BMI (body mass index) is calculated by dividing weight in kilograms by height in metres squared; 18.5–24.9 is normal, 25–29.9 overweight and 30 or above obesity. The obesity threshold depends on height: at 1.70 m it begins at 86.7 kg (191 lb). The number does not separate fat from muscle and is read together with waist circumference and examination findings. In aesthetic surgery planning most surgeons expect a stable weight with a BMI below 30.
- Formula: BMI = weight (kg) ÷ height (m)²; for 1.70 m and 80 kg the result is 27.7 kg/m² (overweight).
- WHO chart: below 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and above obesity (class I–III); the same for women and men.
- Am I obese: obesity begins at height² × 30, i.e. 76.8 kg at 1.60 m, 86.7 kg at 1.70 m, 97.2 kg at 1.80 m.
- A healthy weight is a range for your height: 50.4–67.8 kg at 1.65 m, 53.5–72.0 kg at 1.70 m.
- A waist above 80/88 cm in women or 94/102 cm in men raises the risk within the same BMI class (WHO).
A BMI calculator divides your weight in kilograms by your height in metres squared and places the result on the World Health Organization chart: underweight, normal, overweight or obese. Body mass index, BMI and Quetelet index are names for the same calculation. This page gives the formula with worked examples, the WHO chart, the answer to “am I obese” as a weight for your height, a healthy weight table for women and men, how age changes the reading, and why plastic surgeons ask for this number before body-contouring surgery.
What Is Body Mass Index (BMI)? Is It the Same as the Quetelet Index?
Body mass index is a screening measure that shows whether an adult’s weight falls within the range expected for their height; it is weight divided by height squared and is written in kg/m². BMI, body mass index and the older term Quetelet index describe the same formula, the same chart and the same result; the Belgian mathematician Adolphe Quetelet described the ratio in the 19th century, which is why some sources still use his name.
The measure is used so widely because it needs only height and weight, and because large population studies have linked high values with type 2 diabetes, high blood pressure, cardiovascular disease and several cancers. At the same time the number does not tell fat from muscle, does not show where fat is stored and never makes a diagnosis on its own; a diagnosis of obesity and a weight plan are completed by a physician’s examination, waist circumference and, where needed, body-composition measurement. The calculator on this page is designed for adults; children and teenagers are not assessed with the adult chart.
How Is BMI Calculated? Formula and Worked Examples
BMI is calculated by dividing weight in kilograms by height in metres squared: BMI = weight (kg) ÷ [height (m) × height (m)]. The most common mistake is entering height in centimetres; 170 cm must be entered as 1.70 m. In imperial units the equivalent formula is BMI = 703 × weight (lb) ÷ [height (in) × height (in)]; the calculator at the top of this page switches between the two systems with one tap.
Two examples show how the calculation runs. For a woman who is 1.65 m (5 ft 5 in) tall and weighs 60 kg (132 lb), height squared is 1.65 × 1.65 = 2.72; 60 ÷ 2.72 = 22.0, inside the normal range (18.5–24.9). For a man who is 1.80 m (5 ft 11 in) tall and weighs 95 kg (209 lb), height squared is 3.24; 95 ÷ 3.24 = 29.3, close to the upper limit of the overweight range (25–29.9). For the frequently searched “5 ft 7 in and 176 lb” (1.70 m, 80 kg) the result is 80 ÷ 2.89 = 27.7, which is overweight; at this height obesity begins at 86.7 kg (191 lb). The calculator above uses exactly this formula; the result should be read together with the chart and the thresholds below.
Measurement tips for an accurate BMI
Measurement conditions change the result. Weigh yourself in the morning before breakfast, in light clothing and preferably on the same scale, so readings on different days can be compared. Measure height without shoes, on a hard floor, head level and heels against the wall. Daily fluid shifts move weight by a few hundred grams, so the trend of repeated measurements under the same conditions is more reliable than a single reading. A 2 cm error in height shifts the number by roughly 0.6 points.
