How centimetres become pixels
The conversion is pixels = size in inches × DPI, and one inch is 2.54 cm or 25.4 mm. So 3.5 cm at 300 DPI is 3.5 ÷ 2.54 × 300 ≈ 413 pixels, and 4.5 cm is ≈ 531 pixels. The same physical size at 96 DPI — a screen resolution — would be only 132 × 170 pixels, which is why choosing the wrong DPI produces a photo that looks right but fails a form's pixel check. When a form gives centimetres without a DPI, 300 is the safe default. The cm to pixels calculator shows the conversion for any size.
Common sizes at 300 DPI
3.5 × 4.5 cm (35 × 45 mm) → 413 × 531 px, the standard rectangular passport and application photo in much of Asia and Europe. 2 × 2 in (about 51 × 51 mm) → 600 × 600 px, the square US format. 5 × 5 cm → 591 × 591 px. 6 × 2 cm → 709 × 236 px, a common signature box. Millimetres work the same way — select "mm" and type the figure your form gives.
Crop, stretch or pad
If your photo's shape doesn't match the target, choose how to handle it: Crop to fill keeps proportions and trims the edges (best for portraits), Stretch forces the exact size and can distort faces, and Fit inside, pad white keeps the whole image and adds white borders. After resizing, switch to the Exact KB tab on the same image if the form also sets a file-size limit — resize first, then compress.
Frequently asked questions
How do I resize a photo to 3.5 cm × 4.5 cm?
Open this page — it's already set to 3.5 × 4.5 cm at 300 DPI. Choose your photo, keep 'Crop to fill exactly', and press Resize. The result is 413 × 531 pixels, which is 3.5 × 4.5 cm when printed at 300 DPI.
Is 35 × 45 mm the same as 3.5 × 4.5 cm?
Yes. They are identical sizes in different units, and both produce 413 × 531 pixels at 300 DPI.
What DPI should I choose?
Use the DPI your form states. If it gives only a physical size, choose 300 DPI — the standard for photo prints and what most upload validators assume.
Does resizing change the file size in KB?
Yes — fewer pixels usually means fewer kilobytes. If the form also has a KB limit, resize first, then switch to the Exact KB tab and compress to the limit.