A ratio is in its simplest form when its parts share no common factor other than 1 — just like a fraction in its lowest terms. To simplify, divide every part by their highest common factor. For example, 6 : 8 both divide by 2 to give 3 : 4. Keep dividing until no common factor is left.
Worked example
Problem Express the ratio 18 : 24 in its simplest form.
Find the highest common factor of 18 and 24 — it is 6.
Divide each part by 6: 18 ÷ 6 : 24 ÷ 6.
18 : 24 = 3 : 4. The parts now share no common factor, so this is simplest form.
Tip A ratio is in its simplest form when its parts share no common factor other than 1.