What Should My BMI Be? The WHO BMI Chart
For adults, BMI should be between 18.5 and 24.9; the World Health Organization classes this range as normal, 25–29.9 as overweight and 30 or above as obesity, which is divided into three classes. The chart is the same for women and men.
| BMI (kg/m²) | Category | Short interpretation |
|---|---|---|
| Below 18.5 | Underweight | Nutrition and underlying causes may need assessment. |
| 18.5 – 24.9 | Normal | Weight is within the expected range for height. |
| 25 – 29.9 | Overweight | Interpreted together with waist circumference and metabolic tests. |
| 30 – 34.9 | Obesity, class I | Health risks rise; a weight-management plan is advised. |
| 35 – 39.9 | Obesity, class II | Risks rise markedly; medical assessment is needed. |
| 40 and above | Obesity, class III (morbid obesity) | Medical or surgical weight treatment may be considered. |
Single values that people often ask about are easy to place: 20.7 and 22.5 are normal; 27 is overweight, 30 is where obesity begins, 35 is the class II threshold and 40 the class III threshold. The short answer to “what BMI is dangerous” runs in two directions: above 30 the risk of cardiovascular and metabolic disease rises clearly, and below 18.5 malnutrition, bone loss and weakened immunity become concerns. The number is first placed in its range; what that range means for a particular person is decided together with waist circumference, blood pressure, fasting glucose, blood lipids and activity level. Two people with a value of 27 can be very different: one a regular exerciser with high muscle mass, the other sedentary with a wide waist.
Am I Obese? Obesity Thresholds by Height
Your BMI answers the question: according to the World Health Organization, adults with a BMI of 30 kg/m² or more are obese and those between 25 and 29.9 are overweight. “At what weight am I obese” depends on height; the obesity threshold is height squared multiplied by 30. The table below shows, for each height, the weight at which overweight (BMI 25), obesity (BMI 30) and morbid obesity (BMI 40) begin.
| Height | Overweight from (BMI 25) | Obesity from (BMI 30) | Morbid obesity (BMI 40) |
|---|---|---|---|
| 1.55 m (5′1″) | 60.1 kg (132 lb) | 72.1 kg (159 lb) | 96.1 kg (212 lb) |
| 1.60 m (5′3″) | 64.0 kg (141 lb) | 76.8 kg (169 lb) | 102.4 kg (226 lb) |
| 1.65 m (5′5″) | 68.1 kg (150 lb) | 81.7 kg (180 lb) | 108.9 kg (240 lb) |
| 1.70 m (5′7″) | 72.3 kg (159 lb) | 86.7 kg (191 lb) | 115.6 kg (255 lb) |
| 1.75 m (5′9″) | 76.6 kg (169 lb) | 91.9 kg (203 lb) | 122.5 kg (270 lb) |
| 1.80 m (5′11″) | 81.0 kg (179 lb) | 97.2 kg (214 lb) | 129.6 kg (286 lb) |
| 1.85 m (6′1″) | 85.6 kg (189 lb) | 102.7 kg (226 lb) | 136.9 kg (302 lb) |
| 1.90 m (6′3″) | 90.3 kg (199 lb) | 108.3 kg (239 lb) | 144.4 kg (318 lb) |
The second measure in assessing obesity is waist circumference, because fat stored inside the abdomen carries a higher risk at the same BMI. According to the World Health Organization’s expert report on waist circumference, a waist above 80 cm in women and 94 cm in men indicates increased risk, and above 88 cm in women and 102 cm in men substantially increased risk. The waist is measured at the level of the navel, after breathing out, without the tape compressing the skin. A person with a normal BMI but a wide waist is assessed on that finding; in a muscular person a high BMI is read together with waist and body-fat percentage. The diagnosis of obesity and the treatment plan are made by a physician using these measurements together; the calculator on this page is a screening tool.
Ideal Weight by Height: Healthy Weight Range for Women and Men
An ideal weight is not a single number but the range obtained by converting the 18.5 and 24.9 thresholds into kilograms for your height: lower limit 18.5 × height², upper limit 24.9 × height². Because the WHO chart is the same for both sexes, the ranges below apply to women and men alike; “how much should a woman of 5 ft 5 in weigh” and “how much should a man of 5 ft 7 in weigh” are both read from this table.
| Height | Normal weight range (BMI 18.5–24.9) |
|---|---|
| 1.50 m (4′11″) | 41.6 – 56.0 kg (92 – 123 lb) |
| 1.55 m (5′1″) | 44.4 – 59.8 kg (98 – 132 lb) |
| 1.60 m (5′3″) | 47.4 – 63.7 kg (105 – 140 lb) |
| 1.65 m (5′5″) | 50.4 – 67.8 kg (111 – 149 lb) |
| 1.70 m (5′7″) | 53.5 – 72.0 kg (118 – 159 lb) |
| 1.75 m (5′9″) | 56.7 – 76.3 kg (125 – 168 lb) |
| 1.80 m (5′11″) | 59.9 – 80.7 kg (132 – 178 lb) |
| 1.85 m (6′1″) | 63.3 – 85.2 kg (140 – 188 lb) |
| 1.90 m (6′3″) | 66.8 – 89.9 kg (147 – 198 lb) |
| 1.95 m (6′5″) | 70.3 – 94.7 kg (155 – 209 lb) |
The range is a frame, not a target: a person with high muscle mass can sit slightly above the upper limit and be healthy, while an older person with reduced muscle can be inside the range yet carry a high fat percentage. For those who want a single number, the Devine formula used in clinical practice gives a reference by sex: 50 kg for men and 45.5 kg for women at 152.4 cm (5 ft), plus 2.3 kg for every 2.54 cm (1 in) above that; at 1.70 m the reference is about 66 kg for a man and 61 kg for a woman. The formula was developed for drug dosing and draws a narrower frame than the WHO range; a personal weight goal is set with a physician and according to body composition.
