Radius Sign: Copy, Meaning, and How to Type It
The radius sign is usually written as lowercase r in mathematics or uppercase R before a numerical measurement in engineering drawings. You can copy either character below. The correct notation depends on the mathematical problem, technical drawing, or drafting convention you are using.
Radius Sign Copy and Paste
Copy the radius notation that matches your needs.
| Character | Meaning | Example |
|---|---|---|
| r | Common mathematical variable for radius | r = 5 |
| R | Common radius prefix in engineering drawings | R10 |
| SR | Spherical radius notation in applicable drafting conventions | SR10 |
Copy the radius characters:
Lowercase radius notation: r
Uppercase radius notation: R
Spherical radius notation: SR
The letters r and R are ordinary alphabetic characters, not special Unicode symbols reserved exclusively for radius measurements. You can type them directly using a standard keyboard or copy them from the examples above.
For a geometry equation such as A = πr², lowercase r is conventional. In an engineering drawing, R10 commonly specifies a radius of 10 drawing units. The units and the applicable drawing conventions determine how that measurement should be interpreted.
What Is the Radius Sign?
The radius sign is a letter or notation used to represent or identify the radius of a circle, arc, sphere, or rounded feature. In mathematics, lowercase r and sometimes uppercase R serve as variables. In engineering dimensioning, uppercase R commonly appears immediately before a numerical radius value.
A radius is the distance from the center of a circle to its circumference. For a complete circle, the diameter is twice the radius. For example, a circle with a radius of 10 inches has a diameter of 20 inches.
The exact notation depends on the context. A math textbook may use r in a formula, while a manufacturing drawing may use R10 to identify a rounded edge or circular feature with a radius of 10 drawing units. Always follow the notation and units specified by the relevant problem, drawing, or drafting standard.
How to Type the Radius Sign
You do not need a special keyboard shortcut to type the ordinary letters used for radius notation.
On a Windows or US keyboard
Type lowercase r by pressing the R key. Type uppercase R by holding Shift and pressing R.
On a Mac keyboard
Type lowercase r by pressing the R key. Type uppercase R by holding Shift and pressing R.
In Microsoft Word and Google Docs
Type r or R directly into your document. You can also copy the characters from the copy-and-paste section and insert them into a formula, explanation, or technical note. For equations, use the application’s equation editor if you need properly formatted mathematical expressions, superscripts, or other mathematical notation.
On an iPhone, iPad, or Android device
Open the keyboard’s letter layout and tap R. Use the Shift key to switch between lowercase and uppercase letters.
If you are working in CAD software or preparing a manufacturing drawing, use the application’s dimensioning tools where appropriate. A radius dimension should be created using the relevant drawing function and conventions rather than relying solely on manually typed text.
Radius Sign Unicode and HTML Codes
The ordinary letters used for radius notation have standard Unicode code points. However, these characters are not dedicated radius symbols.
| Character | Unicode name | Unicode code point | HTML numeric reference |
|---|---|---|---|
| r | LATIN SMALL LETTER R | U+0072 | r |
| R | LATIN CAPITAL LETTER R | U+0052 | R |
| ⌀ | DIAMETER SIGN | U+2300 | ⌀ |
| Ø | LATIN CAPITAL LETTER O WITH STROKE | U+00D8 | Ø |
There is no universally standard, dedicated Unicode radius-sign character comparable to the diameter sign ⌀. For ordinary text, lowercase r and uppercase R are the usual choices, depending on context.
The Unicode character ⌀ is specifically named DIAMETER SIGN. The visually similar character Ø is a different Unicode character: LATIN CAPITAL LETTER O WITH STROKE. Although Ø is used to indicate diameter in some technical contexts, the two characters are not interchangeable in every application, typeface, or drafting standard.
How to Use and Format the Radius Sign
The letter r is widely used in mathematical formulas involving circles. Two common formulas are: Circle area: A = πr². Circle circumference: C = 2πr. In both formulas, r represents the radius of the circle. The radius must be expressed in a consistent unit of length.
For example, if a circle has a radius of 5 centimeters, its area is approximately 78.54 square centimeters and its circumference is approximately 31.42 centimeters.
