How to Check Image Size, Dimensions, and File Format

Last updated: June 28, 2026

Before you resize, compress, or upload an image, you need to know what you are starting with. An image carries several measurements — pixel dimensions, file size, DPI, and format — and each tells you something different about whether the image will work for its destination. This guide covers how to check all of them on every platform, and what the numbers mean for your next step.

Quick answer: how do you check an image's size and dimensions?

On Windows, right-click the file and choose Properties to see dimensions and file size. On Mac, select the file and press Cmd+I for the same info. On iPhone or Android, open the photo and look at the info or details panel. For exact pixel dimensions, DPI, and format in one place, open the image in any browser-based image inspector. Once you know the dimensions and file size, you can decide whether to resize, compress, or convert before publishing.

What do the different image measurements mean?

Images carry several distinct measurements, and confusing them leads to wrong decisions. Dimensions, file size, DPI, and format each answer a different question.

Measurement	What it tells you	Units
Dimensions	The pixel width and height	Pixels (e.g. 4000×3000)
File size	How much storage it uses	KB, MB
DPI / PPI	Print resolution density	Dots per inch
Format	The encoding and capabilities	JPEG, PNG, WebP

• Dimensions decide how large the image can display or print.
• File size decides load time and whether it fits an upload limit.
• DPI only matters for print; screens ignore it.
• Format decides quality, transparency, and compression behavior.

How do you check image dimensions on each platform?

The method varies by device, but every platform exposes dimensions and file size without extra software.

• Windows: right-click the file > Properties > Details tab. Shows dimensions, DPI, and file size.
• macOS: select the file > Cmd+I (Get Info). Shows dimensions and file size.
• iPhone: open the Photos app > swipe up on the photo or tap the info button for dimensions.
• Android: open Google Photos > swipe up for info, or use a file manager for details.
• Browser: open any image inspector or upload to check dimensions, DPI, and format at once.

For a reliable cross-platform method, drag the image into a browser tab and the title bar or a right-click "inspect" shows the pixel dimensions. For DPI and format together, the Properties or Get Info panel is faster than guessing.

How do you check image file size?

File size is the number that decides whether an image will load fast or hit an upload limit. Check it the same way you check dimensions — Properties on Windows, Get Info on Mac — and compare it against your target.

Destination	Target file size
Web hero image	Under 200 KB
Web inline image	Under 100 KB
Email attachment	Under 1 MB each
Social upload	Platform-dependent
Print	Size is less critical than resolution

If the file is larger than the target, the fix is compression — but only after resizing to the display dimensions. Compressing a 4000-pixel image that displays at 800 pixels wastes effort; resize first, then compress. The image file size guide covers the relationship between dimensions, format, and bytes in depth.

Checking DPI and resolution

DPI tells you the print density, and it matters only if the image is going to print. To check it, open the file properties (Windows Details tab) or an image inspector that reads EXIF metadata. For the relationship between DPI and printable size, the DPI guide explains how pixels divided by DPI gives printed inches.

• Screen images ignore DPI; do not optimize it for web.
• Print images need 300 DPI for close viewing.
• A high DPI value does not mean high quality if the pixel dimensions are low.

Checking image format

The format — JPEG, PNG, WebP, GIF, and others — decides how the image compresses, whether it supports transparency, and what tools can open it. Check it in the file properties or by the file extension. For deciding which format to use for a given job, the format comparison lays out the trade-offs. If the format does not match your need, convert it before publishing.

Common mistakes when reading image size

The numbers are easy to misread, and a few errors come up repeatedly. Recognizing them saves wasted work.

• Confusing megapixels with megabytes. A 12-megapixel image is not 12 MB. Megapixels count the total pixels (width times height); megabytes count the stored bytes after compression. A 12 MP JPEG might be 3 MB or 8 MB depending on content and quality.

• Assuming a big file means high resolution. A poorly compressed 2 MB image can be lower resolution than a well-compressed 500 KB one. File size and dimensions are independent.

• Treating DPI as a quality score. A 72 DPI photo with 6000 pixels can print larger and sharper than a 300 DPI photo with 1000 pixels. DPI alone tells you nothing without the pixel dimensions.

• Trusting the filename extension. A file named .jpg can actually be a PNG or WebP internally. An inspector that reads the actual format header is more reliable than the extension.

The reliable way to confirm what you have is to read the file's metadata directly rather than inferring it from the name or the folder. Most inspectors show the real format header, the true pixel dimensions, and the embedded DPI in one panel.

Verifying size for specific destinations

Each destination has its own limits, and checking against the right target prevents rejection or slow loads.

Destination	Check before uploading
Website	Dimensions fit the layout, file under 200 KB
E-commerce listing	Platform's required pixel dimensions
Email	Total attachment under provider limit
Social media	Platform's aspect ratio and size limits
Print	300 DPI at the intended physical size

For website images, the most useful check is the rendered size — open the page in a browser, inspect the image element, and confirm the file being served matches the display dimensions. Serving a 4000-pixel image into a 600-pixel slot is a common performance problem that the file properties alone will not reveal. The responsive images guide covers serving the right dimensions to each screen.

What to do once you know the numbers

Checking the image is the diagnostic step; the fix depends on what you found. The WebAIM guide to image accessibility documents how dimensions and file size interact with accessibility and performance, and the Mozilla image format reference explains what each format supports. Once you have the measurements, the path is straightforward: resize to display dimensions, compress to the target file size, convert to the right format, and the image is ready for its destination.

Frequently asked questions

How do I check an image's dimensions?

Right-click the file and open Properties (Windows) or Get Info (Mac) for pixel dimensions, or load it in any editor. On the web, the browser DevTools Network tab shows the loaded file's dimensions and bytes. The numbers tell you whether the image is the size you expect before you ship it.

What is the difference between file size and dimensions?

Dimensions are pixels (width × height); file size is bytes on disk. A 4000px image can be a small file if well compressed, or huge if not. Both matter: dimensions for display sharpness, file size for load speed.

How do I check DPI?

In an editor's Image Size dialog, or with ImageMagick (identify -format "%x x %y" file.jpg). DPI matters for print; for screens, pixel dimensions are what count. See the resolution and DPI guide.

What format is my image?

Check the file extension first, but verify with a tool — extensions can be wrong. ImageMagick identify or the browser DevTools report the true format. Knowing the format tells you what compression and transparency options apply.

How do I check image size in bulk?

A command-line tool reports dimensions for a whole folder at once — ImageMagick identify *.jpg prints each file's dimensions and bytes. For a quick audit of a web folder, this is far faster than opening each file. Use it to catch the one oversized image dragging down a page before you ship it.