The exact math behind Body Mass Index, worked examples in both metric and imperial units, and the full category chart.
Published August 22, 2026 · Reviewed by the Ihsabha Editorial Team
Body Mass Index, almost always shortened to BMI, is the single most-used screening number in weight-related health conversations — doctors chart it at checkups, insurers reference it, and fitness apps display it on the home screen. Despite how often it's used, most people have never actually calculated it themselves; they've only ever seen the number handed to them. This guide walks through the exact formula in both metric and imperial units, shows the calculation step by step with real numbers, and lays out the standard category chart so you know exactly what your result means.
BMI is a simple ratio of your weight to the square of your height. It was originally developed in the 1830s by the Belgian mathematician Adolphe Quetelet as a population-level statistic, not an individual diagnostic tool — a detail that explains a lot of its modern limitations, which we cover in a separate deep dive linked at the end of this guide. Today it's used as a fast, free, no-equipment screening number that correlates reasonably well with body fat levels across large groups of people, which is exactly why it remains the first number captured at almost every medical visit. If you want to go further, BMI Chart for Women: Healthy Ranges by Age & Life Stage explains this point in more depth. To save time, enter your values into the BMI Calculator and get an instant result.
The metric formula is the cleanest version of the calculation:
Your height needs to be in meters, not centimeters, and it needs to be squared — meaning multiplied by itself — before you divide. This squaring step is the part most people get wrong when trying to calculate BMI by hand, so it's worth double-checking before you move on.
If you're working in pounds and inches, the formula includes a conversion constant:
The 703 multiplier exists purely to convert the pounds-per-square-inch result into the same kg/m² scale used internationally, so that a BMI of 24 means the same thing whether it was calculated in London or Los Angeles.
Let's walk through a full example using a height of 170 cm and a weight of 68 kg.
This person's BMI is 23.5, which falls inside the normal weight range covered in the chart below.
Now the same idea using a height of 5'7" (67 inches) and a weight of 150 lb.
Notice both examples land on the same BMI of 23.5 — that's the point of the 703 constant, and it confirms the two formulas are measuring the exact same thing in different unit systems.
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class I) |
| 35.0 – 39.9 | Obesity (Class II) |
| 40.0 and above | Obesity (Class III) |
This chart applies to adults aged 20 and over and is the same set of cutoffs used by the World Health Organization and adopted, with only minor regional adjustments, by most national health agencies worldwide.
Working through a few examples like these is the fastest way to get comfortable with the formula, since the actual arithmetic never changes — only the two input numbers do. Try the BMI Calculator to run these numbers with your own figures.
The adult BMI chart above does not apply to anyone under 20. Children's bodies change shape naturally as they grow, so pediatric BMI is calculated with the same formula but then plotted against age- and sex-specific growth charts, producing a percentile rather than a fixed category. A 12-year-old and a 30-year-old with an identical BMI number of 22 are not being evaluated against the same standard at all — the child's result is compared to other children of the same age and sex, not to the flat adult cutoffs above.
A handful of small errors account for most incorrect BMI calculations:
The formula behind BMI is nearly 200 years old. Belgian mathematician and astronomer Adolphe Quetelet developed the "Quetelet Index" in the 1830s while studying the statistical distribution of human height and weight across the general population — his goal was to describe the "average man," not to diagnose any single patient. The formula sat largely unused in clinical settings for well over a century. It was American physiologist Ancel Keys who, in a widely cited 1972 study comparing several weight-to-height formulas, found that Quetelet's index correlated better with body fat measurements than the alternatives being used at the time, and it was Keys who coined the term "Body Mass Index." From there, BMI spread quickly into public health guidelines, insurance tables, and eventually the standard medical intake form it appears on today. Understanding this history helps explain both its strengths — it's mathematically simple and was chosen because it performed better than its rivals in a real study — and its weaknesses, since it was never validated as a tool for judging any one individual's health with precision. You may also find it useful to check Is BMI Accurate? Limitations of BMI & What to Track Instead, which covers a related angle.
| Method | What It Measures | Equipment Needed |
|---|---|---|
| BMI | Weight relative to height (a screening estimate) | Scale and tape measure |
| Waist circumference | Abdominal fat specifically | Tape measure |
| Skinfold calipers | Subcutaneous fat at specific sites | Calipers, trained technician |
| Bioelectrical impedance | Estimated body fat percentage | Smart scale or handheld device |
| DEXA scan | Precise fat, muscle, and bone breakdown | Medical imaging equipment |
BMI sits at one end of this spectrum: the least precise method, but also the fastest, cheapest, and most accessible by a wide margin — no equipment beyond a scale and a tape measure is required, which is exactly why it remains the universal first step before more detailed methods are considered.
