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Calculate Your Body Mass Index
The Tools Kit's free BMI Calculator works out your Body Mass Index from your height and weight and tells you which adult weight category the number falls in. Enter your height in centimeters and weight in kilograms, or switch to feet, inches and pounds, and your BMI appears instantly to one decimal place. There is no signup, and the calculation runs entirely in your browser.
BMI = weight (kg) ÷ height (m)². Someone 170 cm tall weighing 70 kg has a BMI of 70 ÷ (1.7 × 1.7) = 24.2, which sits in the normal weight range.
How to Use the BMI Calculator
- 1
Choose your units
Select Metric (cm/kg) to enter height in centimeters and weight in kilograms, or Imperial (ft/lb) to enter feet, inches and pounds.
- 2
Enter your height
Type your height in centimeters, or in feet plus inches. A blank inches box counts as zero, so 6 feet on its own is read as 72 inches. Inches must be under 12 when you also enter feet, and negative values are rejected.
- 3
Enter your weight
Type your weight in kilograms or pounds, matching the unit mode you picked.
- 4
Read your BMI and category
Your BMI appears to one decimal place with its weight category (Underweight, Normal weight, Overweight or Obese) and updates as you type.
Each unit mode keeps its own fields, so switching from metric to imperial does not convert what you already typed. If you know your height in one system but want to enter it in the other, the height converter turns centimeters into feet and inches, and the unit converter handles kilograms to pounds.
BMI Formula in Metric and Imperial Units
The metric BMI formula divides weight in kilograms by height in meters squared. The calculator takes your height in centimeters and divides it by 100 first, so 170 cm becomes 1.7 m and 1.7 squared is 2.89.
The imperial BMI formula divides weight in pounds by height in inches squared and multiplies the result by 703. Height is first converted to total inches: feet × 12 + inches. At 5 ft 8 in (68 inches) and 154 lb, BMI = 154 ÷ 4,624 × 703 = 23.4. The 703 factor converts pounds per square inch into kilograms per square meter, so both formulas give the same BMI for the same person.
- 1.75 m, 70 kg: 70 ÷ 3.0625 = BMI 22.9 (normal weight)
- 180 cm, 90 kg: 90 ÷ 3.24 = BMI 27.8 (overweight)
- 5 ft 8 in, 154 lb: 154 ÷ 4,624 × 703 = BMI 23.4 (normal weight)
BMI Chart: Adult Weight Categories
The calculator uses the standard adult BMI categories below. The weight columns show what each band means for one example height, 170 cm (about 5 ft 7 in) in metric and 5 ft 8 in in imperial, so you can see how a healthy weight for your height is worked out: multiply your height squared by the BMI you are interested in.
| BMI | Category | At 170 cm | At 5 ft 8 in |
|---|---|---|---|
| Below 18.5 | Underweight | Under 53.5 kg | Under 122 lb |
| 18.5 to 24.9 | Normal weight | About 53.5 to 72 kg | About 122 to 164 lb |
| 25.0 to 29.9 | Overweight | About 72.3 to 86.4 kg | About 165 to 197 lb |
| 30.0 and above | Obese | 86.7 kg or more | About 197 lb or more |
The result shows one of these four labels. It does not split obesity into classes I, II and III, and it does not give a target or ideal weight, only your BMI and its category. The category follows the rounded number you see, so a BMI that displays as 25.0 is labelled Overweight even if the unrounded value was 24.96.
What BMI Does and Does Not Tell You
BMI is a quick screening number, not a measure of body fat or a diagnosis. Because it uses only height and weight, it cannot tell muscle from fat, so muscular athletes often read as overweight. It also ignores age, sex, ethnicity, frame size and where fat is carried; waist measurement is often used alongside it for that reason.
The calculator applies the adult categories to every result and does not ask for age or sex, so it is not suitable for children and teens, whose BMI is read against growth percentiles. During pregnancy BMI is not meaningful either; if you are expecting, the due date calculator is the more useful tool. For anyone, treat the result as a starting point for a conversation with a healthcare provider.
