About the Image Ruler Tool
The image ruler measures things you can see in a picture. Drag from one point to another to draw a measuring line, and the tool shows its length in pixels and its angle. Draw as many lines as you like and each is numbered on the picture. To get real lengths, draw a line along something whose size you know, such as a ruler, a coin or a door, type its real length and unit, and press Set scale. Every line is then shown in that unit. Download a PNG of the picture with the numbered lines and their lengths drawn on it. It is useful for quick estimates from photos of plans, objects, maps and screenshots. Your picture stays in your browser.
Updated October 2026
How to use the Image Ruler Tool
- 1Select an image or drop one on the page.
- 2Drag across the picture to draw a measuring line. Draw as many as you need.
- 3Read the length in pixels and the angle in the list below the picture.
- 4To use real units, choose a line of known length, type its length and unit, and press Set scale.
- 5Press Download PNG with measurements to save the numbered lines on the picture.
How to measure with a scale
Put a reference of known size in the picture when you take it: a ruler, a credit card, which is 85.6 millimetres wide, or a coin. In the tool, draw a line along its length, enter its size and press Set scale. The tool works out how many pixels equal one unit and converts every line.
Tips for better results
- Take the photo straight on, with the camera square to the surface.
- Put the reference beside the thing you measure and on the same surface.
- Use the longest reference you have. A long line has a smaller error from a pixel's width.
- Keep the lens far from the object when you can, because wide lenses distort the edges.
- Draw lines from edge to edge, zooming in with the Zoom Tool to place the ends.
What it is good for
Estimating the size of an object, a room or a plant from a photo, checking dimensions on a floor plan or a map with a known scale bar, measuring a gap in a screenshot, working out an angle such as a roof pitch from a photo, and annotating a picture for a report.
Pixels, units and the saved picture
Without a scale, lengths are in pixels of the picture you loaded, so a larger copy of the same picture gives larger numbers. The saved PNG draws every line with its number and length in the unit shown on screen at the time.
Phone and computer
Drag with a finger or the mouse to draw a line. The page does not scroll while you touch the picture, so draw as you would on a drawing app. For paper sizes, the Print Size Calculator turns pixels into centimetres, and the Angle Converter changes degrees to other angle units.
Features
- Lengths in pixels to one decimal place
- Angle of each line, from 0 to 360 degrees
- Set a scale from any line of known length: cm, mm, inches or your own unit
- Numbered lines with labels on the saved picture
- Remove single lines or clear all
- Private: measured in your browser
Frequently asked questions
How do I measure in centimetres or inches?
Draw a line along something of known length in the picture, choose it in Line of known length, type its real length and unit and press Set scale. All lines, including the earlier ones, then show in that unit.
How accurate is it?
It is as accurate as your line placement and the picture. Zoom the picture, or use the Zoom Tool first, if you need to place the ends exactly. Perspective and lens distortion also add error.
Why are my real measurements wrong?
A scale is only right for things at the same distance from the camera and in the same plane as the reference. An object closer to the lens looks bigger, so use a reference that lies flat beside what you are measuring.
What does the angle mean?
It is the direction of the line from its first point to its second, with 0 degrees pointing right and angles counted anticlockwise, so 90 degrees points up.
Can I measure an angle between two lines?
Not directly. Subtract the two angles. For lines at 30 and 75 degrees, the angle between them is 45 degrees.
Can I change a line after drawing it?
Not yet. Remove it and draw it again.
Does it measure the picture on my screen?
No. It measures in picture pixels. To measure something on your screen, see the Online Ruler.
Is my picture uploaded?
No.