BMI by Age: Women, Men, Over 65 and Children
If you are looking for a BMI chart by age, the first thing to know is that the World Health Organization publishes no separate adult thresholds by age; the 18.5–24.9 range applies to all adults from 18 onward, and the “25–34 years: 20–25, over 65: 24–29” style tables that circulate online have no official source such as the WHO or NIH. What changes with age is not the chart but the interpretation: as muscle mass falls the same value corresponds to a higher fat percentage, and hormonal stages and chronic illness shift the balance of muscle and fat.
BMI for women and men
BMI is calculated with the same formula and read on the same chart for women and men; 18.5–24.9 is normal for both. The difference lies in interpretation: at the same value women generally carry a higher fat percentage than men; pregnancy, breastfeeding and menopause change the number temporarily, and the standard chart is not used in pregnancy. Men tend to store fat around the abdomen, so waist circumference complements BMI, which is why the waist thresholds above are given by sex. Choosing your sex in the calculator at the top of this page changes that threshold and the interpretation note; it does not change the formula.
Over 65 and in children
In older age muscle mass declines, height may decrease slightly and the same number corresponds to a higher fat percentage. A meta-analysis of 32 studies in people over 65 found that mortality risk did not rise between BMI 23 and 33 but increased below 23 and above 33; a mildly elevated value in later life is therefore not a problem on its own, while a very low value matters for frailty and malnutrition. In children and teenagers (5–19 years) the adult thresholds are not used; the value is read on the World Health Organization’s growth curves by age and sex: more than +1 standard deviation above the median counts as overweight and more than +2 standard deviations as obesity. Because growth continues, the trend on the curve matters more than a single reading, and the assessment is made by a paediatrician.
Why Do Plastic Surgeons Ask About BMI?
In body-contouring surgery, BMI is asked about because it relates directly to anaesthesia safety, wound healing and how long the result lasts. Surgeons look both at the number itself and at whether weight has been stable for several months; at high values a weight-loss plan is usually discussed first. The thresholds below summarise common surgical practice; the individual decision is always made at examination.
Tummy tuck and liposuction
Published patient series show that complications after abdominoplasty, such as seroma, wound-healing problems and infection, increase as BMI rises, and the difference becomes clear above 30. For this reason many surgeons aim for a BMI below 30 and a weight that has been stable for several months before a tummy tuck; an operation performed before weight loss is complete may be undone by later weight changes. The amount of excess skin and muscle separation is measured at examination, independently of the number.
The American Society of Plastic Surgeons (ASPS) describes good candidates for liposuction as adults within 30% of their ideal weight with good skin elasticity and a stable weight. Liposuction is not a weight-loss method; it contours localised fat that diet and exercise do not remove, so the result shows more in the silhouette than on the scale.
Breast, body lifts and after major weight loss
In breast reduction, breast lift and body-lift surgery a high BMI is also linked with wound-healing and anaesthesia risks and is therefore weighed in planning; for facial procedures the decision depends on facial anatomy and regional fat rather than BMI. People who lose a large amount of weight through bariatric surgery or diet are left with excess skin on the abdomen, arms, thighs and breasts; these operations are planned as post-bariatric surgery and weight is expected to stabilise first so that removed skin does not re-form.
Lipedema, a disproportionate fat accumulation in the legs, is a separate condition that BMI does not explain and is assessed differently. Before any operation, steps such as stopping smoking and adjusting medication are planned independently of BMI.
The Limits of BMI and How to Lower It
BMI does not measure body fat, and the only way to lower it is to reduce body weight; the number does not distinguish fat from muscle and does not show whether fat sits inside the abdomen, under the skin or around the hips and thighs. A strength-trained athlete may read as “overweight”, while a person with little muscle may have a normal value yet a high fat percentage. In pregnancy, in heart, kidney or liver disease that causes fluid retention, and in older age the number may not reflect true body composition. The calculated value is therefore a starting point rather than a conclusion and is complemented by waist circumference, body-composition measurement and laboratory results.
