HesapHanem

Circle Calculator

How do I find the area of a circle?

How it is calculated

From the radius r of a circle the tool derives three measures. Area: A = π × r2. Circumference, the distance around the circle: C = 2 × π × r. Diameter, the distance across through the centre: d = 2 × r. Here π is about 3.14159. Enter the radius in any length unit; the circumference and diameter come out in that same unit, and the area in that unit squared (cm gives cm2). The radius must be greater than zero. Each result is rounded to two decimals. If you only know the diameter, halve it first, and if you know the circumference, divide it by 2π to get the radius.

Worked example

A circle has a radius of 5 cm. Area: π × 52 = π × 25 = 78.54 cm2. Circumference: 2 × π × 5 = 31.42 cm. Diameter: 2 × 5 = 10.00 cm. A useful sanity check is that doubling the radius to 10 cm doubles the circumference to 62.83 cm but quadruples the area to 314.16 cm2.

Try this example →

Common mistakes

  • Entering the diameter in the radius field, which makes the area four times too large.
  • Quoting the area in plain units rather than square units, such as cm instead of cm2.
  • Mixing up area and circumference: the area grows with the square of the radius, the circumference only in proportion to it.
  • Using 3.14 for π in a hand check and comparing with this result, then seeing a small mismatch; the tool uses the full value.
  • Entering a radius in one unit and reading the result in another, for example metres in and centimetres out.

About this calculator

This calculator takes a circle's radius and computes its area, circumference and diameter using the standard geometric formulas built around π (pi) — area equals π times radius squared, circumference equals 2π times radius, and diameter is simply twice the radius. Entering just the radius is enough to derive all three related measurements at once.

It is used across schoolwork, where students verify geometry homework or check their own manual calculations, as well as in practical situations like figuring out how much material is needed to cover or fence a circular area, sizing a round tabletop, pipe or garden bed, or working out how much fencing or edging trim is required around a circular perimeter. Anyone who only knows one dimension of a circle — commonly the radius from a spec sheet or measurement — uses this to get every other figure without doing the π arithmetic by hand.

Was this helpful?

You can pick up to 3 reactions.

Comments (1)

  • Editorial team

    Pick the output by the job: area tells you how much soil, paint or paving a round surface needs, while circumference tells you how much edging or trim goes around it. For a round bed of radius 3 m that is 28.27 m² of soil and 18.85 m of edging. Doubling the radius quadruples the area but only doubles the circumference. What round object are you measuring, and did you start from the radius or the diameter?

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators