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Circle calculator

Compute the area, circumference and diameter of a circle from its radius.

Related tools

All Geometry tools →
Circle sector area calculatorCompute the area and arc length of a circle sector from its radius and angle.Circumscribed Circle CalculatorCircumradius R = abc/4K, circumcentre and circumcircle of a triangle, from three sides or three vertices, with the figure drawn.Great Circle Distance CalculatorGreat-circle distance between two latitude/longitude points by the haversine formula, with initial and final bearing, the midpoint and a globe view.Inscribed Circle CalculatorInradius r = K/s, incentre, tangent points and incircle of a triangle, from three sides or three vertices, with the figure drawn.Annulus (ring) area calculatorCompute the area of a ring from its outer and inner radius.Area calculatorCompute the area of a rectangle, square, circle or triangle.Cone surface area calculatorCompute the total surface area and slant height of a cone from its radius and height.Cylinder volume calculatorCompute the volume and surface area of a cylinder from its radius and height.

The Circle calculator turns Radius into Area, Circumference, Diameter, instantly and for free. For instance, with Radius = 5 it returns Area = 78.54, Circumference = 31.416 and Diameter = 10.

How to use it

  1. Enter your values: Radius.
  2. Read the result instantly: Area, Circumference, Diameter.

Frequently asked questions

What does the Circle calculator actually compute?

It takes Radius and derives Area, Circumference and Diameter from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

A single value: Radius. Nothing else is required — no account, no file upload.

Can you show a worked example?

With Radius = 5, the calculator returns Area = 78.54, Circumference = 31.416 and Diameter = 10. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Radius = 10 instead, Area goes from 78.54 to 314.159 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Radius = 3, Area comes out at 28.274. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Homework and site work alike: how much paint a wall takes, how much water a tank holds, whether a corner is square — and, on the coordinate side, the slope between two points, the distance across a plan, the midpoint of a span.

What is the most common mistake?

Mixing units inside one shape. Entering a length in centimetres and a width in metres gives an area wrong by a factor of a hundred, and the result looks perfectly plausible.

What is the difference between the Circle calculator and the Circle sector area calculator?

This one returns Area and Circumference; the Circle sector area calculator returns Sector area and Arc length. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Circumscribed Circle Calculator is the closest one after this: Circumradius R = abc/4K, circumcentre and circumcircle of a triangle, from three sides or three vertices, with the figure drawn.

What else is worth having open alongside it?

Great Circle Distance Calculator and Inscribed Circle Calculator — they come up in the same task often enough to be worth a second tab.

Further reading

All guides →
How-toCircle: Circumference and AreaCircumference is 2πr, area is πr². Here's how radius, diameter and π fit together, both formulas worked through, and how not to mix them up.ExplainerThe Pythagorean Theorem ExplainedIn a right triangle, a² + b² = c². Here's what the formula means, a worked example, how to find any side, and where it's used in real life.How-toHow to Calculate VolumeVolume is the space inside a shape. Here's how to find it for boxes, cylinders and spheres, and why keeping your units consistent is the whole game.How-toHow to Calculate the Square Metres of a RoomFind a room's area by multiplying length by width. Here's the method for rectangular, L-shaped and round rooms, and why area drives your material orders.ExplainerThe Flat Pattern of a Cone Is a Sector, Not a CircleRoll a 50 mm cone 80 mm tall and its pattern is a 190.8° slice of a 94 mm circle. Flatten a nearly cylindrical lampshade and the pattern radius runs to 839 mm — the shallower the taper, the further away the apex.How-toHow Many Tiles Do You Need? (Floor and Wall)Estimate tiles from the area to cover and one tile's size, then add for cuts and waste. Here's the method, with an example and the 10% rule.