Reducing weight comes down to the balance between energy intake and expenditure. A slow, sustainable pace limits muscle loss and makes the result easier to maintain. A nutrition plan, regular walking and resistance exercise, a sleep routine and, where present, assessment of hormonal causes are parts of this process. In obesity class II and III, medical or surgical weight treatment may be considered; those decisions are made with the relevant specialists. Aesthetic surgery does not replace weight loss; it addresses excess skin and regional contouring once weight has stabilised. Suitability and the path to follow are determined through examination and clinical assessment by Prof. Dr. Şükrü Yazar.
Frequently Asked Questions
How is BMI calculated?
Weight in kilograms is divided by height in metres squared. For 70 kg and 1.75 m: 70 ÷ (1.75 × 1.75) = 22.9 kg/m². In pounds and inches the result is multiplied by 703.
What should my BMI be?
For adults, 18.5–24.9 is considered normal. 25–29.9 is overweight and 30 or above is obesity: 30–34.9 class I, 35–39.9 class II, 40 and above class III.
How do I know if I am obese?
If your BMI is 30 or above you are in the obesity class by the World Health Organization definition; 25–29.9 is overweight. A waist above 88 cm in women or 102 cm in men adds to the risk. The definitive assessment is made by a physician.
At what weight am I obese?
It depends on height: the obesity threshold is height squared multiplied by 30. Obesity begins at 76.8 kg (169 lb) for 1.60 m, 86.7 kg (191 lb) for 1.70 m and 97.2 kg (214 lb) for 1.80 m.
Is 5 ft 7 in and 176 lb obese?
No. 80 ÷ (1.70 × 1.70) = 27.7 kg/m², which is overweight. At 1.70 m obesity begins at 86.7 kg (191 lb), and the normal range is 53.5–72.0 kg (118–159 lb).
What BMI is dangerous?
At 30 and above the risk of cardiovascular disease, type 2 diabetes and high blood pressure rises clearly; 40 and above is called morbid obesity. Below 18.5 also carries risk through malnutrition, bone loss and weakened immunity.
How much should someone 1.70 m (5 ft 7 in) weigh?
The normal range is 53.5–72.0 kg (118–159 lb), corresponding to BMI 18.5–24.9. The same range applies to women and men; people with high muscle mass may sit slightly above the upper limit.
What should BMI be for women?
For women too, 18.5–24.9 is normal; the formula and chart are the same as for men. At the same value women generally carry more fat, and the waist thresholds are 80 cm (increased risk) and 88 cm (substantially increased risk).
Does BMI change with age?
The World Health Organization chart for adults does not change with age. Over 65, muscle loss means the number corresponds to a higher fat percentage; in this age group mortality risk was found not to rise between BMI 23 and 33 and to increase below 23. Children are assessed on growth curves by age.
How is BMI assessed in children?
From 5 to 19 years the adult chart is not used; the value is read on the World Health Organization’s growth curves by age and sex. More than +1 standard deviation counts as overweight and more than +2 as obesity; the assessment is made by a paediatrician.
What BMI is needed for plastic surgery?
There is no single cut-off. For major operations such as a tummy tuck many surgeons aim for a stable weight with a BMI below 30; for liposuction, being within 30% of ideal weight is recommended. Final suitability is decided at examination.
Does muscle mass affect BMI?
Yes. Muscle weighs more than fat for the same volume, so a muscular person may have a high BMI with a low fat percentage. Waist circumference and body-composition measurement clarify the picture.
Sources
- Obesity and overweight (adult BMI thresholds) — World Health Organization
- A healthy lifestyle – WHO recommendations (BMI classification) — World Health Organization
- Waist circumference and waist–hip ratio: report of a WHO expert consultation — World Health Organization
- Classification of Overweight and Obesity by BMI, Waist Circumference, and Associated Disease Risks — NHLBI
- BMI-for-age (5–19 years) — WHO growth reference
- BMI and all-cause mortality in older adults: a meta-analysis — Am J Clin Nutr 2014 (PubMed)
- The origin of the “ideal” body weight equations (Devine formula) — Ann Pharmacother 2000 (PubMed)
- Body mass index — MedlinePlus
- Abdominoplasty and Patient Safety: The Impact of Body Mass Index and Bariatric Surgery on Complications Profile — PubMed
- The Effect of Body Mass Index on Outcome of Abdominoplasty Operations — PubMed
- Who is a good candidate for liposuction? — American Society of Plastic Surgeons
