To calculate BMI, divide your weight in kilograms by your height in metres squared. If you use pounds and inches, multiply your weight in pounds by 703, then divide by your height in inches squared. A result of 18.5 to 24.9 is a healthy weight for most adults aged 20 and over.
Body mass index, or BMI, is one of the simplest health numbers you can work out at home. You need only your weight, your height, and one short calculation. Doctors use it as a first screening step because it is fast and consistent.
This guide shows you how, in kilograms and in pounds, with three worked examples, the 703 explained, and the five mistakes that cause most wrong answers.
What Is BMI?
BMI is a number calculated from weight and height, developed in the 1830s by the Belgian statistician Adolphe Quetelet. The formula adjusts weight for height, so a taller person is not automatically scored as heavier than a shorter person of the same build.
BMI is a screening tool, not a diagnosis. It cannot tell you how much of your weight is muscle and how much is fat, and it cannot diagnose any disease on its own. It places you in a broad category — underweight, healthy weight, overweight, or obesity — and flags whether a fuller health check is worth discussing with a clinician. The free BMI calculator on Calcrio applies the same formulas shown here.
The categories for adults aged 20 and over come from the Centers for Disease Control and Prevention (CDC), and are the same for men and women:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5–24.9 | Healthy weight |
| 25.0–29.9 | Overweight |
| 30.0 and above | Obesity |
A BMI of 30.0 or above is divided into Class 1 (30.0–34.9), Class 2 (35.0–39.9), and Class 3 (40.0 and above). For ages 2 to 19, these adult cutoffs do not apply: the CDC uses BMI-for-age percentiles instead.
How to Calculate BMI in Kilograms and Metres
The metric formula is BMI = weight in kilograms ÷ (height in metres × height in metres), or BMI = kg ÷ m². Follow these steps in order.
Step 1: Weigh yourself in kilograms
Weigh yourself in the morning, after using the bathroom and before eating, in light clothing. If your scale shows pounds, divide by 2.2046: 154 pounds ÷ 2.2046 = 69.9 kilograms, or about 70 kilograms.
Step 2: Measure your height in metres
Stand barefoot against a wall and measure from the floor to the top of your head. If your tape shows centimetres, divide by 100: 175 centimetres is 1.75 metres. Entering 175 instead of 1.75 makes the answer roughly 10,000 times too small.
Step 3: Square your height
Multiply your height in metres by itself. For 1.75 metres, that is 1.75 × 1.75 = 3.0625. Squaring is not optional, and skipping it is the most frequent mistake in BMI calculations.
Step 4: Divide your weight by your squared height
Divide the weight from Step 1 by the number from Step 3: 70 ÷ 3.0625 = 22.9, rounded to one decimal place. That falls in the 18.5–24.9 healthy weight range.
How to Calculate BMI in Pounds and Inches
The imperial formula is BMI = (weight in pounds × 703) ÷ (height in inches × height in inches).
Step 1: Weigh yourself in pounds
Use the same morning routine and record your weight to the nearest pound. If your scale shows kilograms, multiply by 2.2046: 70 kilograms × 2.2046 = 154.3 pounds.
Step 2: Convert your height to inches only
Convert your full height to inches first. Multiply the feet by 12 and add the remaining inches: 5 feet 7 inches is (5 × 12) + 7 = 67 inches, and 5 feet 9 inches is 69 inches. Never enter 5.9 for 5 feet 9 inches — that would mean 5 feet and almost 11 inches.
Step 3: Multiply your weight by 703
Multiply your weight in pounds by 703. For 150 pounds, that is 150 × 703 = 105,450. The number 703 is explained below. Do not round it to 700: that small change matters near a category boundary.
Step 4: Square your height in inches, then divide
Multiply your height in inches by itself: 67 × 67 = 4,489. Then divide: 105,450 ÷ 4,489 = 23.5, in the healthy weight range. Check it with the free body mass index calculator on Calcrio.
What Is the 703 in the BMI Formula?
The 703 often looks like a fudge factor. It is not: it is a unit-conversion constant that makes the imperial calculation give the same answer as the metric one. One pound equals 0.4536 kilograms and one inch equals 0.0254 metres, and combined those conversions give 0.4536 ÷ 0.0254² ≈ 703.07, rounded to 703.
If you use 703 with kilograms, or leave it out with pounds, the answer will be wrong by a factor of about 703. As a check, calculate both ways: 70 kilograms at 1.75 metres gives 22.9 by the metric method, and the same person at about 154 pounds and 69 inches gets about 22.7 by the imperial method, allowing for rounding.
