BMI Calculator for Pregnancy

BMI Calculator: Check Your Body Mass Index and Healthy Weight Range

What Is BMI?

Body mass index (BMI) compares your weight with your height. It gives a quick idea of whether your weight is low, healthy or high for your size. The idea goes back to the Belgian mathematician Adolphe Quetelet in the 1830s, and the name “body mass index” was introduced in 1972. It stays popular because it is free, fast and linked to health risks across large groups of people.

BMI does not measure body fat directly. Think of it as a first step that tells you whether further checks might be useful.

How to Calculate BMI

Metric example: a person weighs 62 kg and is 165 cm tall. Height in metres is 1.65, and 1.65 × 1.65 = 2.7225. So BMI = 62 ÷ 2.7225 = 22.8.

Imperial example: a person weighs 150 lb and is 5 ft 6 in (66 in) tall. BMI = 703 × 150 ÷ 66² = 105,450 ÷ 4,356 = 24.2.

Both results are in the healthy range on the standard scale.

BMI Categories for Adults

The World Health Organization (WHO) scale applies to men and women aged 20 and over. Some guidelines use lower cut-offs for people of Asian descent, as shown in the right-hand column.

Why Some Guidelines Use Different BMI Cut-offs for Asian Adults

At the same BMI, people of South Asian and some other Asian backgrounds tend to carry more body fat, particularly around the abdomen, and face a higher risk of type 2 diabetes, high blood pressure and heart disease. A 2004 WHO expert review kept the international categories but added extra action points at 23.0 and 27.5. Several Asian health bodies, including Indian guidelines, define overweight from 23 and obesity from 25.

The exact cut-offs differ between countries and organisations, so ask your doctor which ones apply to you. A BMI of 24, for example, is “healthy” on the standard scale but “overweight, at risk” on the Asian-descent scale.

Healthy Weight Range by Height

BMI for Children and Teenagers

Adult categories should not be used for anyone under 20. For ages 2 to 20, the US Centers for Disease Control and Prevention (CDC) compares a child’s BMI with other children of the same age and sex, using percentiles:

A paediatrician can plot your child’s measurements on the correct growth chart.

Pre-Pregnancy BMI and Weight Gain in Pregnancy

If you are planning a pregnancy or are newly pregnant, your BMI before pregnancy is the starting point. It helps your doctor suggest how much weight gain is healthy. BMI itself is not meaningful during pregnancy, so always use your weight from before you conceived.

The Institute of Medicine recommends these total gains for a single pregnancy:

These are general ranges. Your doctor or midwife will set a target that suits you. Once you are pregnant, use our due date calculator to find your due date and our pregnancy calculator to see your weeks and milestones.

If you want to lose weight before conceiving, talk to your doctor first. Slow, steady change is safer than crash dieting, and you should not try to lose weight once you are pregnant unless your doctor advises it.

Health Risks of a High or Low BMI

A higher BMI

According to the CDC, carrying excess weight increases the risk of:

• high blood pressure and unhealthy cholesterol,
• type 2 diabetes,
• heart disease and stroke,
• gallbladder disease and osteoarthritis,
• sleep apnoea and breathing problems,
• certain cancers,
• depression and anxiety,
• and fertility problems.

A lower BMI

Being underweight can lead to:

• malnutrition, vitamin deficiencies and anaemia,
• weak bones (osteoporosis),
• a weaker immune system,
• growth problems in children and teenagers,
• irregular periods and reduced fertility,
• and a higher chance of complications after surgery.

Sometimes a low weight points to an underlying condition, such as an eating disorder. See a doctor if you are underweight and the reason isn’t clear.

Limitations of BMI

BMI is useful for groups of people, but it can mislead for individuals because it measures excess weight, not excess fat.

• Muscle: athletes and very muscular people can have a high BMI with little body fat.
• Age: older adults tend to have more body fat than younger adults at the same BMI.
• Sex: women usually carry more body fat than men at the same BMI.
• Fat distribution: BMI cannot show belly fat, which carries the highest risk.
• Height: it may overestimate fat in very tall people and underestimate it in very short people.
• Ethnicity: the same BMI can carry different risks in different groups.
• Pregnancy and breastfeeding: BMI is not a valid measure.

Even so, BMI is a fair guide to body fat for most people, and it works best when combined with other measures.

Why Waist Size Matters Too

In 2025 an international commission published in The Lancet argued that BMI alone should not be used to diagnose obesity. It suggested combining BMI with another measure, such as waist circumference, waist-to-height ratio or a direct body fat measurement. A waist-to-height ratio above 0.5 is a commonly used warning sign.

Other Measures Related to BMI

BMI Prime

BMI Prime is your BMI divided by the upper limit of the healthy range (25 on the standard scale, 23 on the Asian-descent scale). A value below 1.0 means you are within the healthy range. For example, a BMI of 22.8 gives a BMI Prime of 22.8 ÷ 25 = 0.91.

Ponderal Index

The Ponderal Index is similar to BMI, but it divides weight by height cubed instead of squared: weight (kg) ÷ height (m)³. It can be more suitable for very tall or very short people. For the 62 kg, 1.65 m example above, the Ponderal Index is 62 ÷ 4.49 = 13.8.

How to Reach a Healthy BMI Safely

1. Set a realistic goal. Losing 5% to 10% of your body weight can improve blood pressure, blood sugar and cholesterol, even if your BMI is still above the healthy range.
2. Make a moderate change to what you eat. Small, sustainable calorie changes work better than extreme diets.
3. Protect your muscle. Eat enough protein and include strength training twice a week.
4. Move every day. Aim for about 150 minutes of brisk activity a week.
5. Sleep and stress. Seven to nine hours of sleep and some stress management support weight control.
6. If you are underweight, add nutritious, energy-dense foods and ask a doctor to rule out medical causes.

Common BMI Mistakes

• Using adult categories for children or teenagers.
• Ignoring waist size and treating BMI as the whole picture.
• Using the standard scale when Asian-descent cut-offs apply to you, or the other way round.
• Weighing yourself at different times of day or in different clothing.
• Aiming for the very bottom of the healthy range.
• Assuming a normal BMI means there is no health risk.

BMI Calculator FAQs

How do I calculate my BMI?

Divide your weight in kilograms by your height in metres squared. For example, 62 kg at 1.65 m gives 62 ÷ 2.7225 = 22.8.

What is a healthy BMI?

For most adults it is 18.5 to 24.9. People of Asian descent are often advised to stay between 18.5 and 22.9.

Is the BMI range different for men and women?

No, the same categories apply. Women naturally carry more body fat than men at the same BMI.

Is a BMI of 25 overweight?

On the standard scale, yes, 25 is where the overweight range begins. On the Asian-descent scale, overweight begins at 23.

Is BMI accurate for athletes?

Not always. Muscle weighs more than fat, so athletes can have a high BMI with low body fat. Body fat percentage and waist size give a clearer picture.

Can I use this calculator for my child?

No. Children and teenagers need BMI-for-age percentile charts, which a paediatrician can use.

Can I use BMI during pregnancy?

No. Use your pre-pregnancy weight to find your BMI, and follow your doctor’s advice on weight gain.

How often should I check my BMI?

Every one to three months is plenty. Track your waist measurement alongside it.

Final Thoughts

BMI is a useful first check, not the final answer. Use the scale that applies to you, add your waist measurement, and look at your blood pressure and overall health as well. If you are planning a pregnancy, use your pre-pregnancy BMI to talk through healthy weight gain with your doctor. If your results worry you, speak to a doctor or registered dietitian.