When using kg and metres the calculation is weight in kg divided by height (metres) squared, so you only square the height. For example, Lee weighs 78kg and is 180cm tall. So we would first convert centimetres to metres which becomes 1.8m and to square this we would do 1.8*1.8. We would then divide weight (78kg) by the answer of height squared which gives us a BMI of 24.1.
Or you can find an online BMI calculator here: https://www.heartfoundation.org.au/your-heart/know-your-risks/healthy-weight/bmi-calculator.