Circle Diameter Calculator
Find the diameter of any circle from its radius, circumference, or area. The diameter is the full width across the center — the longest straight line inside the circle. Enter one known value below to get all measurements.
1. Select Known Metric
2. Enter Value & Unit
Results
Circle Diameter Calculator
The diameter cuts a circle in half — it's the line segment from edge to edge through the center. In Euclidean geometry, it's the maximum distance between any two points on the circle. Explore the interactive diagram to see how diameter connects to radius, area, and circumference.
Why diameter matters
Diameter is how most people describe circular objects. "A 12-inch pizza," "a 26-inch bike wheel," "a 4-inch pipe" — these are all diameters. It's the most intuitive measurement because you can physically measure it with a ruler across the widest point.
Diameter in manufacturing
Pipe sizing, bearing specifications, bolt circles, and O-ring dimensions are all specified by diameter. Machinists use calipers to measure diameter directly. Converting from circumference (a tape-measure reading) to diameter using d = C/π is a common shop-floor calculation.
Diameter Formula
The diameter formula is the simplest in circle geometry — just double the radius. Click each part to explore.
Click each part to explore
Alternative formulas: d = C / π from circumference, d = 2√(A / π) from area.
How to Calculate Diameter
Three steps from any known value to the diameter. Enter a radius to see it in action.
Identify What You Know
r, C, or ADetermine whether you have the radius, circumference, or area. Each requires a different formula to find the diameter.
Apply the Formula
d = 2rFrom radius: d = 2r. From circumference: d = C/π. From area: d = 2√(A/π). Pick the one that matches your known value.
Read the Result
dThe diameter is in the same linear unit as your input. If your radius was in inches, the diameter is in inches.
Formula Variations
Three routes to the diameter depending on your starting measurement.
The most direct conversion. The diameter is simply double the radius — no Pi needed. If your radius is 7 cm, the diameter is 14 cm.
From radius
From circumference
From area
Example
Find the diameter from a known radius. Change the value to try your own.
The radius of the circle is 9 centimeters.
Frequently Asked Questions
Common questions about diameter and how to find it.