BMI, or body mass index, is a simple way to compare body weight with height. It turns height and weight into one number that is used to screen whether an adult is underweight, in the healthy weight range, overweight, or in an obesity category.
The calculation is straightforward, but mistakes are common. People forget to square height, mix centimetres with metres, use pounds in the metric formula, or read the category as a diagnosis. BMI is useful only when you understand what it measures and what it leaves out.
BMI does not measure body fat directly. It cannot tell you how fit you are, where fat is stored, or whether a weight is healthy for your personal medical situation. It is a screening tool, not a diagnosis.
What BMI Is
BMI is an index based on weight relative to height. Taller people naturally tend to weigh more, so BMI adjusts weight by height squared rather than comparing weight alone.
For adults, the result is usually grouped into categories:
- Underweight
- Healthy weight
- Overweight
- Obesity class I, II, or III
The reason BMI is widely used is practical. It is quick, cheap, and easy to calculate. It can be used in clinics, studies, public health reports, and personal tracking without special equipment.
That convenience is also its weakness. A simple index cannot capture the full complexity of human bodies. BMI should be understood as a rough screening measure that may prompt a closer look.
The Metric BMI Formula
The metric formula is:
BMI = weight in kg / height in metres squared
Written another way:
BMI = kg / m²
The phrase "height in metres squared" means you multiply height by itself first, then divide weight by that result.
If your height is 1.75 m, height squared is:
1.75 x 1.75 = 3.0625
If your weight is 70 kg:
70 / 3.0625 = 22.9
So the BMI is 22.9.
Metric example step by step
Example person:
- Weight: 82 kg
- Height: 1.78 m
Step 1: Square the height.
1.78 x 1.78 = 3.1684
Step 2: Divide weight by height squared.
82 / 3.1684 = 25.88
Step 3: Round sensibly.
BMI is usually shown to one decimal place:
25.9
Step 4: Compare with adult categories.
A BMI of 25.9 falls in the overweight category.
The Imperial BMI Formula
The imperial formula is:
BMI = 703 x weight in pounds / height in inches squared
The number 703 is a conversion factor that makes pounds and inches produce the same kind of BMI result as kilograms and metres.
If your height is 5 ft 9 in, first convert it to inches:
5 x 12 + 9 = 69 inches
If your weight is 165 lb:
BMI = 703 x 165 / (69 x 69)
BMI = 115,995 / 4,761 = 24.4
Imperial example step by step
Example person:
- Weight: 190 lb
- Height: 6 ft 0 in
Step 1: Convert height to total inches.
6 x 12 = 72 inches
Step 2: Square the height.
72 x 72 = 5,184
Step 3: Multiply weight by 703.
190 x 703 = 133,570
Step 4: Divide by height squared.
133,570 / 5,184 = 25.8
The BMI is 25.8, which falls in the overweight category for adults.
Standard Adult BMI Categories
The standard adult categories are:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40.0 or above | Obesity class III |
These categories apply to adults. They are not designed for children and teenagers, who are assessed using age and sex adjusted centiles.
The category boundaries are useful, but they are not sharp biological lines. A BMI of 24.9 and a BMI of 25.0 are almost identical in practical terms. Health risk tends to change gradually as weight, waist size, body composition, and other markers change.
Worked Example: Finding a Healthy Weight Range
BMI can be rearranged to estimate a weight range for a height. This can be helpful when someone asks what weight corresponds to the healthy BMI category.
The formula is:
Weight = BMI x height²
Suppose someone is 1.65 m tall.
Step 1: Square the height.
1.65 x 1.65 = 2.7225
Step 2: Calculate the lower end using BMI 18.5.
18.5 x 2.7225 = 50.4 kg
Step 3: Calculate the upper end using BMI 24.9.
24.9 x 2.7225 = 67.8 kg
For a 1.65 m adult, the standard healthy BMI range corresponds to roughly 50 kg to 68 kg.
This is a broad screening range. It is not a personal target and should not override medical advice, pregnancy guidance, athletic context, disability, age-related changes, or eating disorder care.
Another Example: Checking a Result That Seems Off
Suppose someone is 180 cm tall and weighs 74 kg. They enter 180 as the height in the metric formula and get a tiny number. The problem is the unit. Height must be in metres, so 180 cm should be entered as 1.80 m.
Correct calculation:
1.80 x 1.80 = 3.24
74 / 3.24 = 22.8
The correct BMI is 22.8, not a number close to zero.
Now suppose the same person uses an imperial calculator and enters 5.11 for a height of 5 ft 11 in. That is another common error. Feet and inches are not decimals in the usual sense. Five feet eleven inches means:
5 x 12 + 11 = 71 inches
Small unit errors can move the result into the wrong category, so it is worth checking the input before interpreting the answer.
Common BMI Calculation Mistakes
Using centimetres instead of metres
The metric formula needs height in metres. If you are 170 cm tall, use 1.70 m, not 170.
