Forms and print shops think in centimetres and inches; screens and image files think in pixels. When a notification says "photograph 3.5 cm × 4.5 cm" or "signature 6 cm × 2 cm", it is describing a physical size, and to produce a digital file you need to know how many pixels make up each centimetre. That missing link is DPI — dots (pixels) per inch. Once you know it, the conversion is simple arithmetic, and this tool does it for you and applies it in one step.
This page resizes and crops any image to an exact size in centimetres, millimetres, inches or pixels, at the DPI you choose, and writes that DPI into the file so software that reads it (including print dialogs) knows the intended physical size.
Why do cm and inches need a DPI?
A digital image has no physical size by itself — it is a grid of pixels. "4.5 cm tall" only becomes a pixel count once you decide how densely those pixels are packed. At 100 DPI, a 4.5 cm photo is 177 pixels tall; at 300 DPI it is 531 pixels tall. Both are "4.5 cm" when printed at their own DPI. That is why a requirement that gives centimetres without a DPI is ambiguous. If your notification gives both (for example "3.5 × 4.5 cm, 200 DPI"), use both. If it gives only centimetres, 200 DPI is common for online forms and 300 DPI is standard for printing; many forms also give a pixel size, which removes the ambiguity entirely.
How do I convert cm, mm and inches to pixels?
The formula is the same for every unit — convert to inches, then multiply by DPI:
- From cm: pixels = cm ÷ 2.54 × DPI
- From mm: pixels = mm ÷ 25.4 × DPI
- From inches: pixels = inches × DPI
- Back to cm: cm = pixels ÷ DPI × 2.54
Results are rounded to whole pixels, so a size can come out a pixel different from a table that rounds differently. Portals that check exact pixels will quote the pixel size themselves; use that when it is given.
What are common sizes like 3.5 × 4.5 cm and 6 × 2 cm in pixels?
| Physical size | At 100 DPI | At 200 DPI | At 300 DPI | At 600 DPI |
|---|
| 3.5 × 4.5 cm (passport / form photo) | 138 × 177 | 276 × 354 | 413 × 531 | 827 × 1063 |
| 3.5 × 1.5 cm (signature) | 138 × 59 | 276 × 118 | 413 × 177 | 827 × 354 |
| 6 × 2 cm (signature) | 236 × 79 | 472 × 157 | 709 × 236 | 1417 × 472 |
| 5 × 6 cm | 197 × 236 | 394 × 472 | 591 × 709 | 1181 × 1417 |
| 2 × 2 inch (5.08 × 5.08 cm) | 200 × 200 | 400 × 400 | 600 × 600 | 1200 × 1200 |
| 2 × 2.5 cm (stamp size) | 79 × 98 | 157 × 197 | 236 × 295 | 472 × 591 |
Calculated with pixels = cm ÷ 2.54 × DPI, rounded to the nearest pixel.
These are the documents in our data set that give a physical size, with the DPI and pixel size we use for them. The table is generated from the same data file as the editor's presets:
Documents in data/presets.json specified in cm/mm. Verify with the official notification.
A pattern worth noticing: most of them use 200 DPI for online uploads. The new SSC module's signature, for example, is 6.0 × 2.0 cm at 200 DPI, which is 472 × 157 px.
Crop, fit or stretch — which should I choose?
- Crop to fill scales the image until it covers the whole target, then trims whatever sticks out. Nothing is distorted and there are no borders. This is right for photos, as long as the part you care about is inside — use the crop position to choose which side to keep.
- Fit inside scales the image until it fits entirely, and fills the leftover area with white. Nothing is cut off, which suits signatures, documents and logos whose shape differs from the target.
- Stretch forces the image into the target shape. It distorts faces and text and is almost never what you want for an application form — it is there for cases where the source already has the right proportions.
What DPI should I use?
Use the DPI the notification gives. If it gives only a physical size, think about where the image will end up:
- Online upload forms: 200 DPI is the value most often paired with centimetre sizes in our data. It gives a readable face at 3.5 × 4.5 cm (276 × 354 px) while keeping the file small enough for tight KB limits.
- Printing at a shop or at home: 300 DPI is the usual standard for photo prints. A 35 × 45 mm print at 300 DPI needs 413 × 531 px; the passport size photo maker lays those out on A4 or 4×6 sheets.
- Very large prints or archiving: higher DPI (600) only helps if your source photo actually has that much detail. Upscaling a small phone crop to 600 DPI adds pixels but not sharpness.
Remember that DPI is also stored as a small metadata value inside JPG and PNG files. Some portals read it; most ignore it and look only at the pixel count. This tool sets both consistently, so the file is correct either way. WebP has no standard DPI field, which is why the DPI is not shown for WebP output.
How do I resize in cm and KB at the same time?
Set the size and DPI first, then type a maximum KB. The tool searches for the highest JPEG (or WebP) quality that fits under the limit and tells you whether it made it. If it cannot, reduce the DPI or the size — halving the DPI quarters the pixel count. If the form also has a minimum KB and your file comes out too small, run it through increase image size in KB, which adds bytes without touching the pixels. PNG ignores the KB limit because it is lossless.
Can I resize in cm in Paint, Word or Canva?
Paint's resize dialog works in percent and pixels, not centimetres, so you have to do the conversion yourself, and it will stretch the image if the proportions differ. Word lets you set a picture's size in centimetres on the page, but that changes how it is displayed, not the pixels in the file, so a portal still sees the original image. Canva can create a design in cm, but exporting at a chosen DPI and file size takes extra steps. This tool changes the actual pixels, crops without distortion, embeds the DPI and can cap the KB — in one place, in your browser.
Physical sizes and DPI requirements vary between forms and are revised between notifications. Verify with the official notification before you upload.