Need the diameter of a circle or want to calculate circumference from diameter? This page explains every formula you need to go between diameter (d), radius (r), circumference (C), and area (A) — plus clear examples and a quick-reference table. Perfect for school, engineering, machining, and exam revision.
Diameter from radius: d = 2r
Circumference from diameter: C = π · d
Diameter from circumference: d = C / π
Area from diameter: A = (π/4) · d² (because A = πr² and r = d/2)
Radius from diameter: r = d / 2
Radius from circumference: r = C / (2π)
Diameter from area: d = 2 · √(A/π)
| Known | Formula | Example Input | Result |
|---|---|---|---|
| Circumference → Diameter | d = C / π | C = 50 | d ≈ 15.9155 |
| Diameter → Circumference | C = π · d | d = 12 | C ≈ 37.6991 |
| Diameter → Radius | r = d / 2 | d = 12 | r = 6 |
| Radius → Diameter | d = 2r | r = 7.5 | d = 15 |
| Diameter → Area | A = (π/4) · d² | d = 10 | A = 25π ≈ 78.5398 |
| Area → Diameter | d = 2·√(A/π) | A = 100 | d ≈ 11.2838 |
Example 1 — Area of a circle with diameter 10A = (π/4)·d² = (π/4)·100 = 25π ≈ 78.5398
Example 2 — How to calculate diameter from circumference 50d = C / π = 50 / π ≈ 15.9155
Example 3 — How to find circumference with diameter 24C = π·d = π·24 ≈ 75.3982
Example 4 — How to find the diameter from area 20d = 2·√(A/π) = 2·√(20/π) ≈ 5.0462
r directly (e.g., A = πr²).d = 2r and r = d/2.Q: How do I find the diameter of a circle?
Use d = 2r if you know the radius, or d = C/π if you know the circumference.
Q: How to calculate a diameter from a circumference?
Divide by π: d = C/π.
Q: How do you find the circumference from the diameter?
Multiply by π: C = π·d.
Q: What is the area of a circle with diameter d?A = (π/4)·d².
Q: How do I get diameter from area?
Rearrange the area formula: d = 2·√(A/π).
Q: What’s the difference between diameter and radius?
Diameter is twice the radius (d = 2r); radius is half the diameter (r = d/2).