The plural of radius is radii or radiuses.
A radius is a straight line from the center of a circle or sphere to any point on its circumference.
The plural for radius comes in two different forms, both of them acceptable in both American and British English.
Being a word of Latin origin, the correct way to pluralize the word radius according to Latin rules is to replace the suffix -us with -i, to form radii.
The word radius can also be pluralized according to the regular English rule of adding -es to the end of the word to form radiuses.
Both radiuses and radii are acceptable although the word radii is much more commonly used in written English as you can see from the graph of written instances of each word since 1900.
Singular Example: The radius of a circle can be calculated by dividing its circumference by 2π.
Plural Example: The radii of any two circles are equal in length if and only if their diameters are also equal in length.
Plural Example: By measuring the radiuses of all 8 circles, the student was able to approximate their areas.
The graph shows the occurances of the plural of radius in written English since 1800 using Google's Ngram Viewer.
The value of pi can be estimated by approximated by dividing the circumference of any circle by its diameter.