Circle calculator
Convert radius, diameter, or circumference into the full set of circle measurements.
What this calculator covers
Use this circle calculator when you know one circle measure and want the rest without switching formulas by hand.
The walkthrough starts by converting everything back to radius, which makes the later diameter, circumference, and area formulas easier to follow.
Frequently asked questions
- Can I start with circumference instead of radius or diameter?
- Yes. Select circumference as the starting measurement and enter its value. The calculator divides by 2Ï€ to recover the radius, then derives diameter and area from there.
- What units does the output use?
- The calculator is unit-neutral — enter any consistent unit and the outputs share that same unit. If you enter radius in centimeters, circumference is in centimeters and area is in square centimeters. No unit conversion is performed.
- Why is area expressed in square units while circumference is in linear units?
- Circumference is a one-dimensional measurement — the distance around the boundary of the circle. Area measures the two-dimensional space enclosed, so it requires squared units. This is why the area formula uses radius squared multiplied by π.
- How precise are the results?
- All four outputs are displayed to up to six decimal places, which is sufficient for most practical geometry, drafting, or homework purposes. Internal calculations use JavaScript's floating-point arithmetic with π at full double-precision.
Tool
Run the calculation
Result
RESULT · CIRCLE AREA
â„–114
Primary result
78.539816 sq units
Starting from a radius of 5, the circle has radius 5, diameter 10, circumference 31.415927, and area 78.539816.
- Radius
- 5
- Diameter
- 10
- Circumference
- 31.415927
- Area
- 78.539816
Step-by-step solution
- 1.Convert the provided radius into radius 5, since radius is the common input for the main circle formulas.
- 2.Use radius to calculate diameter 10 and circumference 31.415927.
- 3.Square the radius and multiply by pi to get the area 78.539816.
Walkthrough
Visual walkthrough
Every circle formula can be reached once the radius is known, so the calculator converts the starting measurement into radius first.
01
Normalize to radius
r = 5
Radius is the one value that feeds both circumference and area formulas directly.
02
Compute the boundary measures
d = 2r, C = 2pi r
Diameter doubles the radius, and circumference wraps one full turn around the circle.
03
Compute the area
A = pi r^2 = 78.539816
Squaring the radius captures how much two-dimensional space the circle covers.
78.539816 square units