Circle Area Calculator — Area, Radius, Diameter, Circumference
Type any one of radius, diameter, or circumference. Get the other three plus the area.
- Radius
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- Diameter
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- Circumference
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- Area
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What is circle area calculator?
The area of a circle is one of the first formulas every geometry student commits to memory: A = π·r². The radius is the distance from the center to the edge; the diameter is twice the radius and passes through the center; the circumference is the distance around the edge and equals 2π·r. Knowing any one of those three lets you derive the other two and the area in a single step, which is what this calculator does.
To use it, pick which measurement you already have — radius, diameter, or circumference — type the value, and select a unit (in / ft / cm / m). The headline shows the area in the squared unit; the stats list shows all four values rounded to four decimal places. The unit and mode toggles remember your last choice across visits via localStorage, so a fabricator who always works in inches and circumference inputs doesn’t reconfigure the tool every time.
Privacy is straightforward: every calculation runs locally in your browser. The values you type never leave your device. There is no server, no API call, no analytics on input values. The only persisted state is your unit and mode preference.
When to use a circle area calculator
- Geometry homework — area from radius — The most common textbook prompt: given a radius, find the area. Type the radius, leave the picker on Radius, and read π·r² rounded to four decimals. The diameter and circumference appear alongside for the variants of the same question.
- Fabrication — diameter from circumference — Wrap a flexible tape around a cylinder, read the circumference, and you can recover the diameter without trying to bisect the cross-section. Switch the picker to Circumference, type your measurement, and read the diameter directly.
- Quick conversion — radius ↔ diameter — If you have one and need the other, the calculator does the doubling or halving for you and shows the area in the same step. Useful when you're moving between a spec sheet that lists one and a tool that asks for the other.
How to use the Circle Area Calculator — Area, Radius, Diameter, Circumference
- Pick which measurement you have — Use the Input picker to choose Radius, Diameter, or Circumference. The label on the input field updates to match. Your last choice persists across visits.
- Type the value and the unit — Enter a positive number, then select your unit from the segmented control (in / ft / cm / m). The choice is cosmetic — it appends the unit suffix to every output and is remembered for next time. Switching units does not convert the value you typed.
- Read the four results — The headline shows the area. Below it, the stats list shows the radius, diameter, circumference, and area, each rounded to four decimal places. Tap the copy button to put a one-line summary on your clipboard.
Worked examples
Radius = 1 cm
Input: Mode Radius, value 1 cm
Output: Area = 3.1416 cm², diameter = 2 cm, circumference = 6.2832 cmThe reference unit circle. π ≈ 3.1416 rounded to four decimals.
Diameter = 10 in
Input: Mode Diameter, value 10 in
Output: Area = 78.5398 in², radius = 5 in, circumference = 31.4159 inCircumference = 6.2832 m
Input: Mode Circumference, value 6.2832 m
Output: Area ≈ 3.1416 m², radius ≈ 1 m, diameter ≈ 2 mRound-trip of example 1 in different units — the circumference of a unit circle is 2π.
Frequently asked questions
What is the formula for the area of a circle?
A = π·r², where r is the radius. For a unit circle (r = 1), the area is π ≈ 3.1416. If you start from the diameter, the formula becomes A = π·d²/4; from the circumference, A = C²/(4π). This calculator does the conversions internally so you only need one of the three inputs.How do I find the radius from the area?
r = √(A/π). This calculator doesn't accept area as a starting input, but you can plug area into a hand calculation or use the diameter/circumference path if those measurements are easier to obtain.How do I find the circumference from the diameter?
C = π·d. For a diameter of 10, the circumference is 10π ≈ 31.4159. Switch the picker to Diameter, enter your value, and the circumference appears in the stats list automatically.Does this work for ellipses or ovals?
A = πr² and C = 2πr only describe a true circle. An ellipse has two distinct semi-axes (a and b), and its area is π·a·b with no closed-form circumference. We may add a dedicated ellipse calculator in the future.Why is the answer not exact?
Math.PI, so chained computations don't accumulate visible rounding error.What units are supported?
Are my numbers stored or sent anywhere?
localStorage so the toggles remember your last choice.