Circle Diameter
Calculate circle diameter, circumference, and area instantly from radius. Free geometry tool for students, engineers, and DIY projects with chart visualization.
About This Calculator
The Circle Diameter Calculator helps you compute the diameter, circumference, and area of a circle when you know its radius. Whether you are a student learning geometry, an engineer designing circular components, or a DIY enthusiast planning a project, this tool provides quick and accurate results.
The calculations are based on fundamental circle formulas: diameter D = 2r, circumference C = 2pir, and area A = pir^2, where r is the radius. The tool uses the mathematical constant pi (pi) approximated to high precision for all computations.
Regional Notes
India (IN): Metric units (meters, centimeters) are commonly used. The calculator works with any consistent unit system.
United States (US): Imperial units (inches, feet) are common for DIY and construction projects. Enter the radius in your preferred unit.
United Kingdom (UK): Both metric and imperial units are used. The calculator is unit-agnostic -- just enter the radius value and read the results in the same unit.
Frequently Asked Questions
How do you calculate the diameter of a circle?
The diameter of a circle is calculated by doubling the radius: D = 2 x r. If you have the circumference, use D = C ÷ pi. If you have the area, use D = 2 x sqrt(A ÷ pi).
What is the formula for circumference from diameter?
The circumference is calculated as C = pi x D, where D is the diameter. Since D = 2 x r, the formula can also be written as C = 2pir.
How do you find the area of a circle from the diameter?
The area of a circle is A = pi x r^2, where r is the radius. Using the diameter, this becomes A = pi x (D ÷ 2)^2 = piD^2 ÷ 4.
Can I use any unit of measurement?
Yes, you can use any unit (meters, inches, feet, etc.). The calculator works with any consistent unit -- just enter the radius and the results will match the same unit for diameter and circumference, with area in square units.
What is the relationship between radius and diameter?
The diameter is always twice the radius. The radius is the distance from the center of the circle to any point on its edge, while the diameter is the distance across the circle through the center.
Is this circle calculator free to use?
Yes, all calculators on Calculy, including the Circle Diameter Calculator, are completely free to use with no limits or registration required.