Enter one circle measure and this calculator finds the other main circle values with the right length and square units.
How to use our Circle Calculator
- Choose Known measure to tell the calculator which circle value you already have: radius, diameter, circumference, or area.
- Enter that number in Value using a positive amount greater than 0.
- Pick the matching Length unit such as mm, cm, m, in, ft, or yd.
- Choose Decimal places to control how much rounding you want in the results.
- Click Calculate to show Radius, Diameter, Circumference, and Area.
- Read the units carefully: Radius, Diameter, and Circumference use the selected length unit, while Area uses the matching square unit.
- Do a quick check: Diameter should be 2 times Radius, and if your known measure was radius, Circumference should be a little more than 6 times the radius.
Definitions
Known measure: The one circle value you already know and want to start from.
Value: The number for the chosen known measure.
Length unit: The unit used for Radius, Diameter, and Circumference, such as cm or ft.
Decimal places: How many digits appear after the decimal point in the displayed results.
Radius: The distance from the center of the circle to the edge.
Diameter: The distance across the circle through the center. It is 2 times the radius [3].
Circumference: The distance around the circle, like the circle's perimeter [2].
Area: The amount of space inside the circle, shown in square units such as cm^2 or ft^2.
Pi (π): The constant used in circle formulas, about 3.14159.
Common mistakes and quick fixes
Mistake: Typing 0, a negative number, or leaving Value blank.
Fix: Enter a positive number greater than 0 in Value .
Mistake: Choosing Known measure as Area but thinking the Value is a length like radius or diameter.
Fix: If you know a length, change Known measure to radius, diameter, or circumference. Use Area only when the number is in square units.
Mistake: Forgetting that Area is shown in square units, then comparing it directly to Radius or Circumference .
Fix: Read the output labels carefully. Radius , Diameter , and Circumference are lengths, but Area is square units such as cm^2 or ft^2.
Mistake: Picking the wrong Length unit , such as inches when your number is really in feet.
Fix: Set Length unit to the same unit used by your entered Value before you calculate.
Mistake: Entering more digits than you want to read, then thinking the answer changed when only rounding changed.
Fix: Adjust Decimal places to the display precision you want. The circle size stays the same; only the shown rounding changes.
Limitations & Key Assumptions / Boundary Conditions
- The calculator uses one known value at a time. It does not compare multiple given measures for consistency.
- Value must be greater than 0. Blank, zero, negative, or non-numeric entries produce an error instead of a result.
- The selected Length unit is assumed to match the entered Value for radius, diameter, or circumference. If you choose the wrong unit, every output will be wrong by the same scale.
- When Known measure is Area, the entered number is treated as square units tied to the selected Length unit. For example, choosing cm means the area input is read as cm^2.
- Results are rounded for display based on Decimal places, so shown values may look slightly different from exact calculator memory values.
- This tool covers only the four core circle measures: radius, diameter, circumference, and area. It does not calculate arcs, sectors, chords, or spheres.
Methodology
How the calculator works
The calculator first converts your chosen Known measure into radius. After that, it uses the radius to find the other three circle measures.
Formulas used
d = 2r
C = 2 * pi * r
A = pi * r^2
r = d / 2
r = C / (2 * pi)
r = sqrt(A / pi)
Here, r is radius, d is diameter, C is circumference, and A is area. The calculator uses JavaScript Math.PI for pi, which is 3.141592653589793.
Worked mini-example
Suppose Known measure is radius, Value is 10, and Length unit is cm.
r = 10 cm
d = 2 * 10 = 20 cm
C = 2 * pi * 10 = 62.831853... cm
A = pi * 10^2 = 314.159265... cm^2
If Decimal places is 2, the displayed results become 10.00 cm, 20.00 cm, 62.83 cm, and 314.16 cm^2.
Unit handling
Radius, Diameter, and Circumference stay in the selected length unit. Area is shown in the matching square unit. For example, if the length unit is ft, the area unit is ft^2.
Error handling and result checks
The calculator blocks blank, zero, negative, non-numeric, NaN, and Infinity inputs because a real circle measure must be greater than 0. A quick self-check is that diameter should always equal 2 times radius.
Assumptions that affect results
The math assumes a perfect circle and exact input values. Real measured objects may differ a little because of measurement error, rounding, or because the shape is not perfectly circular.