The short answer
Why use Measure Radius?
Measuring across a hole or a wheel by eye tends to miss the widest point. Fitting a full circle to the edge shows straight away whether the size is right all the way round.
When you open an image, the center point sits in the middle and the radius is set to 20% of the shorter side. Drag the center onto the middle of the round object, then raise or lower Radius (px) until the ring traces its edge. The status line shows the radius and the diameter, which is simply twice the radius, both to two decimals.
The Radius box is always in pixels of the original image. Pixels per unit only converts the reported numbers, so with a scale of 40 pixels per centimeter, typing 200 gives a 5 cm radius and a 10 cm diameter. Round things seen at an angle look like ovals, and a circle will not fit an oval, so photograph them face on.
To get the distance between two specific points instead, such as across a gap that is not round, use Measure Distance. If you only need the pixel position of the center, for code or an image map, Find Coordinates places a single crosshair and reads its x and y.
Where Measure Radius fits
Pipe sizes
Check the outer diameter of a pipe end against a ruler laid beside it.
Wheel and tyre
Compare the diameter of two wheels photographed next to each other.
Drilled holes
Confirm a hole matches the drill size before fitting a dowel or bolt.
Start to finish
How to use Measure Radius
- 01Tap Choose image and pick a face-on photo of the hole, wheel, pipe end or round part.
- 02Drag the center point to its middle, then change Radius (px) until the red ring sits on the edge.
- 03Read the radius and diameter in the status line, then tap Apply changes and Download result for the PNG.
What to expect from Measure Radius
| Check | What it means here |
|---|---|
| Supported input | JPEG, PNG, WebP, GIF, BMP, SVG and browser-supported AVIF images |
| Main controls | Draggable points, exact X and Y fields, and an optional pixels-per-unit scale |
| Output | The measurement on screen and a marked-up copy of the image |
| Working limit | 50 MB and 50 megapixels per image |
Measure Radius tips
- The ring starts at 20% of the image's shorter side; adjust from there.
- Radius (px) stays in image pixels even after you set Pixels per unit.
- If the ring fits one side but not the other, the object was shot at an angle.
- Raise the radius in steps of 1 near the edge for a tight fit.
Measure Radius FAQ
Something missing? Ask us.
Can I drag the circle's edge to resize it?
Does it find the circle automatically?
Does Measure Radius upload my file?
Is Measure Radius free to use?
Why did Measure Radius fail or look wrong?
Related tools
More in Measurement.