Circle Calculator
Enter any one of radius, diameter, circumference or area and get the rest — plus sector area, arc length, and chord length. Free, runs in your browser.
Circle Calculator
Enter any one value — the others are calculated.
How to Use the Circle Calculator
Enter one measurement
Type any single value you know — the radius, diameter, circumference, or area. You only need one.
Get the rest instantly
The calculator works out every other property of the circle at once, so you never have to rearrange formulas by hand.
Add a sector angle
Type an angle in degrees to also get the sector area, arc length, and chord length for that slice of the circle.
Use the results
Apply the numbers to flower beds, pizzas, pipes, wheels, or any round object — the calculator handles any units consistently.
Everything About a Circle
One Number Tells You Everything
A circle is the simplest curved shape, and it has a remarkable property: every measurement you might want is determined by a single number, the radius. The diameter is just twice the radius; the circumference (the distance around) is 2πr; and the area (the space inside) is πr². Because all four are linked, knowing any one of them tells you the other three — so if you measure the circumference of a tree trunk you can find its diameter, or if you know the area of a circular field you can find how much fence its edge needs. This calculator lets you enter whichever measurement you happen to have and instantly gives the rest, doing the algebra of rearranging the formulas for you. At the centre of all of it sits π (pi), the constant ratio of any circle's circumference to its diameter — about 3.14159 — one of the most important and most studied numbers in mathematics.
Beyond the whole circle, the calculator also handles a slice of one — a sector, like a wedge of pizza or a slice of pie chart. Give it an angle and it finds three things: the sector area (the area of the wedge), the arc length (the curved edge of the wedge), and the chord length (the straight line across the open end). These come up constantly in design and engineering — the metal needed to make a curved bracket, the length of a curved path, the geometry of gears and cams — and they each scale neatly with the angle: a 90° sector is exactly a quarter of the circle's area and a quarter of its circumference. Seeing the radius, the whole-circle properties, and the sector properties together makes the relationships concrete rather than a set of formulas to memorise.
"A circle hides all its secrets in one number. Give the calculator the radius — or anything you can derive it from — and every other measurement falls out."
Circles Everywhere
Circular measurement is one of the most practically useful pieces of geometry because round things are everywhere. Gardeners size circular beds and work out edging; cooks scale recipes between pizza and cake tins of different diameters (and discover that a 16-inch pizza has far more than twice the area of a 12-inch one, because area grows with the square of the radius); engineers and plumbers calculate the cross-section of pipes and the circumference of wheels and pulleys; and designers lay out logos, dials, and pie charts. The sector tools add the slices: clock faces, fan blades, curved staircases, and any "pie chart" proportion. Because this calculator runs entirely in your browser and accepts any single input, it is an instant, private way to get from one circular measurement to all the others, with no formula-shuffling and no unit assumptions — the numbers come out in whatever units you put in.
10 Facts About Circles
Every circle measurement comes from one number: the radius.
The diameter is twice the radius; the circumference is 2πr.
The area is πr² — it grows with the square of the radius.
So a 16″ pizza has ~1.8× the area of a 12″ one, not 1.3×.
π (pi) ≈ 3.14159 is the ratio of circumference to diameter.
A 90° sector is exactly a quarter of the circle.
The arc length is the curved edge of a sector.
The chord is the straight line across a sector's opening.
π has been computed to over 100 trillion digits.
The circle encloses the most area for a given perimeter of any shape.
Frequently Asked Questions
- The area is π times the radius squared (πr²). Enter the radius — or the diameter, circumference, or even the area itself — and the calculator gives the area along with every other measurement. For example, a radius of 5 gives an area of about 78.54.
- The circumference — the distance all the way around — is 2πr, or equivalently π times the diameter. Enter any single measurement and the calculator computes it. A radius of 5 gives a circumference of about 31.42.
- Yes. You can enter whichever value you actually have — radius, diameter, circumference, or area — and the calculator works backwards to the radius and then forwards to everything else. This saves you rearranging the formulas yourself.
- A sector is a "pie slice" of a circle, bounded by two radii and an arc. Its area is the fraction of the full circle equal to its angle over 360°. Enter a sector angle and the calculator gives the sector area, plus the arc length (curved edge) and chord length (straight line across the opening).
- Because area grows with the square of the radius, not in proportion to it. A 16-inch pizza has a radius one-third bigger than a 12-inch one, but (4/3)² ≈ 1.8 times the area. The calculator makes this easy to check — enter each diameter and compare the areas.
- The arc length is the length of the curved part of a sector — the portion of the circumference that the sector spans. It is the same fraction of the full circumference as the sector angle is of 360°. Enter a radius and an angle to see it.
- Whatever units you put in. If you enter the radius in centimetres, the circumference and arc come out in centimetres and the area in square centimetres. The calculator is unit-agnostic, so just keep your inputs consistent.
- Yes. The calculation runs in your browser — nothing is uploaded, stored, or logged — and it works offline once loaded.
- Completely free, with no account, sign-up, or limit. It runs entirely in your browser and collects no data. Use it as often as you like.
- It uses a high-precision value of π and shows results to ten significant figures, which is far more than enough for any practical purpose. The shared, tested math engine handles the calculations.
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