Equation of a Circle Calculator
📈 Coordinates & graphs. Use the Equation of a circle formula to find radius r, x, y, centre a, centre b: pick the value you want, type the others, and the answer updates as you go.
A circle with centre (a, b) and radius r.
Pick the ○ next to the value you want to find, fill in the others, and it solves as you type.
What each letter means
| Quantity | Unit |
|---|---|
| Radius r | no unit |
| x | no unit |
| y | no unit |
| Centre a | no unit |
| Centre b | no unit |
Worked example
Find the radius r when x = 4, y = 3, Centre a = 0, Centre b = 0.
Using (x - a)² + (y - b)² = r²: Radius r = 5.
Frequently asked questions
What is the formula for Equation of a circle?
(x - a)² + (y - b)² = r². A circle with centre (a, b) and radius r.
What do the letters stand for, and what units are used?
Radius r, x, y, Centre a, Centre b.
How do you calculate radius r?
Put the values into (x - a)² + (y - b)² = r². For example, with x = 4, y = 3, Centre a = 0, Centre b = 0, radius r = 5.
Can I solve for a different value?
Yes. In the calculator above, pick the circle next to any value (radius r, x, y, centre a, centre b) and fill in the others: it rearranges the formula for you.
