Circle Calculator
Enter Known Value
Results
| Radius (r) | — |
|---|---|
| Diameter (d) | — |
| Circumference (C) | — |
| Area (A) | — |
How to Use This Circle Calculator
Round features show up everywhere—pipe OD, pizza size, pulley diameter—but homework and shop notes rarely label all four numbers at once. Tell the tool what you actually measured (radius, diameter, circumference, or area) and it back-solves the rest.
- Pick Given Value. Choose radius, diameter, circumference, or area from the dropdown. The input label updates to match.
- Enter one positive number. Zero and negatives are rejected— a circle cannot have zero size.
- Press Calculate. The table fills radius, diameter, circumference, and area together.
- Read the step block. Each mode shows how radius was recovered before the other measures were computed.
For flat disk area only, the circle tab on the Area Calculator works the same way. For sphere surface area, open the Surface Area Calculator.
Circle Formulas and Practical Applications
Every circle measure ties back to radius—the spoke from center to edge. Once r is known, the other three fall out in one pass.
Core relationships
d = 2r
C = 2πr = πd
A = πr²
A 5 ft radius pipe cap has diameter 10 ft, circumference about 31.42 ft, and area about 78.54 ft². Enter r = 5 above to match.
Working backward from C or A
r = C / (2π)
r = √(A / π)
Fabricators often know belt length (circumference) or plate square footage (area) before radius. Those reverse forms are exactly what the circumference and area input modes use internally.
Where circles show up on the job
- Sizing gaskets from measured OD (diameter mode).
- Checking how much trim fits around a round table (circumference mode).
- Converting a floor circle's square footage back to radius for layout (area mode).
Standard Units and Conversion Tables
Use one length unit for the input. Area outputs in square units of that measure.
| If input is in… | Area output |
|---|---|
| Meters (m) | Square meters (m²) |
| Feet (ft) | Square feet (ft²) |
| Inches (in) | Square inches (in²) |
| Centimeters (cm) | Square centimeters (cm²) |
See the Volume Calculator for spheres and cylinders in 3D. Browse all math calculators.
Frequently Asked Questions
What are the key circle formulas?
d = 2r, C = 2πr, A = πr². One known value determines the other three.
Can I start from circumference or area?
Yes—select those from Given Value. Radius is recovered first via C / (2π) or √(A / π).
What if I only know the diameter?
Choose Diameter, enter d, and calculate. Radius is half of that; circumference and area follow.
Does the calculator accept π in the input?
Type the numeric measurement you measured. π is built into the formulas—not something you paste into the value field.
What unit should I use?
Stay consistent: feet in, feet and ft² out. Mixing units in one run would skew area.
Disclaimer. RapidRatio is informational only—not fabrication or engineering sign-off. Confirm critical dimensions on site.