Glossary
circumference
The circumference is the distance around a circle, equal to π times the diameter, or 2πr. Use 3.14 for π unless the question says otherwise.
The circumference is the distance around a circle — the circle's perimeter. The two formulas are C = πd (pi times the diameter) and C = 2πr (twice pi times the radius), which say the same thing because the diameter is twice the radius. With a radius of 5, the circumference is 2 × 3.14 × 5 = 31.4.
Three things decide whether the question works out. Choose the right form: pi times the diameter if the diagram labels a diameter, 2πr if it labels a radius, and halve or double if it labels the other one. Keep pi as 3.14 unless the question offers 22/7 or asks for the answer in terms of pi, in which case leave the symbol in. And check that the units are consistent before multiplying.
The usual error is quoting the area instead. Area is πr² and takes squared units; circumference is 2πr and takes ordinary ones, and a question about a wheel traveling a distance, a circular fence or a ring's edge wants the circumference. Since the two formulas differ only in the square and the 2, reading the question for whether it asks about space inside or distance around is the whole check.
Taught in
Related terms
- perimeter — The perimeter is the distance all the way around a shape — add the lengths of every side. For a rectangle the formula is 2(length + width).
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)