How do you calculate ideal body weight?
The Devine formula estimates ideal body weight (IBW) from height and sex only. Actual weight is not an input.
IBW (kg) = 50 (male) or 45.5 (female) + 2.3 × (height in inches − 60)
Height in inches is height in cm ÷ 2.54. The base value of 50 or 45.5 kg is the ideal weight at exactly 5 feet (152.4 cm). Every inch above that adds 2.3 kg.
How to use the ideal body weight calculator
- Enter the Height in cm.
- Choose the Sex: Female or Male. This sets the base value.
- Read the Ideal body weight, rounded to one decimal place. The steps show the height in inches and the multiplication.
- If the height is under 152 cm, read the warning: the formula is unreliable there.
Worked examples
Example 1 – a woman, 165 cm. 165 ÷ 2.54 = 64.96 in; 64.96 − 60 = 4.96. IBW = 45.5 + 2.3 × 4.96 = 56.91 → 56.9 kg.
Example 2 – a man, 178 cm. 178 ÷ 2.54 = 70.08 in; 70.08 − 60 = 10.08. IBW = 50 + 2.3 × 10.08 = 73.18 → 73.2 kg.
Example 3 – a woman under 5 feet, 148 cm. 148 ÷ 2.54 = 58.27 in; 58.27 − 60 = −1.73. IBW = 45.5 + 2.3 × (−1.73) = 41.52 → 41.5 kg. The result is below the 45.5 kg base, and the calculator warns that the formula is unreliable at this height.
Quick reference
Devine ideal body weight in kg, rounded to one decimal place as in the calculator.
| Height | Female | Male |
|---|---|---|
| 155 cm | 47.9 | 52.4 |
| 160 cm | 52.4 | 56.9 |
| 165 cm | 56.9 | 61.4 |
| 170 cm | 61.4 | 65.9 |
| 175 cm | 66 | 70.5 |
| 180 cm | 70.5 | 75 |
| 185 cm | 75 | 79.5 |
| 190 cm | 79.5 | 84 |
For the same height, the male value is always 4.5 kg higher than the female one.
Common mistakes
- Height in metres or feet. The calculator expects centimetres: 1.65 m is 165. And 5 ft 6 in is 66 inches, which is 66 × 2.54 = 167.6 cm, not 56 or 66 cm.
- Using IBW when actual weight is lower. If someone weighs less than their IBW, many protocols use the actual weight instead. It’s worth checking which weight your protocol asks for.
- Treating IBW as a target weight. It is a calculation aid for dosing, not a judgement about what someone should weigh.
- Rounding the inches too early. Keep the decimals until the end. Rounding 64.96 in to 65 adds almost 0.1 kg.
Which weight to use
IBW is one of three weights used in dosing, alongside actual and adjusted weight. When actual weight is well above IBW, some protocols ask for an adjusted body weight instead. The creatinine clearance calculator is a common place where the choice matters. Our guide to actual, ideal and adjusted body weight sets out when each one is used.
A quick sense check
IBW grows by about 0.9 kg for every centimetre and does not depend on actual weight at all. If two people of the same height and sex give different IBWs, one of the heights was entered wrongly. A result far below the base value means a very short height or a unit error.
Frequently asked questions
What is the Devine formula?
Ideal body weight = 50 kg for men, or 45.5 kg for women, plus 2.3 kg for every inch of height over 5 feet (60 inches). The calculator converts height in cm to inches by dividing by 2.54. A woman of 165 cm, for example, has an IBW of 56.9 kg.
Why does the formula use inches?
Because it was published in the United States in 1974, in imperial units. The maths still works in centimetres: each extra centimetre adds about 0.9 kg, because 2.3 ÷ 2.54 = 0.906. The calculator does the conversion for you and shows the height in inches in its steps.
What happens below 152 cm?
The formula starts subtracting weight. 152.4 cm is exactly 60 inches, so below it the inches over 60 become negative. The results are not well supported, and the calculator shows a warning in this range. At 148 cm, a woman’s IBW comes out at 41.5 kg, below the 45.5 kg base.
Is ideal body weight the weight a person should aim for?
No. Despite the name, it is a dosing estimate linked to height, not a health target. Its origins are poorly documented, and it is used because it is simple and widely known. For a person’s weight in relation to health, BMI and a clinical assessment are the usual starting points.
Can I use it for children?
No, the Devine formula was designed for adults. Children are usually dosed on actual weight or with paediatric references, following local guidance. Below 152 cm the formula also becomes unreliable, which rules out most children anyway. The calculator gives a result but cannot tell whether the person is a child.
Is ideal body weight the same as predicted body weight?
Almost. Predicted body weight, often used for ventilator settings, is written as 50 or 45.5 + 0.91 × (height in cm − 152.4). That is the Devine formula expressed in centimetres, so the two give practically the same number for the same height and sex.
Sources
- Pai MP, Paloucek FP. The origin of the ideal body weight equations. Ann Pharmacother 2000
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm 1974;8:650–655