Common uses
- A quick check of where your weight sits for your height before a doctor or gym visit
- Pairing your BMI with the calorie calculator to estimate how much to eat while moving toward a weight goal
- Tracking BMI over time as part of a fitness routine, alongside a habit tracker or a written plan in the goal planner
- Working out how far a change in weight moves your BMI; the percentage calculator shows that change as a percent
- Filling in a BMI field on a health, insurance or fitness form
More health, money and everyday calculators from The Tools Kit are collected on the calculator tools page.
BMI terms explained
| Term | What it means here |
|---|---|
| BMI (Body Mass Index) | Weight divided by height squared, expressed in kg/m²; one number that relates weight to height |
| kg/m² | The unit of BMI. Imperial results are converted into the same unit by the 703 factor, so metric and imperial BMIs are directly comparable |
| 703 conversion factor | The multiplier that turns pounds per square inch into kilograms per square meter in the imperial BMI formula |
| Underweight | A BMI below 18.5 in the standard adult classification |
| Normal weight | A BMI from 18.5 up to 24.9; some sources call this the healthy weight range |
| Overweight | A BMI from 25 up to 29.9 |
| Obese | A BMI of 30 or above; clinical guidance further splits this into classes, which this calculator does not show |
| Body composition | The split between fat, muscle, bone and water in the body; BMI cannot see it, only total weight |
Frequently Asked Questions
BMI is weight in kilograms divided by height in meters squared. In imperial units, it is weight in pounds divided by height in inches squared, multiplied by 703. Someone 170 cm tall weighing 70 kg has a BMI of 70 ÷ (1.7 × 1.7) = 24.2.
Convert your height to total inches (feet × 12 + inches), square it, divide your weight in pounds by that number, then multiply by 703. At 5 ft 8 in (68 inches) and 154 lb, BMI = 154 ÷ 4,624 × 703 = 23.4. Choose Imperial (ft/lb) in the calculator to have this done for you.
For adults, a BMI from 18.5 to 24.9 falls in the normal weight range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These are screening bands, not a diagnosis.
Multiply your height in meters squared by 18.5 and by 24.9 to get the normal-weight range in kilograms. At 170 cm, that is about 53.5 kg to 72 kg. At 5 ft 8 in, it is about 122 lb to 164 lb. The calculator shows your BMI and category rather than a target weight.
BMI is a useful population-level screening measure but an imperfect individual one. It uses only height and weight, so it cannot tell muscle from fat and ignores age, sex, ethnicity, frame size and where fat is carried. A muscular athlete can read as overweight while having low body fat.
Not reliably. The calculator applies the standard adult categories to every result. For children and teens aged 2 to 19, BMI is interpreted with age- and sex-specific growth percentiles, so ask a pediatrician or use a percentile chart instead.
The formula and the adult category cut-offs are the same for men and women, and this calculator does not ask for sex or age. Women typically carry more body fat than men at the same BMI, which is one reason BMI is best read alongside other measures.
703 converts pounds per square inch into kilograms per square meter, so a BMI calculated from feet, inches and pounds matches the metric result. The exact factor is about 703.07, which makes no difference once BMI is rounded to one decimal place.
Yes. Use the Metric (cm/kg) and Imperial (ft/lb) buttons above the form. Each mode has its own fields, so values typed in one mode are not converted into the other; re-enter your height and weight after switching.
No. The BMI is calculated by JavaScript in your browser as you type. Nothing is sent to a server, saved to an account, or kept after you close the tab.
Reviewed by Raja Jahangir · Last reviewed: October 7, 2026
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How is BMI calculated?
BMI is weight in kilograms divided by height in metres squared. Someone 1.75 m tall weighing 70 kg has a BMI of 22.9. The standard adult bands are under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, and 30 or above obese. BMI ignores muscle mass and body composition.