Wrong:
70 / (170 x 170) = 0.0024
Correct:
70 / (1.70 x 1.70) = 24.2
Forgetting to square height
BMI divides by height squared, not height. If you divide 70 by 1.75, you get 40, which is not the correct BMI.
Mixing metric and imperial units
Do not use pounds in the metric formula or kilograms in the imperial formula. If using pounds and inches, include the 703 conversion factor.
Rounding too early
If you round height squared too aggressively, the final BMI may shift slightly. Keep a few decimal places during the calculation, then round the final BMI to one decimal place.
Reading BMI as body fat percentage
A BMI of 27 does not mean 27% body fat. BMI has no percentage unit. It is a weight-for-height index.
Using adult categories for children
Children's BMI is interpreted differently because growth changes body composition and proportions. Use paediatric centile charts and professional guidance for children.
What BMI Can Tell You
BMI can give a quick indication of whether your weight is low, typical, or high for your height according to adult population categories. It can be useful for:
- Tracking broad weight changes over time
- Estimating whether weight may be outside the common healthy range
- Starting a conversation with a health professional
- Public health research
- Comparing weight categories across groups
It is most useful when it is part of a bigger picture. If your BMI changes from 23 to 29 over a few years, that trend may be worth noticing. If your BMI is 31 and your waist measurement and blood pressure have also increased, that combination may be more meaningful than the BMI alone.
What BMI Cannot Tell You
BMI cannot tell you:
- Your body fat percentage
- Your muscle mass
- Your bone density
- Your waist size
- Your cardiovascular fitness
- Your diet quality
- Your blood pressure
- Your cholesterol or blood sugar
- Whether you have a medical condition
It also cannot tell where body fat is carried. Fat around the abdomen can have different health implications from fat stored elsewhere. That is why waist circumference is often used alongside BMI.
Limitations of BMI
BMI is less reliable for some people and situations.
Athletes and strength-trained people
Muscle adds weight. A muscular person may have a high BMI without having a high level of body fat.
Older adults
Older adults may lose muscle while maintaining weight, which can make BMI look more reassuring than it should. Strength, mobility, nutrition, and medical history matter.
Pregnancy
BMI categories are not interpreted the same way during pregnancy. Pre-pregnancy BMI may be used in care planning, but pregnancy weight changes need specific guidance.
Children and teenagers
Adult BMI categories do not apply. Children need age and sex adjusted centiles.
Differences between populations
Health risk can appear at different BMI levels across populations. BMI cut-offs are broad and may not fit every individual equally well.
Use the BlinkCalc BMI Calculator
The BMI Calculator can calculate BMI for you using metric or imperial inputs. It is useful if you want to avoid unit mistakes or quickly compare results.
To use it well:
- Enter your current height and weight accurately.
- Check that the units are correct.
- Read the BMI and category.
- Treat the result as screening information.
- Consider waist measurement, health history, and trends over time.
The calculator is for educational use and does not diagnose health conditions.
Practical Uses for BMI
BMI can be useful when you need a quick baseline. For example, someone starting a fitness plan may record BMI alongside waist measurement, resting heart rate, and strength markers. Over time, the trend can show whether weight is moving in the intended direction.
It can also help someone understand a health form or insurance document that asks for BMI. Rather than guessing, they can calculate it directly.
Clinicians may use BMI as one input during routine checks, but they do not have to rely on it alone. A sensible interpretation considers symptoms, history, exam findings, and other measurements.
FAQ
What is BMI?
BMI is body mass index, a number that compares weight with height. It is used as an adult weight category screening tool.
How do you calculate BMI in metric units?
Divide weight in kilograms by height in metres squared. For example, 70 kg and 1.75 m gives 70 / 3.0625 = 22.9.
How do you calculate BMI in imperial units?
Use 703 x weight in pounds divided by height in inches squared. For example, 165 lb and 69 inches gives a BMI of 24.4.
What are the standard BMI categories?
Below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25.0 to 29.9 is overweight, and 30.0 or above is grouped into obesity classes.
Why can BMI be inaccurate?
BMI does not distinguish muscle from fat, does not show fat distribution, and does not account for age, pregnancy, ethnicity, or individual health history.
Is BMI the same as body fat percentage?
No. BMI is not a percentage and does not measure body fat directly.
Should I worry about a BMI just over 25?
Not automatically. A BMI just over 25 should be interpreted with waist size, body composition, health markers, and personal context.
Can BMI be too low?
Yes. A BMI below 18.5 is classed as underweight for adults and may warrant further review, especially if it is unexpected or linked with symptoms.
Conclusion
BMI is simple: weight divided by height squared, with a conversion factor for pounds and inches. Its simplicity is the reason it is useful and the reason it has limits.
Use BMI to understand the broad category your weight falls into, not to judge your overall health. Calculate it correctly, read it in context, and treat unusual or concerning results as a reason to look more closely.