r = 7 cm
d = 2 × 7 = 14 cm
One circle measurement unlocks the rest
Diameter of a Circle Calculator
Enter radius, circumference, or area to find the diameter, then see the circle’s complete measurement set and the exact rearranged formula.
Free to use. No account required. Your inputs stay in this browser.
The essential relationship
d = 2r = C/π = 2√(A/π)
A diameter is a straight segment through a circle’s center with endpoints on the circle. Its length is twice the radius. The calculator keeps full precision internally and rounds only the displayed result.
Worked examples
Follow the numbers, one step at a time
C = 31.416 m
d = C/π ≈ 10 m
A = 78.54 in²
d = 2√(A/π) ≈ 10 in
Built for understanding
More than an answer box
Every result includes the rearranged formula, substituted values, and a reasonableness check. Use the visual to connect the symbols to the measurement.
- Instant, private, browser-based calculation
- Decimal-friendly input and validation
- Responsive layout for phones and classrooms
- Copy-ready answer with units
Method and references
Formula basis
The formula follows standard textbook definitions. Unit symbols follow SI guidance; results are educational estimates, not engineering certification.
- OpenStax, Circles
- NIST SP 811, SI units
Quick answers
Frequently asked questions
What is the diameter formula?
Use d = 2r, d = C/π, or d = 2√(A/π), depending on what you know.
Does a circle have more than one diameter?
It has infinitely many diameter segments, all with the same length.
Is diameter the same as circumference?
No. Diameter crosses the circle; circumference is the distance around it.
How do I measure a real object?
Measure straight across its widest point through the center.
Keep learning with Lab101
This focused tool is part of the Lab101 collection of free math and science references.