Ideal weight calculator: a range, not one perfect number

At 170 cm, BMI 18.5 to below 25 works out at roughly 53.5 kg to below 72.3 kg. That is a wide band, and it is wide on purpose: no height-and-sex formula can hand you one perfect number. This calculator puts the adult BMI range beside four classic estimates so you can see how they compare.

Where you belong inside that band, if anywhere, depends on your muscle, your history, your health and what you want the change for.

Why so many formulas exist

Robinson, Miller, Devine and Hamwi each came at height and weight from a different era and for a different job. Some were built for clinical work such as drug dosing, and they are still used there. None was built to tell you what to weigh.

They take height and sex and nothing else. Two people at the same height can differ a lot in muscle, age and medical history, and so can the weight each of them can hold comfortably.

A paper comparing these approaches makes the same case: a range tied to BMI tells you more than one “ideal” point. Peterson et al., ideal body weight and BMI

The formulas shown here

These are the versions you will find in most tables, written for heights above five feet. Below 152.4 cm the calculator leaves them blank rather than stretch the per-inch rule past where it was meant to go.

The Hamwi result is converted to your chosen units. There is no “frame size” adjustment. Male and female are the labels the original equations used; they say nothing else about the person standing on the scale.

Robinson's original paper drew its estimates from height-weight tables so that doctors could dose drugs. It is worth asking what an equation was built for before you let it plan your diet. Robinson et al., original paper

A worked comparison

For a woman at 170 cm, height is about 6.93 inches above five feet:

• Robinson: approximately 60.8 kg.
• Miller: approximately 62.5 kg.
• Devine: approximately 61.4 kg.
• Hamwi: approximately 61.1 kg.

They cluster tightly here, and it is tempting to read that as agreement. It is not. The four share the same two inputs and much of the same history; they are not four clinicians who examined her.

Now set them beside the BMI range of 53.5 to below 72.3 kg. A neat point with a decimal looks more certain than a wide band. It is not more true.

What the BMI range adds, and misses

The range is 18.5 × height in metres squared up to, but not including, 25 × height squared. The categories follow the CDC's convention for adults aged 20 and older. CDC BMI categories

BMI cannot tell fat from muscle or say where the fat sits. A weight inside the range does not make you healthy, and a weight outside it does not say why or what to do about it.

The BMI calculator shows your current number and what it can and cannot tell you. The body fat calculator adds a tape-measure view; imperfect too, but imperfect in a different way.

Choosing a goal you can evaluate

Start with why you want to change: blood markers, mobility, a sport, comfort, or something a clinician asked for. Your first goal can be a stage you can reach, not the bottom of the range.

HeyCoach will not set a target weight below the adult BMI 18.5 line. That is a floor, not a suggestion. Some people should aim well above it, and some should not be tracking weight loss at all.

Once you have a goal, the calorie calculator gives you a starting intake. Then watch your energy, hunger, strength and the weight trend over weeks. Unexpected loss, lasting fatigue or distress around food is a reason to talk to someone, not to try a new setting.

FAQ

Which ideal-weight formula is best?

None of them. Height and sex are not enough to find your healthiest weight. They are reference points from older tables, and that is all they can be.

Why do the historical formulas disappear below five feet?

Because they are written as increments above five feet. Stretching them below that gives you a number that looks precise and means little.

Should I aim for the lowest BMI-based weight?

No. The bottom of the range is where screening starts to flag underweight, not a prize. Pick a target that fits your health and your history.

The easy way to do this

Once you have picked a goal, tell HeyCoach in WhatsApp; there is no app to download. It sets calorie and protein targets in about 30 seconds and projects the date you would reach that weight at a sensible pace. From then on a photo or one line logs a meal, and a reply fixes an estimate even days later. This page gives you a range; the coach works with the number you choose inside it.