Body Mass Index — BMI — is the most widely used health screening measurement in the world. Doctors use it during checkups, fitness coaches use it to set goals, and insurance companies use it to assess risk. Yet most people who type their numbers into a BMI calculator have no idea what the number actually means, or whether they are calculating it correctly in the first place.
This guide fixes that. You will learn the BMI formula step by step (in both metric and imperial units), how to read the official BMI chart, what your result really tells you about your health — and just as importantly, what it does not tell you.
What Is BMI?
BMI stands for Body Mass Index. It is a simple numerical value calculated from your weight and height that places you into a weight category: underweight, healthy weight, overweight, or obese.
The concept was developed in the 1830s by Belgian mathematician Adolphe Quetelet, and it was adopted by the World Health Organization (WHO) as a standard screening tool in the 20th century. Today it is used globally because it is fast, free, and requires nothing more than a scale and a tape measure.
One thing to understand from the start: BMI is a screening tool, not a diagnosis. It flags potential weight-related health risks, but it cannot tell you how much of your weight is fat versus muscle. We will come back to this important limitation later.
Try it now: Skip the manual maths — use our free BMI Calculator for an instant result with your full weight-status breakdown. Then explore our BMR Calculator, Calorie Calculator and Body Fat Calculator to build your complete health picture.
The BMI Formula (Step by Step)
There are two versions of the formula depending on your units.
Metric formula
BMI = weight (kg) ÷ height (m)²
Worked example: You weigh 70 kg and you are 1.75 m tall.
- Square your height: 1.75 × 1.75 = 3.0625
- Divide your weight by that number: 70 ÷ 3.0625 = 22.86
Your BMI is 22.9.
Imperial formula
BMI = 703 × weight (lb) ÷ height (in)²
Worked example: You weigh 154 lb and you are 69 inches tall.
- Square your height: 69 × 69 = 4,761
- Multiply your weight by 703: 703 × 154 = 108,262
- Divide: 108,262 ÷ 4,761 = 22.7
Your BMI is 22.7. (Notice it matches the metric result — 154 lb ≈ 70 kg and 69 in ≈ 1.75 m.)
Don’t want to do the maths by hand every time? Our free BMI calculator does it instantly — but now you know exactly what is happening behind the scenes.
BMI Chart for Adults
Once you have your number, find where it falls on the official adult BMI chart:
BMI Range | Category |
|---|---|
Below 18.5 | Underweight |
18.5 – 24.9 | Healthy weight |
25.0 – 29.9 | Overweight |
30.0 and above | Obese |
Obesity is further divided into classes:
BMI Range | Obesity Class |
|---|---|
30.0 – 34.9 | Class I (moderate) |
35.0 – 39.9 | Class II (severe) |
40.0 and above | Class III (very severe) |
These cutoffs come from the World Health Organization and are used by health authorities in most countries.
BMI for Children and Teenagers
The adult chart does not apply to anyone under 18. Children’s bodies change rapidly as they grow, so BMI for kids and teens is interpreted using percentiles that compare the child to others of the same age and sex:
- Below the 5th percentile: underweight
- 5th to 85th percentile: healthy weight
- 85th to 95th percentile: overweight
- 95th percentile or above: obese
If you are checking BMI for a child, use a dedicated child BMI calculator or percentile chart, and discuss the result with a pediatrician rather than interpreting it yourself.
What Your BMI Result Actually Means
A number on a chart is only useful if you know what to do with it. Here is what each category generally indicates:
Underweight (below 18.5). May indicate nutritional deficiency, weakened immunity, or an underlying health condition. If your BMI is here, it is worth discussing with a doctor — especially if the weight loss was unintentional.
Healthy weight (18.5–24.9). Associated with the lowest risk of weight-related disease. The goal here is maintenance: balanced nutrition, regular activity, and keeping an eye on the trend over time rather than a single reading.
Overweight (25–29.9). Statistically linked to a higher risk of heart disease, high blood pressure, type 2 diabetes, and joint problems. This is the range where small, sustainable lifestyle changes make the biggest difference.