For most adults tracking general health, recalculating BMI every few months is more useful than checking it daily or weekly. Body weight naturally fluctuates day to day due to water retention, food intake, and other short-term factors that have nothing to do with actual fat gain or loss, so a single day's number can be misleading either way. If you're actively working toward a weight-related goal, a monthly check is usually enough to see a meaningful trend without over-focusing on normal daily noise. If your weight or height changes for another medical reason, recalculating right away naturally makes sense so your other tools — like an ideal weight range or calorie target — stay based on current numbers rather than outdated ones.
If your BMI lands close to a category boundary — say, 24.7 or 25.3, right around the normal-to-overweight line — a few practical steps help make sure the number you're reading is actually reliable before drawing any conclusions from it: For an instant, practical check, the BMI Calculator is ready to go.
Given everything BMI can't measure, it's fair to ask why it remains the default first number captured at almost every medical visit. The honest answer is that, at a population level and as a fast initial screen, it correlates reasonably well with health outcomes across large groups of people, it costs nothing beyond a scale and a tape measure, and it gives clinicians a consistent, universally understood starting point for a conversation that then moves on to more specific measurements when needed. BMI was never meant to be the final word for an individual patient — it's the first filter, not the full diagnosis, and most clinical guidelines explicitly say so even while continuing to record it at every visit.
Rather than calculating from scratch every time, it can help to see the approximate weight range that lands in the "normal weight" BMI category (18.5–24.9) for a few common heights.
| Height | Normal Weight Range (18.5–24.9 BMI) |
|---|---|
| 150 cm (4'11") | 42 – 56 kg (93 – 123 lb) |
| 160 cm (5'3") | 47 – 64 kg (104 – 141 lb) |
| 165 cm (5'5") | 50 – 68 kg (111 – 150 lb) |
| 170 cm (5'7") | 53 – 72 kg (118 – 159 lb) |
| 175 cm (5'9") | 57 – 76 kg (125 – 168 lb) |
| 180 cm (5'11") | 60 – 81 kg (132 – 178 lb) |
| 185 cm (6'1") | 63 – 85 kg (140 – 188 lb) |
These ranges are a useful quick reference, but they're rounded for readability — for your exact figure at your precise height, use the step-by-step formula above or the calculator linked below.
If your BMI result has you thinking about a gradual, healthy change, the underlying principles that actually work are well established and don't require extreme measures:
Whatever your starting BMI, the goal worth working toward is sustainable, long-term health — not chasing a specific number as fast as possible.
Skip the manual arithmetic. Ihsabha's BMI Calculator takes your height and weight in either metric or imperial units and instantly returns your exact BMI, your category, and a healthy weight range for your height — all in one step, with no rounding errors.
Once you know your category, the most useful next step is usually to look at your result alongside other numbers rather than in isolation. If your BMI falls in the overweight or obese range, Ihsabha's Ideal Weight Calculator can show you a target range for your height, and the TDEE Calculator can estimate your daily calorie needs if you're planning a gradual, sustainable change. If you're an athlete or have a muscular build, it's also worth reading about BMI's known limitations before drawing conclusions from the number alone.
In metric units, BMI equals weight in kilograms divided by height in meters squared (kg / m²). In imperial units, BMI equals weight in pounds divided by height in inches squared, multiplied by 703.
For most adults, a BMI between 18.5 and 24.9 falls in the normal weight range. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 or above as obese.
Yes. Convert your height to meters, square that number, then divide your weight in kilograms by the squared height. It only takes one division and one multiplication, though an online calculator avoids rounding errors.
The formula requires height in meters, not centimeters. If you only have your height in centimeters, divide by 100 first to convert it to meters before squaring it.
BMI is a fast screening number, not a diagnosis. It doesn't distinguish between muscle and fat, doesn't account for where fat is distributed on the body, and can misclassify very muscular or very lean individuals. It remains useful precisely because it's quick and consistent, but if your number surprises you or you have specific health concerns, it's worth discussing your full picture — not just the BMI figure — with a doctor or qualified health professional. Used the right way — as a quick starting signal rather than a verdict — the formula you've just learned to calculate by hand remains one of the most practical, evidence-informed health numbers available to absolutely anyone, anywhere, with nothing more than a scale and a tape measure.
Get your personal numbers with Ihsabha's BMR Calculator and Daily Calorie Calculator. For related reading on Ihsabha's blog, see How to Calculate Your Daily Calorie Needs: The Complete TDEE Guide, Calorie Deficit for Weight Loss: How Many Calories Should You Eat? and Calorie Surplus for Muscle Gain: How Many Calories to Build Muscle.