BMI Calculator for Adults: Formula, Chart

Introduction

Body Mass Index, or BMI, is one of the most widely used screening tools in medicine and public health. It gives clinicians, researchers, and individuals a quick numerical snapshot of whether a person’s weight is proportionate to their height. Understanding how BMI is calculated — and what it actually tells you — puts you in a much stronger position to interpret your own health data and have informed conversations with your doctor.

This guide covers everything you need to calculate BMI manually using both the metric and imperial formulas, interpret your result against WHO categories, and understand where the measure falls short. By the end, you will know exactly how to work through the math, spot the most common errors, and decide what to do with the number you get.

What Is BMI and Why Does It Matter?

BMI is a ratio of weight to height squared. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and later adapted by public health bodies as a population-level screening tool. The key word there is screening — BMI does not diagnose disease. It flags people who may warrant further evaluation.

Despite its limitations (which we will cover later), BMI remains the standard first-pass metric because it requires no lab work, no specialist equipment, and no clinical training. You need only a scale and a measuring tape. That accessibility makes it genuinely useful when used correctly.

The BMI Formula

There are two versions of the formula depending on which unit system you use. Both produce the same result.

Metric Formula

BMI = weight (kg) / height (m)²

You divide your weight in kilograms by the square of your height in metres. For example, if you are 1.75 m tall and weigh 70 kg:

BMI = 70 / (1.75 × 1.75)
BMI = 70 / 3.0625
BMI = 22.9

Imperial Formula

BMI = (weight (lb) / height (in)²) × 703

Because pounds and inches are not metric units, a conversion factor of 703 is applied to bring the result into the same numerical range as the metric formula. Without the 703 multiplier, the raw ratio would be far too small to match the standard BMI scale.

Both formulas are mathematically equivalent — the 703 factor simply accounts for the unit conversion between the two systems.

Step-by-Step Worked Example

Walking through a real calculation reinforces the formula far better than reading it in the abstract. Here are two complete examples, one metric and one imperial.

Metric Example

Person: 34-year-old woman, 163 cm tall, 68 kg

Step 1 — Convert height to metres 163 cm ÷ 100 = 1.63 m

Step 2 — Square the height 1.63 × 1.63 = 2.6569 m²

Step 3 — Divide weight by height squared 68 ÷ 2.6569 = 25.6

Result: BMI = 25.6 (just inside the “Overweight” category — see chart below)

Imperial Example

Person: 41-year-old man, 5 ft 10 in, 185 lb

Step 1 — Convert height entirely to inches 5 feet × 12 = 60 inches + 10 inches = 70 inches total

Step 2 — Square the height in inches 70 × 70 = 4,900 in²

Step 3 — Divide weight by height squared 185 ÷ 4,900 = 0.03776

Step 4 — Multiply by 703 0.03776 × 703 = 26.5

Result: BMI = 26.5 (Overweight category)

Working through these steps by hand is a good way to verify any online tool you use and to catch entry errors before they mislead you.

WHO BMI Categories for Adults

The World Health Organization defines four primary BMI categories for adults aged 18 and over. These thresholds apply regardless of age or sex for standard adult BMI screening.

BMI Range	Category	Health Risk Level
Below 18.5	Underweight	Increased risk (malnutrition, bone loss, immune suppression)
18.5 – 24.9	Normal weight	Lowest risk for weight-related conditions
25.0 – 29.9	Overweight	Moderate increased risk
30.0 – 34.9	Obese Class I	High risk
35.0 – 39.9	Obese Class II	Very high risk
40.0 and above	Obese Class III	Extremely high risk

A few points worth knowing about these ranges:

The 18.5 and 25.0 cutoffs are population-level thresholds, not hard biological boundaries. A BMI of 24.9 is not meaningfully different from 25.1 in clinical terms.
Some national guidelines (particularly for people of Asian descent) use adjusted thresholds — for example, overweight starting at 23.0 rather than 25.0 — because metabolic risks appear at lower BMI values in certain ethnic groups.
Athletes and very muscular individuals often fall into the “overweight” range despite low body fat, because muscle is denser than fat (more on this in the Limitations section).