Obese (30+). Linked to significantly higher risk of cardiovascular disease, diabetes, sleep apnea, and certain cancers. A result here is a strong signal to build a plan with a healthcare professional.
Remember: these are population-level risks, not personal verdicts. Two people with a BMI of 27 can have very different health profiles.
The Limitations of BMI (Read This Before You Worry)
BMI is useful, but it is a blunt instrument. Here is what it gets wrong:
1. It cannot tell muscle from fat. A muscular athlete can have a BMI of 28 — technically “overweight” — while having very low body fat. This is the most common criticism of BMI.
2. It ignores fat distribution. Belly fat carries more health risk than fat stored on hips and thighs, but BMI treats all weight the same. Waist circumference is a useful companion measurement.
3. It is less accurate for the elderly. Older adults naturally lose muscle mass, so BMI can underestimate health risk in this group.
4. It doesn’t account for ethnicity. Research shows that Asian populations tend to develop weight-related health risks at lower BMIs. The WHO has suggested lower action thresholds — around 23 for overweight and 27.5 for obesity — for Asian adults.
5. It says nothing about fitness. You can have a “healthy” BMI and still be sedentary with poor cardiovascular health.
The takeaway: use BMI as a starting point, then add context — waist measurement, body fat percentage, activity level, and how you actually feel.
5 Common BMI Calculation Mistakes
- Mixing up units. The most frequent error: using centimeters in the metric formula without converting to meters first. 175 cm must become 1.75 m.
- Measuring height with shoes on. Even 2–3 cm changes the result. Measure barefoot, standing straight against a wall.
- Using an old weight. Weigh yourself in the morning, after using the bathroom, before eating — and use that number.
- Forgetting the 703 in the imperial formula. Without it, your result will be wildly wrong.
- Treating one reading as final. Weight fluctuates daily. Track the trend over weeks, not a single number.
How to Reach (and Keep) a Healthy BMI
If your BMI is outside the healthy range, crash diets are not the answer. What works is boring, sustainable change:
- Know your numbers. Calculate your BMR (Basal Metabolic Rate) to find how many calories your body burns at rest, then build your eating plan around it. Our BMR calculator and calorie calculator walk you through this.
- Aim for 0.5 kg per week. Faster weight loss usually means muscle loss and rebound weight gain.
- Prioritise protein and strength training. This preserves muscle while you lose fat — which matters far more than the scale number.
- Track body fat, not just weight. A body fat calculator gives you a clearer picture of real progress than BMI alone.
- Sleep 7–9 hours. Poor sleep disrupts hunger hormones and is consistently linked to weight gain.
Small note of encouragement: moving from a BMI of 31 to 28 cuts health risk substantially, even though both numbers sit outside the “healthy” band. Progress counts.
Frequently Asked Questions
What is a good BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. This range is associated with the lowest risk of weight-related diseases.
How do I calculate BMI without a calculator?
Use the metric formula: weight in kilograms divided by height in meters squared. Example: 70 kg ÷ (1.75 × 1.75) = 22.9. For pounds and inches, use: 703 × weight (lb) ÷ height (in)².
Is BMI accurate?
BMI is accurate as a population screening tool, but imperfect for individuals. It doesn’t distinguish muscle from fat and is less reliable for athletes, the elderly, pregnant women, and some ethnic groups. Use it alongside waist measurement and body fat percentage.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese: Class I (30–34.9), Class II (35–39.9), and Class III (40+).
Does BMI differ for men and women?
The formula and chart are the same for adult men and women. However, women naturally carry a higher body fat percentage at the same BMI, which is one reason BMI should be interpreted with context.
How often should I check my BMI?
Once a month is plenty for most people. Daily checks mostly capture water-weight noise. Track the monthly trend instead.
Disclaimer: This article is for informational purposes only and is not medical advice. Consult a healthcare professional for personalized guidance.
