in) are all naming the same square photo.
No. 35 × 45 mm is a smaller portrait rectangle — the shape the UK and the Schengen states use — while 51 × 51 mm is a square. The two specs also place the head at different heights, so a photo composed for one cannot be trimmed into the other; it needs recropping around the face.
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