The short answer
Why use Online Ruler?
Pixel measurements settle questions that are hard to answer by eye. Calibration also lets a known scale turn pixels into a real-world unit.
Online Ruler focuses on measuring a distance in pixels or calibrated units. Points can be placed on the image or entered numerically. The calculation uses those coordinates and the optional pixels-per-unit value.
The selected file stays in this browser tab and is released when you leave the page. Results are only as accurate as the selected points and calibration. Leave calibration at 1 when the answer should stay in pixels.
What Online Ruler does
Open the source photo
Drop, choose, or paste a supported file into Online Ruler. Measuring a distance in pixels or calibrated units. The source stays on your device; we work on a copy.
Read the numbers
Online Ruler is for measuring a distance in pixels or calibrated units. The preview is the place to decide if the setting went too far. The original file is not overwritten.
Useful controls
Main controls: Point coordinates and an optional pixels-per-unit calibration. Working limit: 50 MB and 50 megapixels per image. Watch the preview at full size before you save. A current desktop browser usually has more memory than a phone.
Copy the result
Export is an on-screen measurement and an annotated image where the tool supports one. The download is a new file. Keep the original until you have opened the copy once.
Where Online Ruler fits
These are ordinary jobs where the focused controls save time. They also show where a full editor or a manual review may still be the better choice.
Design checks
Confirm spacing, radius, or alignment before copying a layout.
Reference photos
Measure a known feature after entering a reliable image scale.
Pixel inspection
Use Online Ruler when a visual estimate is not precise enough.
Start to finish
How to use Online Ruler
- 01Choose, drop, or paste a supported file.
- 02Adjust the online ruler controls and check the preview at full size.
- 03Download the finished copy and open it once before using it elsewhere.
What to expect from Online Ruler
This table covers the parts that matter before you begin: what goes in, which controls do the work, what comes out, and where the browser draws a hard line.
| Check | What it means here |
|---|---|
| Supported input | JPEG, PNG, WebP, GIF, BMP, SVG and browser-supported AVIF images |
| Main controls | Point coordinates and an optional pixels-per-unit calibration |
| Output | An on-screen measurement and an annotated image where the tool supports one |
| Working limit | 50 MB and 50 megapixels per image |
| Privacy | Files and entered values stay in this tab. Filenames, pixels, metadata, OCR text, QR contents, locations, and passwords are not sent to analytics. |
Before you export
- Zoom in before placing points on a hard edge.
- Use the longest reliable calibration distance available.
- Keep calibration at 1 for pixel-only answers.
- Perspective can distort real-world measurements in ordinary photographs.
Frequently asked questions
If you cannot find what you are looking for, write us a message and we will get back to you.
What should I check before saving with Online Ruler?
Does my file get uploaded?
Can I use Online Ruler on a phone?
Will Online Ruler overwrite my original file?
Why might the Online Ruler result look different in another app?
Is Online Ruler free to use?
How does Online Ruler work?
What can I put into Online Ruler?
How do I save the result from Online Ruler?
Why did Online Ruler fail or look wrong?
Will Online Ruler keep a copy of my work?
Related tools
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Continue the workflow
These tools handle common next steps after online ruler.