Circle calculator from radius, diameter, circumference or area
Choose any known circle measurement and enter its value. HitEquals solves radius, diameter, circumference and area without requiring a second input.
Inputs used by this calculator
Circle Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Known measurement | Radius | Radius · Diameter · Circumference · Area | Supplies the known measurement quantity used by the published equation or result breakdown. |
| Known value | 5 | 1e-12 to 1000000000000 | Supplies the known value quantity used by the published equation or result breakdown. |
Circle measurements
Radius runs from center to edge. Diameter crosses the center and equals two radii. Circumference is boundary length; area is enclosed surface.
Circle formulas
Every measure can be derived from radius.
d=2rC=2πr=πdA=πr²Solve backward
From circumference, divide by 2π for radius. From area, divide by π and take the positive square root.
Units and precision
Radius, diameter and circumference share a linear unit. Area uses the corresponding square unit. π remains unrounded internally.
Worked example
For radius 5, diameter is 10. Multiplying 10 by π gives circumference; multiplying 25 by π gives area.
What the Circle Calculator answers
Solve radius, diameter, circumference and area from any one circle measurement.
Use Circle Calculator when you know Known measurement and Known value and need the relationship expressed by the published equation.
Convert a known measure to radius
| Known | Radius |
|---|---|
| Radius r | r |
| Diameter d | d/2 |
| Circumference C | C/(2π) |
| Area A | √(A/π) |
Frequently asked questions
What is circle circumference?
It is the distance around the circle: 2πr or πd.
How do I find radius from area?
Divide area by π and take the positive square root.
Is diameter twice radius?
Yes.
Does area use square units?
Yes, such as square inches or square meters.
Is π rounded?
The engine uses JavaScript's full numeric π value and rounds only the display.
What question does this Circle Calculator answer?
Solve radius, diameter, circumference and area from any one circle measurement.
What to keep in mind
Results use the same linear unit as the input and its square for area.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.