How do I measure the radius of a circle in an image?

Upload your photo, select the Circle tool, and click three points along the curved rim of the object. The tool fits an exact circle through the points and calculates center coordinates, radius, diameter, circumference, and area.

Why use 3 points to define a circle?

Any three non-collinear points uniquely define a circumscribed circle in 2D Euclidean space. This allows measuring round objects even when parts of the rim are obscured, off-center, or partially cropped.

Can I measure pipe, coin, or bore diameters from a picture?

Yes. Position your camera looking directly down the bore or rim with a known scale reference, click three points on the circular edge, and read the exact diameter in millimeters or inches.

How do I calculate circle circumference and circular area?

Once the 3 perimeter points are placed, the tool calculates circumference (C = pi * d) and surface area (A = pi * r^2) automatically in calibrated units.

Can I measure round objects when part of the circle is obscured?

Yes. As long as you can place 3 distinct points along any visible curved arc of the rim, the engine reconstructs the full circle geometry.

Area & Geometry

Image Circle Measurement — Measure Radius, Diameter & Circumference from Photos

Measure circular objects, radius, diameter, arc, circumference, and circular area from photos using 3-point circular fitting and calibrated units.

TL;DR: Image Circle Measurement is a 100% free client-side measurement utility. Calibrate scale with reference objects, inspect dimensions, and measure angles or areas with zero file uploads and zero telemetry.

Active Image: Sample Architectural Blueprint (Preloaded)

(Or paste Ctrl+V)

Unit:

Zoom: 100% X: 0, Y: 0

Live Measurement Result

500.00 cm

196.85 in • 500 px

Recorded Measurements (1)

Export CAD & Data

Download vector SVG, AutoCAD DXF, PNG, or CSV table.

Understanding Image Circle Measurement

Measure circular objects, radius, diameter, arc, circumference, and circular area from photos using 3-point circular fitting and calibrated units. Designed for architects, web designers, engineers, students, and homeowners who need instant, high-precision dimensional analysis directly in their web browser without installing heavy software or paying subscription fees.

Mathematical Formula & Technical Methodology

d = 2r, \quad C = 2\pi r = \pi d, \quad A = \pi r^2

Formula Variables Explained:
Fits a circumscribed circle across three perimeter points using determinant matrix geometry, computing radius, diameter, circumference, and area.

Step-by-Step Worked Example

Clicking 3 points around a circular gear with radius 50 pixels (calibrated at 10 px/cm) yields diameter: 10.0 cm, circumference: 31.42 cm, area: 78.54 cm².

Direct Answer: Image Circle Measurement Overview

Quick Answer: To measure the diameter or radius of a circular object in a photo, upload your image to the Image Circle Measurement tool and click any three points along the curved perimeter. The tool fits an exact circle through the points and calculates center coordinates, radius, diameter, circumference, and area.

Connected Measurement Guides & Tools

Complete Image Measurement & Scale Guide — Master reference calibration, perspective compensation, and precision measuring.
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Image Area Measurement — Calculate polygon surface area and boundary perimeter.
Online Ruler (Actual Size) — 1:1 on-screen virtual ruler in centimeters, millimeters, and inches.

100% Client-Side Privacy Guarantee

All image rendering, camera streaming, coordinate mapping, and mathematical calculations execute locally inside your web browser using HTML5 Canvas 2D and sub-pixel JavaScript algorithms. Zero files or camera feeds are ever sent to remote servers.

Frequently Asked Questions

: Upload your photo, select the Circle tool, and click three points along the curved rim of the object. The tool fits an exact circle through the points and calculates center coordinates, radius, diameter, circumference, and area.
: Any three non-collinear points uniquely define a circumscribed circle in 2D Euclidean space. This allows measuring round objects even when parts of the rim are obscured, off-center, or partially cropped.
: Yes. Position your camera looking directly down the bore or rim with a known scale reference, click three points on the circular edge, and read the exact diameter in millimeters or inches.
: Once the 3 perimeter points are placed, the tool calculates circumference (C = pi * d) and surface area (A = pi * r^2) automatically in calibrated units.
: Yes. As long as you can place 3 distinct points along any visible curved arc of the rim, the engine reconstructs the full circle geometry.

Embed This Tool

Add the live Image Circle Measurement to your own website with this embed code:

<iframe src="https://nexlove.org/embed/image-circle-measurement.html" width="100%" height="650" style="border:1px solid #e2e8f0;border-radius:12px" title="Image Circle Measurement — NexLove" loading="lazy"></iframe>

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