3 Worked Examples of BMI Calculation
Worked examples are the fastest way to check your method.
Example 1: 70 kg, 1.75 m — metric
Height squared: 1.75 × 1.75 = 3.0625. Divide: 70 ÷ 3.0625 = 22.857…, which rounds to 22.9. Category: 22.9 is between 18.5 and 24.9, so this is a healthy weight.
Example 2: 78 kg, 1.75 m — metric
Height squared: 1.75 × 1.75 = 3.0625. Divide: 78 ÷ 3.0625 = 25.469…, which rounds to 25.5 — overweight (25.0–29.9). Just 8 extra kilograms moves the result from 22.9 to 25.5, across the boundary. A result just above or below 25.0 should be treated as borderline.
Example 3: 150 lb, 5 ft 7 in — imperial
Height in inches: (5 × 12) + 7 = 67. Weight × 703: 150 × 703 = 105,450. Height squared: 67 × 67 = 4,489. Divide: 105,450 ÷ 4,489 = 23.494…, which rounds to 23.5 — a healthy weight (18.5–24.9). All three examples use the same CDC categories, defined by the BMI number itself, not by the units used.
Common Mistakes When Calculating BMI
Most wrong BMI answers come from one of five errors:
| Mistake | What goes wrong | How to fix it |
|---|---|---|
| Forgetting to square height | Dividing by height once gives a number about 1.75 times too large | Always multiply height by itself first |
| Using centimetres instead of metres | Entering 175 instead of 1.75 makes the answer about 10,000 times too small | Divide centimetres by 100 |
| Using 703 with kilograms | Multiplying kilograms by 703 inflates the answer about 703 times | Use 703 only with pounds and inches |
| Entering 5 ft 9 in as 5.9 ft | 5.9 feet is 5 ft 10.8 in, not 5 ft 9 in | Convert to 69 inches first |
| Mixing units | Using pounds with metres, or kilograms with inches | Pick metric or imperial and convert both measurements to match |
Two habits improve accuracy. Measure your height rather than guessing it: most adults overestimate it by 1 to 2 centimetres, which lowers the calculated BMI. And weigh yourself at the same time of day on the same scale, since body weight normally fluctuates by 1 to 2 kilograms during a day.
What Your BMI Result Means — and What It Does Not
A BMI result places you in one of four adult categories, each linked to average health risk across large populations. People in the healthy range of 18.5–24.9 have, on average, the lowest risk of conditions such as type 2 diabetes, heart disease, and high blood pressure. Risk rises gradually through the overweight range and more steeply at 30.0 and above. A BMI below 18.5 is also linked to higher risk, including nutritional deficiency and bone loss.
Those are population averages, not individual predictions. BMI does not distinguish muscle from fat, so a very muscular person can have a BMI above 25.0 while carrying little body fat, and it does not measure where fat is stored. Clinicians use BMI alongside waist circumference, blood pressure, blood sugar, and cholesterol before drawing conclusions. If your result surprises you, check it with a free BMI calculator to rule out an arithmetic error, then discuss it with a healthcare professional if it sits outside the healthy range.
FAQ: How to Calculate BMI
What is the formula to calculate BMI?
The metric formula is BMI = weight in kilograms ÷ height in metres squared. The imperial formula is BMI = (weight in pounds × 703) ÷ height in inches squared. Both give the same result, and 703 is a unit-conversion constant.
How do I calculate BMI without a calculator?
Weigh yourself, measure your height, square your height, and divide your weight by that number. At 70 kg and 1.75 m, that is 70 ÷ 3.0625 = 22.9. A free online BMI calculator checks your working in seconds.
What is a healthy BMI?
For adults aged 20 and over, a healthy BMI is 18.5 to 24.9, according to the CDC. Below 18.5 is underweight, 25.0 to 29.9 is overweight, and 30.0 or above is obesity. The cutoffs do not apply to children and teenagers.
Why is there a 703 in the BMI formula?
The 703 converts pounds and inches into the metric units the formula uses. It comes from 0.4536 ÷ 0.0254², about 703.07. Use it only with pounds and inches.
Can I calculate BMI from feet and inches directly?
No, not without converting first. Convert your height to total inches: multiply the feet by 12 and add the remaining inches. A height of 5 feet 9 inches is 69 inches, not 5.9 feet.
Is BMI accurate?
BMI is accurate as a population screening tool, but it is not a diagnosis for an individual. It does not distinguish muscle from fat or measure where fat is stored. Use it as a first check, alongside waist circumference and blood pressure.