How to Calculate Ideal Body Weight: Devine Formula Explained

Learning how to calculate ideal body weight gives you a sensible target to aim for, instead of a number copied from a chart or guessed from a friend’s weight. The method in this guide uses the Devine formula, which is the formula behind the Body Weight Calculator, together with a healthy BMI weight range for the same height. Devine gives a single target; the BMI range gives room to move.

Ideal body weight is not a verdict on your health, and it is not the weight you must reach to be fit. It is a starting point for setting a goal, planning food and training, and judging whether your current weight sits near, above, or below the range usually considered healthy for your height. It uses only sex and height, so treat it as a guide, not a verdict.

The Method / Formula

The Devine formula was originally created for estimating drug dosages, and it is now the most quoted ideal-weight formula. It starts from a base weight at five feet tall, then adds a fixed amount for every inch above five feet. Men and women have different base weights.

Men: IBW = 50 kg + 2.3 kg per inch above 5 ft.

Women: IBW = 45.5 kg + 2.3 kg per inch above 5 ft.

The healthy BMI weight range uses the same height differently. Body mass index, or BMI, is weight in kilograms divided by height in metres squared. Turning that around gives a weight range: the lowest healthy weight is 18.5 times height in metres squared, and the highest is 24.9 times height in metres squared.

Healthy BMI weight range = 18.5 x height(m)^2 to 24.9 x height(m)^2.

Here is what each part means, term by term:

• Base weight: 50 kg for men and 45.5 kg for women. This is the Devine figure at exactly five feet tall, before any extra height is added.
• 2.3 kg per inch: the amount added for each inch above five feet. It is the same for both sexes; only the base weight differs.
• Inches above 5 ft: your height in inches minus 60, because five feet is 60 inches. At exactly five feet, this part is zero.
• Height in metres: your height in centimetres divided by 100. This is the height used for the BMI range, not for the Devine part.
• 18.5 and 24.9: the lower and upper BMI limits that define the healthy range. The weights matching those limits form the band.

The key conversion is centimetres to inches, because Devine works in inches while most people measure height in centimetres. Divide your height in centimetres by 2.54 to get inches, and the rest is simple subtraction, multiplication, and addition.

How to Calculate Ideal Body Weight Step by Step

1. Choose the correct sex for the formula, because the base weight is 50 kg for men and 45.5 kg for women.
2. Measure your height in centimetres, standing barefoot against a wall.
3. Convert your height to inches by dividing the centimetre figure by 2.54.
4. Work out how many inches you are above five feet by subtracting 60 from your height in inches, then multiply that number by 2.3 kg.
5. Add the result to the base weight for your sex. That total is your Devine ideal body weight in kilograms.
6. For the healthy range, convert your height to metres by dividing centimetres by 100, square that number, and multiply it by 18.5 for the lower end and by 24.9 for the upper end.
7. Compare your current weight with both figures, using the Devine figure as a single target and the BMI range as the band to aim inside.

Worked Example

Take the example used on the tool page: a man who is 175 cm tall. Follow the steps above and you should land on exactly the figures the calculator gives.

• Convert height to inches: 175 / 2.54 = 68.90 inches, to two decimal places.
• Find the inches above five feet: 68.90 – 60 = 8.90 inches. Using the unrounded figure, this is 8.8976 inches.
• Multiply by 2.3 kg: 8.8976 x 2.3 = 20.46 kg.
• Add the men’s base weight: 50 + 20.46 = 70.46 kg. That is the Devine ideal body weight.
• For the BMI range, first convert height to metres: 175 cm = 1.75 m, and 1.75 squared is 3.0625.
• Lower end of the range: 18.5 x 3.0625 = 56.66 kg.
• Upper end of the range: 24.9 x 3.0625 = 76.26 kg.

So, for a man of 175 cm, the Devine ideal body weight is 70.46 kg and the healthy BMI weight range is 56.66 kg to 76.26 kg. The Devine figure sits inside the BMI range, toward its upper half, and the range itself is almost 20 kg wide. That width is the point: a healthy weight for this height is a band, not a single point. For comparison, a woman of the same height would have a Devine figure of 45.5 + 20.46 = 65.96 kg, while her BMI range would be exactly the same, because the BMI range depends on height alone.

Common Mistakes / Good to Know

• Using centimetres as if they were inches. If you multiply a centimetre figure by 2.3 kg, the result will be far too high. Always divide by 2.54 first, and only then subtract 60.
• Using the wrong base weight. The 50 kg base is for men and the 45.5 kg base is for women, and swapping them shifts the answer by 4.5 kg. Double-check the sex you selected before you trust the result.
• Treating the Devine figure as an exact or harsh target. It is an estimate from height and sex alone, originally made for drug dosages, and it takes no account of frame size, muscle, or body composition. A large-framed person may sit healthily near the top of the BMI band and a small-framed person near the bottom, so aim for the range rather than chasing one decimal place.
• Applying the formula where it does not fit. It is not suitable for children, who need growth-chart percentiles, or for muscular athletes, who can sit above the BMI band while still being lean. In those cases a body-composition measure is more useful than a height-only target.

For a fuller picture of what that weight is actually made of, try the Lean Body Mass Calculator, and read our related guide How to Calculate Lean Body Mass: Formulas with Example.

Frequently Asked Questions