How to Resize a Photo in cm or Inches

A form asks for a photo "3.5 x 4.5 cm". A print shop wants an image "at 10 x 15 cm, 300 DPI". But image editors talk in pixels, and your phone photo is something like 4032 x 3024 pixels. How do you get from one to the other?

The short answer: a digital image has no physical size until you decide how many pixels go into each inch. That number is the DPI. Once you know it, the maths is simple.

Pixels, print size and DPI, explained

Pixels are the tiny squares an image is made of. A 4032 x 3024 photo is 4032 pixels wide and 3024 tall. This is the only size a digital file really has.

Print size is the physical size on paper, in centimetres, millimetres or inches. The same file can be printed as a stamp or a poster.

DPI (dots per inch, often used interchangeably with PPI, pixels per inch) is how many pixels are placed in each inch of paper. More pixels per inch means a smaller, sharper print; fewer means a bigger, softer one.

• 300 DPI is the common target for photos and documents you print and look at up close, including passport and ID photos.
• 150 DPI is often fine for larger prints viewed from further away.
• 72 DPI is a value you will see on screen images; for screens, only the pixel size matters.

A JPG or PNG file can store a DPI value inside it. It does not change the pixels; it tells printing and layout apps what physical size to use by default. That is why a print shop may ask for "300 DPI": they want enough pixels for the size, and often the right value written in the file.

The formula

To work out the pixels you need:

1. Convert the size to inches: cm / 2.54, or mm / 25.4.
2. Multiply by the DPI: pixels = inches x DPI.
3. Round to the nearest whole pixel.

To go the other way (what print size will my photo give?): inches = pixels / DPI, then cm = inches x 2.54.

Worked example: 3.5 x 4.5 cm at 300 DPI

This is the size many countries use for passport and ID photos.

• Width: 3.5 cm / 2.54 = 1.378 inches, and 1.378 x 300 = 413.4, so 413 pixels.
• Height: 4.5 cm / 2.54 = 1.7717 inches, and 1.7717 x 300 = 531.5 (531.496 before rounding), so 531 pixels.

So the photo should be 413 x 531 pixels with 300 DPI written in the file. A few more common sizes at 300 DPI:

Always check the official page for an ID photo: some forms set their own pixel size for uploads, and that wins over any conversion.

Why your photo might come out stretched

A 4032 x 3024 phone photo has a 4:3 shape. A 3.5 x 4.5 cm photo is taller than it is wide (7:9). If you force one into the other, faces get squashed. Two ways to avoid it:

• Crop first, then resize. Use Crop Image to cut the photo to the right shape around what matters, then resize to the exact size.
• Lock the aspect ratio. Set one side and let the other follow. You get the right width or height, keeping the shape, and can crop the rest.

For passport photos specifically, head size and position also matter, which a simple resize does not handle. Filo's ID Photo Maker crops around your face to the chosen country's format.

Resize on a Mac with Preview

Preview, built into macOS, can resize an image and change its resolution. Apple's steps:

1. Open the image, then click the Adjust Size button in the Markup toolbar (show the Markup toolbar first if it is hidden), or choose Tools > Adjust Size.
2. Enter new values for width and height, or choose a size from the Fit into menu. You can also choose percent from the menu next to the Width and Height fields.
3. The Resolution field sets the DPI. Apple notes that if you want to change the width or height without losing detail, deselect Resample image first.

Apple's guide only names percent among the units for width and height, so check the menu in your version for centimetres or inches.

Resize in cm, mm or inches with Filo

Filo's Resize Image works in Chrome, Edge, Firefox and Safari on Windows, Mac, Chromebook, iPhone and Android. Resizing runs in your browser, so the image stays on your device.

Step 1: Add the photo

Open Resize Image and add a JPG, PNG or WebP. You can add several; the same settings apply to all of them.

Step 2: Choose the unit

Under Resize by, choose Size, then pick Pixels, Centimetres, Inches or Millimetres. (Choose Percent instead to scale by a percentage.)

Step 3: Enter the size and DPI

Type the width and height, for example 3.5 and 4.5 cm, and set the DPI: type any value from 1 to 1200, or tap a common one (72, 150, 300 or 600). Under the fields, Filo shows the pixel size this gives, for example 413 x 531 px at 300 DPI. Lock the aspect ratio to keep the photo's shape, or unlock it if you have already cropped to the right shape.

Step 4: Download

Save the resized image. It keeps its original format, and for JPG and PNG, the DPI is written into the file, so print and layout apps open it at the size you chose. WebP files have no standard place for a DPI value, so for print sizes choose a JPG or PNG source.

A few honest details:

• Filo refuses a size it cannot make rather than quietly shrinking it: more than 20,000 pixels on a side, or more than the roughly 16.7 megapixels a browser can create. It tells you to pick a smaller size or a lower DPI.
• On the web you get 3 free conversions in total, and each image uses one. Files made with a free conversion carry a small Filo mark in the corner.
• The Filo app for iPhone and Android is free for life, with no daily limit.

Making the file smaller too

Resizing changes the pixel size, which usually makes the file smaller, but some upload forms also set a maximum file size, such as 250 KB. If the resized photo is still too big, use Image Compress and set a size limit in KB.

FAQ

Sources checked

• Apple Support, "Crop, resize, or rotate an image in Preview on Mac": https://support.apple.com/guide/preview/crop-resize-or-rotate-an-image-prvw2015/mac (checked 2026-11-12)
• Arithmetic: 1 inch = 2.54 cm = 25.4 mm (exact, by definition)