Image Size, Resolution & DPI Explained: Resize in cm, mm, Inches

A digital image has no real size in centimetres. It only has pixels. A size in cm, mm or inches only appears when you also choose a DPI (dots per inch), which says how many of those pixels go into each inch on paper. So "3.5 cm x 4.5 cm" really means "this many pixels at this DPI". The formula is:

pixels = (size in inches) x DPI, and 1 inch = 2.54 cm = 25.4 mm.

If you just want the job done, enter the cm, mm or inch size and the DPI in our resize image in cm tool and it works out the pixels for you.

The three terms, in plain words

Image size (pixel dimensions)

This is the width and height in pixels, for example 1200 x 1600. It is the only "real" size of a digital photo. More pixels means more detail.

File size

This is how much storage the file takes, in KB or MB. It depends on the pixel count, the file format (JPG, PNG, WebP) and how much compression is applied. Two photos with the same pixel size can have very different file sizes. Exam forms usually set a limit on both, for example "200 x 230 pixels, 20 to 50 KB".

Resolution / DPI

DPI (often written PPI for screens) is a number stored inside the file that tells a printer how densely to place the pixels. A 600 x 600 pixel image printed at 300 DPI comes out 2 x 2 inches. The same image at 150 DPI prints 4 x 4 inches, but looks softer because each pixel is bigger.

How to convert cm, mm and inches to pixels

1. Convert the size to inches. Divide cm by 2.54, or mm by 25.4.
2. Multiply by the DPI.
3. Round to the nearest whole pixel.

Worked examples

Notice that 3.5 x 4.5 cm and 35 x 45 mm are the same size, so they give the same pixels.

Going the other way: pixels to cm

cm = pixels / DPI x 2.54. A 600 pixel wide image at 300 DPI is 600 / 300 x 2.54 = 5.08 cm wide.

Which DPI should you choose?

• Online forms that give a size in cm: check whether the notice also gives pixels. If it does, the pixels are what the portal actually checks. If it only gives cm, 200 or 300 DPI are common choices, but a larger pixel size also means a larger file, which can break the KB limit.
• Printing photos: 300 DPI is the usual target for prints you hold in your hand.
• Screens and websites: DPI is ignored. Only the pixel dimensions matter.

How to resize an image to a cm size, step by step

1. Open the resize image in cm tool and choose your photo.
2. Pick the unit (cm, mm or inches) and type the width and height.
3. Set the DPI. Use the value in your form notice if there is one, otherwise 300 for print.
4. If the shape of your photo does not match the target, crop first with crop image so the face is not stretched.
5. Download, then check the file size. If it is above the limit, run it through compress image or a fixed target like compress image to 50 KB.

The resizing happens in your browser, so the photo is not uploaded anywhere.

For exam photos where the portal gives exact pixels and KB, the exam photo resizer is quicker because it handles size and compression in one go. For passport-style prints, use the passport size photo tool.

Why "changing DPI" often does nothing

Many people open an image editor, change 72 DPI to 300 DPI, and expect a better photo. If the pixel dimensions stay the same, nothing about the picture changes. Only the print size label changes. To make an image physically larger in print without losing quality you need more pixels, which means a better original photo, not a different DPI number.

The reverse also catches people out. Some editors, when you change DPI, quietly add or remove pixels ("resample"). If the tool shows the pixel count changing, it is resampling. That is fine when you want a specific cm size, but enlarging a small image this way only adds blur.

Aspect ratio: why photos get squashed

Aspect ratio is width divided by height. A 3.5 x 4.5 cm photo has a ratio of about 0.78, a portrait shape. A typical phone photo is 3:4 (0.75) or 9:16 (0.56). If you force a 9:16 photo into 3.5 x 4.5 cm without cropping, the face stretches sideways. Always crop to the right shape first, then resize.

Common mistakes

• Typing cm values into a pixel box. Entering "3.5 x 4.5" as pixels produces a tiny, unusable image. Check the unit.
• Ignoring the KB limit. Getting the dimensions right is half the job. The upload can still fail if the file is too heavy or too light.
• Enlarging a small image. Turning a 150 pixel wide photo into 600 pixels makes it blurry. Start from the original camera photo, not a WhatsApp forward or a screenshot.
• Mixing up mm and cm. 35 mm is 3.5 cm, not 35 cm. A factor of ten error gives a huge file.
• Assuming 72 DPI means low quality. For screens and most upload portals the DPI tag is irrelevant. Pixel count is what matters.

Quick reference

• 1 inch = 2.54 cm = 25.4 mm
• pixels = inches x DPI
• inches = cm / 2.54 = mm / 25.4
• cm = pixels / DPI x 2.54

Keep these four lines handy and any size instruction, from a visa photo to an A4 scan, becomes simple arithmetic.

Questions