Circle Calculator

Which measurement can I start from?

Pick radius, diameter, circumference or area from the dropdown and enter one value. The calculator converts it to a radius first (r = d ÷ 2, r = C ÷ 2π, or r = √(A ÷ π)) and then derives the other three from that radius, so the four results always agree with each other.

What does the calculator return for a radius of zero or a negative number?

It shows the error "Value must be positive". A circle needs a radius greater than zero, so 0 and negative entries are rejected for every input type, including area and circumference. A cleared field counts as 0, so it triggers the same message.

Why is the description talking about arc length when I only see four results?

The results are area, circumference, diameter and radius. Arc length and sector area need a central angle, which this form does not ask for. Use the formulas arc = 2πr × θ ÷ 360 and sector area = πr² × θ ÷ 360 with θ in degrees, taking r from the Radius line of the output.

How many decimal places are shown, and does it round?

Results are rounded to at most six decimal places and trailing zeros are dropped, so a radius of 10 gives an area of 314.159265 and a diameter of 20. Because of that rounding, a circle with a radius far smaller than 0.001 will show a tiny area as 0 or a heavily truncated number. Scale the units up (millimetres instead of metres) for very small circles.

Which value of π does it use?

JavaScript’s built-in Math.PI, which is accurate to about 15 to 16 significant digits. That is far more precise than 3.14 or 22/7, so your answer can differ slightly from a textbook that rounds π. For a radius of 7, 22/7 gives an area of 154 while the calculator gives 153.93804.

What units do the answers come in?

The same unit you typed, squared for area. Enter the radius in centimetres and the circumference and diameter are in centimetres while the area is in square centimetres. The calculator does not convert between units and never labels them, so keep track yourself.

Can I find the radius from an area?

Yes. Choose Area and enter the value. An area of 50 gives a radius of 3.989423, a diameter of 7.978846 and a circumference of 25.066283. The inverse uses the square root, so the area must be positive.

Does it handle ellipses, semicircles or rings?

No, only a full circle. For a semicircle halve the area (πr² ÷ 2); its perimeter is πr + 2r because the straight edge counts. For a ring, run the calculator twice, once for the outer radius and once for the inner, and subtract the two areas.