It is also used as a symbol in engineering and mathematics:

- In engineering, the diameter symbol is often referred to as "phi". This symbol is used to indicate the diameter of a circular section, for example means the diameter of the circle is 14 inches or millimeters, depending on the context.
- It is used to represent the Golden mean and the Euler totient function.
- In spherical coordinates phi is usually used to represent the angle to the z axis.
- The golden mean (1+√5)/2 is often called Φ, φ being 1/Φ.