The healthy weight range
The range at the top is the weight that gives a body mass index between 18.5 and 24.9 at your height, which the World Health Organization classes as a healthy weight for adults. For someone 170 cm tall that is about 53.5 to 72 kg. It is the most widely used guide, it is the same for men and women, and it leaves room for differences in build.
The four ideal weight formulas
Doctors use these formulas mainly to dose medicines, but they are a useful single number to aim for. Each starts with a weight for a height of 5 feet and adds a set amount for every inch above it. The Devine formula (1974) gives men 50 kg plus 2.3 kg per inch and women 45.5 kg plus 2.3 kg per inch. Robinson and Miller (both 1983) and Hamwi (1964) use slightly different numbers, so the results spread over a few kilograms. The average is shown as a middle point.
Why your ideal weight may differ
None of these methods measures body fat. People with more muscle, a larger frame or who are very active can be healthy above these numbers, and older adults may be healthier slightly above the lower end. Some health bodies use lower BMI cut-offs for people of South Asian, East Asian and Middle Eastern backgrounds, because the risk of diabetes and heart disease starts at a lower weight. Waist size is a helpful extra check: a waist above half your height is a warning sign.
Who should not use it
The calculator is for adults aged 18 and over. It does not apply during pregnancy, and children and teenagers are assessed with growth charts. If you have a health condition or want to lose or gain a lot of weight, ask a doctor or dietitian to set a realistic target with you.