How many pixels is 3.5 × 4.5 cm?

At 300 DPI it is about 413 × 531 pixels. Use pixels = cm × 300 ÷ 2.54 for any other size, and resize to that with the resize tool.

Does changing DPI change the file size?

Changing only the DPI tag does not resample the image, so the file size barely moves. What changes the file is the pixel count (resizing) and the compression — see compressing without quality loss.

Why does my image look fine on screen but blurry in print?

On screen only pixels matter, so a small image looks crisp. Stretched to a physical size below 300 DPI it has too few pixels per inch and blurs. Resize up to the 300 DPI pixel count before printing.

What is the difference between DPI and PPI?

PPI (pixels per inch) describes the digital image; DPI (dots per inch) describes the printer. In everyday tools the two terms are used interchangeably for the same setting.

Are my images uploaded to a server?

No. The resize, DPI, crop and compress tools all run in your browser, so your image stays on your device.

How to resize an image in cm or mm

Screens think in pixels; printers and forms think in centimetres. The bridge between them is DPI, and once you understand it, resizing to an exact 3.5 × 4.5 cm or 5 × 5 cm becomes simple arithmetic. This guide gives you the formula, a ready-made conversion table, and the steps to hit any physical size with the resize tool and the DPI tool.

Updated June 23, 20266 min read

Resize to exact pixel dimensionsEnter the pixel size from the table below and resize in your browser — nothing uploaded.

The cm-to-pixel formula

An image has no fixed physical size on its own — only a pixel count. It gains a centimetre size the moment you pair it with a DPI (dots per inch). The conversion is:

pixels = centimetres × DPI ÷ 2.54
centimetres = pixels × 2.54 ÷ DPI

So a 3.5 × 4.5 cm photo at 300 DPI is (3.5 × 300 ÷ 2.54) × (4.5 × 300 ÷ 2.54) ≈ 413 × 531 pixels. Resize to that pixel size and the physical dimensions are correct. For millimetres, divide by 10 to get centimetres first.

What DPI should you use?

Choosing a DPI for the job
Use	DPI	Why
Print (photos, documents, exam forms)	300	The standard for crisp print; most forms assume it
Large posters viewed from afar	150	Lower detail is fine at distance; smaller file
Screen / web only	72–96	DPI is irrelevant on screen; only pixels matter

Choosing a DPI for the job

Set or check the embedded DPI value with the DPI tool. Remember: changing the DPI tag alone does not resample the image — to truly hit a physical size you resize the pixels to the value from the formula above.

Ready conversion table (at 300 DPI)

Common physical sizes in pixels at 300 DPI
Physical size	Pixels (300 DPI)	Typical use
3.5 × 4.5 cm	≈ 413 × 531 px	Passport / exam photo
2 × 2 in (5.08 × 5.08 cm)	600 × 600 px	US visa, square photo
4 × 2 cm	≈ 472 × 236 px	Signature box
5 × 5 cm	≈ 591 × 591 px	Larger ID photo
10 × 15 cm (4 × 6 in)	1200 × 1800 px	Standard print
21 × 29.7 cm (A4)	2480 × 3508 px	Full-page document

Common physical sizes in pixels at 300 DPI

Step by step

Decide the physical size you need in cm or mm, and the DPI (300 for print, unless told otherwise).
Convert to pixels with the formula, or read it straight off the table above.
Open the resize tool and enter those pixel dimensions. Crop first with the crop tool if the aspect ratio does not match.
If a printer or form needs the DPI tag set to 300, apply it with the DPI tool.
If a form also caps the file size, compress to the KB limit last, after the dimensions are correct.

How to resize an image in cm or mm

The cm-to-pixel formula

What DPI should you use?

Ready conversion table (at 300 DPI)

Step by step

Frequently asked questions

Related guides

Related tools