In engineering drawings, uppercase R commonly appears before a numerical value. For example, R5 indicates a radius of 5 drawing units under the drawing’s applicable conventions. A radius callout may also include tolerance information. When documenting a manufactured part, use the complete drawing, including its units, notes, and tolerances, rather than interpreting a dimension in isolation.
Radius Sign vs. Diameter Notation
Radius and diameter describe different measurements of a circle. Radius: the distance from the center of a circle to its boundary. Diameter: the distance across a complete circle through its center. For a complete circle: d = 2r.
| Notation | Meaning | Example interpretation |
|---|---|---|
| r = 10 | Mathematical radius | Radius is 10 units |
| R10 | Engineering radius dimension | Radius is 10 drawing units |
| ⌀20 | Diameter sign followed by a value | Diameter is 20 drawing units |
| Ø20 | Common alternative diameter notation | Intended diameter is 20 units |
| SR10 | Spherical radius notation | Spherical radius is 10 drawing units |
| S⌀20 | Spherical diameter notation | Spherical diameter is 20 drawing units |
The dedicated diameter sign ⌀ and the letter Ø are distinct Unicode characters. Follow the relevant technical drawing standard and software conventions when selecting diameter notation.
Radius Sign in Engineering Drawings
In engineering drawings, uppercase R commonly appears immediately before the numerical radius dimension. A callout such as R12 identifies a radius of 12 drawing units under the drawing’s applicable conventions. This notation helps engineers, machinists, designers, and inspectors identify rounded edges, arcs, and curved features on a manufactured component.
For example, a drawing may specify R8 for a rounded corner and use a diameter callout for a circular hole. Radius dimensions may include tolerances, surface requirements, or other notes. Spherical features may use prefixes such as SR for spherical radius in applicable drafting conventions.
Radius Sign Examples and Quick Comparison
| Example | Interpretation |
|---|---|
| r = 4 cm | Radius is 4 centimeters |
| d = 2r = 8 cm | Diameter of the complete circle is 8 centimeters |
| C = 2πr | Formula for circle circumference |
| A = πr² | Formula for circle area |
| R8 | Engineering radius of 8 drawing units |
| ⌀16 | Diameter of 16 drawing units |
| SR10 | Spherical radius notation |
Practical example: Suppose a complete circular plate has a diameter of 30 inches. Its radius is half the diameter: r = d ÷ 2 = 30 ÷ 2 = 15 inches.
Radius Sign FAQ
What is the radius sign?
The radius is commonly represented by lowercase r in mathematics. In engineering drawings, uppercase R is commonly placed before the numerical radius dimension, as in R10.
What is the symbol for radius in geometry?
The usual mathematical variable is r, although R may also be used. Follow the notation defined in the particular equation, textbook, or geometry problem.
Is there a special Unicode radius sign?
There is no universally standard, dedicated Unicode radius-sign character comparable to the diameter sign ⌀. Ordinary text generally uses r or R.
What is the difference between a radius and a diameter?
The radius measures the distance from the center of a circle to its boundary. The diameter crosses the circle through its center and is twice the radius: d = 2r.
Does the radius sign go before or after the number?
In conventional engineering dimensioning, uppercase R commonly appears before the numerical value. For example, R15 indicates a radius of 15 drawing units.
Is Ø the same as the radius sign?
No. Ø is commonly used to indicate diameter in technical contexts, while R commonly indicates radius in engineering drawings.
How do I type the radius sign on a keyboard?
Press the R key to type lowercase r. Hold Shift and press R to type uppercase R. These are ordinary keyboard characters.
What does R20 mean on an engineering drawing?
R20 generally identifies a radius dimension of 20 drawing units. For a complete circle with a radius of 20 units, the diameter would be 40 units.
What is the formula for calculating radius from diameter?
Divide the diameter by two: r = d ÷ 2. For example, a circle with a diameter of 30 inches has a radius of 15 inches.
Conclusion
The radius sign is generally written as r in mathematical formulas and R before a numerical radius dimension in engineering drawings. Understanding radius notation alongside diameter symbols, formulas, drawing units, and technical standards helps prevent mistakes in schoolwork, CAD design, manufacturing, and technical documentation.
Jack Harrison is a contributor at Square Root Signs, sharing creative ideas, useful information, and engaging content to help readers discover something valuable with every visit.
