BMI = weight (kg) ÷ height (m)²
70 kg and 170 cm give 70 ÷ 1.70² = 24.22. That sits in the 18.5 to 24.9 band. The page also accepts pounds and inches and converts them into the same formula.
The bands describe adults. Athletes, pregnancy, children and older adults can need a different clinical reading. The number is not body fat and it does not replace a visit with a clinician.
Questions
Which height is used?
Meters, after dividing centimeters by 100. 170 cm is 1.70 m.
Is 18.5 underweight?
No. Below 18.5 is the first band. 18.5 is already in the middle band.
Can I use this for a child?
WHO uses age-based charts for children. This page applies only the four adult bands.
Is the weight uploaded?
The weight is sent with this request to calculate BMI and is not stored.