Diameter to Radius Calculator

Convert between diameter and radius instantly. Enter either measurement, pick your unit (mm, cm, m, in or ft), and this calculator works out the radius, diameter, circumference and area of the circle.

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Reference formula Standard geometry: radius = diameter / 2. Uses full-precision pi.

1. Enter Your Measurement

Convert from

Diameter

Radius

Unit

2. What You Get

Enter the diameter or radius of any circle and this tool converts it to the other measurement straight away. It also works out the circumference (distance around the circle) and the area (space inside the circle) using the same unit you entered.

This is useful for pipe sizing, wheel and tyre measurements, round garden beds, pizza sizes, or any job where a spec sheet gives you one measurement and you need the other.

Conversion Results

Radius
-
Centre to edge

Diameter
-
Edge to edge through centre

Circumference

2 × π × radius

Area

π × radius²

Full Breakdown

Radius-

Diameter-

Circumference-

Area-

Formulas Used

Radius from diameterr = d ÷ 2

Diameter from radiusd = 2 × r

CircumferenceC = 2πr

AreaA = πr²

How to Convert Diameter to Radius

The radius of a circle is always exactly half of its diameter. The diameter is the straight-line distance across a circle, passing through its centre, from one side to the other. The radius runs from the centre point to any point on the edge. Because the centre sits exactly halfway along the diameter, the relationship between the two is simple and fixed.

The Formula

Conversion	Formula	Example
Diameter to radius	r = d ÷ 2	20 cm diameter = 10 cm radius
Radius to diameter	d = 2 × r	10 cm radius = 20 cm diameter
Circumference (from radius)	C = 2πr	10 cm radius = 62.83 cm circumference
Area (from radius)	A = πr²	10 cm radius = 314.16 cm² area

Common Uses

Plumbing and pipes: pipe sizes are often given as diameter, but flow and fitting calculations sometimes need the radius.
Wheels and tyres: tyre diameter is printed on the sidewall, but suspension and clearance calculations often use radius.
Round garden beds and pools: working out materials or fencing length around a circular area.
Manufacturing and engineering: converting between spec sheets that use diameter and formulas that require radius.

Frequently Asked Questions

How do you convert diameter to radius? Divide the diameter by 2. A circle with a diameter of 20 cm has a radius of 10 cm.

How do you convert radius to diameter? Multiply the radius by 2. A circle with a radius of 7 m has a diameter of 14 m.

Can I find the circumference and area from the radius or diameter? Yes, once you have the radius this calculator automatically works out the circumference (2 x pi x radius) and area (pi x radius squared) for you.

Related Calculators

Essential Calculators: the full toolkit of everyday conversion and maths calculators.
Circumference to Diameter Calculator: work backwards from circumference.
Circumference Calculator: find the distance around any circle.
Area of a Circle Calculator: work out the area from radius or diameter.
Circle Calculator: solve for any circle measurement.

Sources: Standard Euclidean geometry definitions of radius, diameter, circumference and area, as used in NZ secondary school mathematics curricula (nzmaths.co.nz).

This calculator uses exact geometric formulas and full-precision pi. Results are rounded for display. Always double-check critical measurements (such as pipe fittings or structural work) with a physical measurement or a qualified tradesperson.

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