Healthy Weight Ranges by Height

One practical question people ask is: what weight range puts me in the “Normal” BMI category? The table below answers that for a selection of heights using metric values. To find your healthy weight range, you are looking for the weights that produce BMI values between 18.5 and 24.9.

The formula to find these boundaries is:

Lower healthy weight = 18.5 × height (m)²
Upper healthy weight = 24.9 × height (m)²

Height	Lower Healthy Weight	Upper Healthy Weight
155 cm (5’1”)	44.4 kg (97.9 lb)	59.8 kg (131.8 lb)
160 cm (5’3”)	47.4 kg (104.5 lb)	63.7 kg (140.4 lb)
165 cm (5’5”)	50.3 kg (110.9 lb)	67.7 kg (149.3 lb)
170 cm (5’7”)	53.5 kg (117.9 lb)	71.9 kg (158.6 lb)
175 cm (5’9”)	56.7 kg (125.0 lb)	76.3 kg (168.2 lb)
180 cm (5’11”)	59.9 kg (132.1 lb)	80.7 kg (177.9 lb)
185 cm (6’1”)	63.3 kg (139.6 lb)	85.2 kg (187.8 lb)
190 cm (6’3”)	66.8 kg (147.3 lb)	89.9 kg (198.2 lb)

These ranges give you a practical target, but they are not a prescription. Your ideal weight depends on factors including muscle mass, bone density, and individual metabolic health — none of which BMI captures directly.

Common Mistakes When Calculating BMI

Small errors in the input data or the math can shift your BMI by a full category. Here are the mistakes that come up most often.

1. Forgetting to Square the Height

The most frequent arithmetic error is dividing weight by height instead of height squared. If you are 1.70 m tall, you need to use 2.89 (1.70 × 1.70), not 1.70. Using the unsquared value produces a BMI roughly twice the correct figure.

2. Mixing Up Units Mid-Calculation

Using kilograms in the imperial formula (without the 703 multiplier) or using pounds in the metric formula produces a nonsensical result. Before you start, confirm which unit system you are using and stick to it throughout. If you are working in metric, your weight must be in kilograms and your height must be in metres.

3. Converting Height Incorrectly

When using the imperial formula, height must be expressed entirely in inches, not in feet and inches. Five feet nine inches is 69 inches (5 × 12 = 60, plus 9 = 69). Using “5.9” as a decimal directly in the formula is incorrect — that would represent 5.9 feet, which is not the same as 5 feet 9 inches.

4. Not Converting Centimetres to Metres

A common slip in the metric formula is entering height in centimetres (e.g., 175) instead of metres (1.75). The result will be absurdly small — around 0.23 instead of 22.9. Always divide your centimetre value by 100 first.

5. Weighing Yourself Inconsistently

BMI is only as accurate as the inputs. Weighing yourself in the morning versus the evening, clothed versus unclothed, or before versus after eating can shift your recorded weight by 1–3 kg. For a consistent baseline, weigh yourself in the morning, unclothed, after using the bathroom.

The Limitations of BMI: What the Number Does Not Tell You

BMI is a useful tool, but it has real limitations that every adult should understand before drawing conclusions from their number.

It does not distinguish fat from muscle. Two people with identical BMI values could have very different body compositions. A 90 kg athlete with 12% body fat and a 90 kg sedentary person with 30% body fat will have the same BMI, but dramatically different health profiles. BMI cannot tell these individuals apart.

It does not account for fat distribution. Visceral fat — fat stored around abdominal organs — is more metabolically harmful than subcutaneous fat. Waist circumference and waist-to-hip ratio are better predictors of cardiometabolic risk than BMI alone, particularly for people in the “normal” BMI range who carry excess abdominal fat.

It may not apply uniformly across ethnicities. Research has shown that people of South Asian, East Asian, and some other ethnic backgrounds face higher cardiometabolic risk at BMI values that would be considered “normal” by standard WHO cutoffs. Several health organisations now recommend lower thresholds for these populations.

