Circle Diameter Gauge
Hold a round object against the screen, match the circle, and read its diameter, radius, circumference and nearest standard size
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Hold a round object such as a coin, button, washer or drill bit against the screen and match the circle to read its diameter, radius, circumference and area, along with the nearest standard sizes.
How to use
- On your first visit, press "Calibrate screen" and match the screen to a card or a coin.
- Put the object against the left and bottom guide lines, and drag the circle or use the − / + buttons until the yellow circle matches its edge.
- For a straight width such as a watch strap, change "Shape to measure" to "Line (width or length)".
- Under "Nearest standard sizes", choose a "Category" to see the 5 closest sizes with their "Diff. mm".
How it works
- Circumference = diameter × π and area = π × radius², shown in mm, cm and inches. The diameter is set in steps of 0.1 mm.
- A screen reading can be off by 0.5 to 1 mm, so every size within ±0.5 mm of the reading is highlighted as a candidate.
- The washer and wire tables follow ISO 7089, ASME B18.21.1 and ASTM B258, checked in October 2026. Watch strap widths have no formal standard.
- A screen cannot be held against a hole, so trace its rim on paper with a pencil and match the tracing.
Questions and answers
- How do I find the circumference from the diameter?
- Multiply the diameter by π (about 3.1416). A circle 20 mm across has a circumference of about 62.8 mm.
- Where do I measure a drill bit?
- Measure the smooth shank, not the cutting part. Number and letter drills are only about 0.1 mm apart, so check the marking on the shank as well.
- How many mm is a button size in ligne (L)?
- 40L is 1 inch, so 1L is 0.635 mm. A 20L button is about 12.7 mm across.
- Can I measure a circle larger than the screen?
- No. Only sizes that fit on the screen can be drawn; if you type a larger diameter, the page shows the largest size it can draw.