Forms and print shops describe sizes in different units. One application says "photograph of 3.5 cm × 4.5 cm", another says "200 × 230 pixels", a visa site mentions "2 × 2 inches", and a printing job asks for "300 DPI". Digital images, however, only have pixels. To get the right physical size, you need to understand how centimetres, millimetres and inches relate to pixels through DPI. This guide explains the conversion in plain language, gives a ready reference table, and shows how to resize an image accurately.
Use the official specification for your form. The sizes below are examples to explain the maths, not official requirements. Exams, passport and visa authorities each publish their own dimensions; always follow the current official notice.
Pixels, DPI and physical size
A digital image is a grid of pixels, for example 600 × 800. It has no physical size until it is printed or displayed. DPI (dots per inch, often used interchangeably with PPI, pixels per inch) tells a printer how many of those pixels to place in one inch of paper.
- At 300 DPI, 600 pixels print as 2 inches.
- At 150 DPI, the same 600 pixels print as 4 inches, but less sharply.
So the formula is simple:
pixels = inches × DPI, and since 1 inch = 2.54 cm = 25.4 mm:
pixels = (cm ÷ 2.54) × DPI or pixels = (mm ÷ 25.4) × DPI.
Worked examples
| Physical size | At 300 DPI | At 200 DPI |
|---|---|---|
| 3.5 × 4.5 cm (35 × 45 mm) | 413 × 531 px | 276 × 354 px |
| 2 × 2 inches (51 × 51 mm) | 600 × 600 px | 400 × 400 px |
| A4 page (21 × 29.7 cm) | 2480 × 3508 px | 1654 × 2339 px |
| 4 × 6 inch photo print | 1200 × 1800 px | 800 × 1200 px |
| 5 × 3.5 cm signature box | 591 × 413 px | 394 × 276 px |
Pixel values are rounded to the nearest whole number. If a form gives both pixels and centimetres, the pixel value is what the upload system checks.
Step-by-step: resize to an exact size
- Work out the target pixel size using the formula above, or take it directly from the form if pixels are given.
- Crop the image to the right aspect ratio first. A 3.5 × 4.5 cm photo has a ratio of 7:9. Use Crop Image, which offers common presets, or type a custom ratio.
- Open Resize Image and enter the width and height in pixels. Keep "maintain aspect ratio" on if you already cropped correctly.
- Download and check the dimensions in your file properties.
- If a KB limit applies, compress afterwards with Compress Image, or do both at once with Photo & Signature Resize.
For passport-style photos specifically, Resize Photo for Passport uses the 413 × 531 px preset described in our passport photo guide.
Why crop before resizing
If you resize a 4:3 phone photo directly to 7:9, the image is either stretched (faces look wider or thinner) or padded with borders. Cropping to the correct shape first, then resizing, keeps proportions natural. This is the most common cause of distorted photos in applications.
Does DPI metadata matter?
Image files can store a DPI value in their metadata, but most upload systems ignore it and look only at pixel dimensions and file size. Printers and print shops do use it to decide the default print size. If you print at home, set the print size explicitly in the print dialog (for example 3.5 × 4.5 cm) or print a PDF at "Actual size". Our guide to passport photos explains how to fit several on one sheet.
Upscaling: making small images bigger
You can increase pixel dimensions, but you cannot add real detail. A 200 × 250 px image enlarged to 600 × 750 px will look soft. If a form asks for larger dimensions than your image has, it is better to retake the photo or rescan the document at a higher resolution. For documents, scanning at 300 DPI gives enough pixels for almost any purpose.
Screen sizes are different
For social media, websites and WhatsApp, physical size and DPI do not matter; only pixels do. Common targets are 1080 × 1080 px for Instagram posts, 1280 × 720 px for YouTube thumbnails and 500 × 500 px for profile pictures. Ocr4you has presets for these: Instagram, YouTube thumbnail and WhatsApp DP.
Format and file size
After resizing, choose the right format. Photos are best saved as JPG; graphics and screenshots with sharp edges as PNG. Our PNG vs JPG guide explains why. If the image comes from a website as WEBP, convert it first with Image to JPG.
Quick reference
- 1 inch = 2.54 cm = 25.4 mm.
- pixels = cm × DPI ÷ 2.54.
- 300 DPI is standard for photo prints; 150–200 DPI is acceptable for documents.
- Crop to the aspect ratio first, then resize, then compress.
- Upload systems check pixels and KB, not DPI metadata.
Common questions
The form says 200 DPI but gives no pixel size
Multiply the physical size in inches by 200, or use the cm formula. If no physical size is given either, contact the issuing body or check the FAQ section of the official notice.
My image is the right size but the portal says invalid dimensions
Some portals require exact values, others ranges. Check whether width and height are swapped, and whether the portal expects a specific orientation.
Need the Hindi version of these steps for photos? Read फोटो का साइज़ KB में कैसे कम करें.
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