It does not reflect age-related changes in body composition. As adults age, muscle mass typically decreases and fat mass increases, even if overall weight stays the same. An older adult with a “normal” BMI may have a higher body fat percentage than the number suggests.

It does not capture fitness level. Cardiorespiratory fitness is one of the strongest predictors of all-cause mortality — stronger than BMI. A physically active person with a BMI of 27 may have a significantly lower health risk than a sedentary person with a BMI of 22.

The takeaway is not that BMI is useless — it is that BMI works best as one data point among several. Use it alongside waist circumference, blood pressure, blood glucose, cholesterol levels, and physical activity habits for a more complete picture.

Frequently Asked Questions

Q: Is BMI calculated differently for men and women?

The standard adult BMI formula is the same for men and women. The same weight-to-height-squared calculation and the same WHO category cutoffs apply to both sexes. However, at the same BMI, women typically have a higher body fat percentage than men — a biological difference that the formula does not reflect. Some researchers argue for sex-specific adjusted thresholds, but these are not yet part of standard WHO guidelines.

Q: What BMI is considered obese?

According to WHO definitions, a BMI of 30.0 or above is classified as obese. This is further divided into three classes: Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 and above, sometimes called severe or morbid obesity). Each class is associated with progressively higher risk of conditions including type 2 diabetes, cardiovascular disease, and certain cancers.

Q: Can I use the same BMI formula for children?

No. The standard adult BMI formula is not appropriate for children and teenagers. For people under 18, BMI-for-age percentiles are used, which compare a child’s BMI against reference data for their age and sex. A child is typically classified as overweight if their BMI falls at or above the 85th percentile for their age and sex, and obese at or above the 95th percentile. Adult cutoffs do not apply.

Q: How often should I check my BMI?

For most adults, checking BMI once or twice a year is sufficient — more frequent checks are rarely informative because weight changes meaningful enough to shift your BMI category take weeks to months to develop. Daily weigh-ins are more useful for tracking trends over time, but calculating your full BMI that frequently adds little value beyond knowing your weight.

Q: If my BMI is in the normal range, does that mean I am healthy?

Not necessarily. A “normal” BMI (18.5–24.9) reduces the likelihood of several weight-related conditions, but it does not guarantee good health. You can be within a normal BMI range and still have high blood pressure, poor blood glucose control, elevated cholesterol, or low cardiovascular fitness. Conversely, some people with BMI values just outside the normal range have excellent metabolic health. BMI is a screening tool, not a health certificate.

Conclusion

BMI is a straightforward calculation: divide your weight (in kg) by the square of your height (in metres), or use the imperial equivalent with the 703 conversion factor. The result places you in one of several WHO categories that correlate with population-level health risk. Knowing how to do the math yourself — rather than relying entirely on tools you cannot verify — gives you a clearer understanding of what the number represents and where it comes from.

That said, no single number tells the whole story of your health. Treat BMI as a starting point and combine it with other measures — waist circumference, physical activity habits, blood markers — for a fuller picture.

If you want to skip the manual calculation entirely, NovaCalculator’s free online BMI and health calculators handle both metric and imperial inputs instantly, and display your result against the full WHO category chart so you can see exactly where you stand.

Frequently Asked Questions

Is BMI calculated differently for men and women? +

What BMI is considered obese? +

Can I use the same BMI formula for children? +

No. The standard adult BMI formula is not appropriate for children and teenagers. For people under 18, BMI-for-age percentiles are used, which compare a child's BMI against reference data for their age and sex.

How often should I check my BMI? +

For most adults, checking BMI once or twice a year is sufficient. Daily weigh-ins are more useful for tracking trends over time, but calculating your full BMI that frequently adds little value beyond knowing your weight.

If my BMI is in the normal range, does that mean I am healthy? +

Not necessarily. A 'normal' BMI (18.5–24.9) reduces the likelihood of several weight-related conditions, but it does not guarantee good health. You can be within a normal BMI range and still have high blood pressure, poor blood glucose control, elevated cholesterol, or low cardiovascular fitness.