From a radius
Radius r
A = πr²; C = 2πr
Open calculator ↗GEOMETRY, ONE FIGURE AT A TIMECircle.
Circle.
Understand every measure.
One radius. A whole world of relationships. Four free tools, clear formulas and a little discovery along the way.
Find your calculator ↗FIGURE 012D / REFERENCE DRAWING
CIRCLENOT TO SCALE
THE CALCULATOR DESK
Start with what you know.
4 calculators available
From a diameter
Diameter d
r = d/2
Open calculator ↗Find radius from area
Area A
r = √(A/π)
Open calculator ↗Sector of a circle
Radius r + Central angle θ
Sector area = πr²θ/360; arc = 2πrθ/360
Open calculator ↗MEET THE FIGURE
A circle, explained.
A circle is the set of points in a plane at a fixed distance from a centre. That fixed distance is the radius. In everyday language, “circle area” means the area of the disk enclosed by the circle. The diameter passes through the centre and measures two radii.
Understand the properties ↗LOOK A LITTLE CLOSER
More than a formula.
The ratio of circumference to diameter is π for every Euclidean circle.
Do not enter the diameter in a field asking for radius. A sector boundary includes two radii as well as its arc.
Explore three geometry facts ↗These tools use ideal Euclidean geometry. Every input must use the same unit. Numerical results are rounded and do not replace measurement, specialist design or safety checks. See worked examples.