How to read the results
The calculator gives you two views. The first is the result and average of four clinical formulas originally developed to dose medication. The second is the weight range that keeps your body mass index between 18.5 and 24.9 at your height.
The formulas point to a single number, while the BMI range shows a broad healthy band. If your weight is already inside that band, you do not need to reach the formula average exactly. Ideal weight is a reference, not a requirement.
Which formulas are used?
Each formula starts from a base weight at 5 feet (152.4 cm) and adds a fixed amount for every inch (2.54 cm) above that:
Devine: men 50 kg + 2.3 kg, women 45.5 kg + 2.3 kg Robinson: men 52 kg + 1.9 kg, women 49 kg + 1.7 kg Miller: men 56.2 kg + 1.41 kg, women 53.1 kg + 1.36 kg Hamwi: men 48 kg + 2.7 kg, women 45.5 kg + 2.2 kg
The Devine formula is also used in the body analysis of the Kilotaki app. Below 5 feet the formulas are extended downward with the same slope, so results for very short heights are less reliable.
Why ideal weight is not one number
Two people of the same height can have very different frames, muscle mass and fat distribution, which is why the formulas disagree by a few kilograms. For someone with a large frame or who lifts weights regularly, values near the top of the range are more realistic.
In practice, a good approach is to consider the formula average and the healthy BMI range together. If your waist is under 94 cm (37 in) for men or 80 cm (31.5 in) for women and you feel energetic, your weight is very likely in a healthy range.
Limitations
These formulas were developed between the 1960s and 1980s, mostly for drug dosing and kidney function estimates. They do not measure body composition and ignore age, muscle mass, ethnicity and frame size. Error grows for very short or very tall people.
Muscular people can sit well above the formula result and still be perfectly healthy. In older adults, a slightly higher weight can even protect against frailty. Use the results as a general frame of reference, not a precise target.
When to see a doctor
If your current weight is far above or below the healthy range, talk to a doctor or registered dietitian before starting a plan to lose or gain weight. This is especially important if you have a chronic condition, are pregnant, are under 18 or have a history of disordered eating.