How BMI is calculated
Body mass index compares your weight to your height. In metric units it is weight in kilograms divided by height in meters squared. With pounds and inches the same formula becomes BMI = 703 x weight (lb) / height (in)2; the 703 simply converts the units. For the default 5 ft 9 in (69 inches) and 160 lb: 703 x 160 / 692 = 112,480 / 4,761 = 23.6, which falls in the healthy range. The same person in metric is 175.3 cm and 72.6 kg, and 72.6 / 1.7532 gives the same 23.6.
The CDC and the World Health Organization use the same adult cutoffs: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25.0 to 29.9 is overweight, and 30 or above is obesity, which is further divided into class 1 (30-34.9), class 2 (35-39.9) and class 3 (40 and up, sometimes called severe obesity). These categories apply to adults 20 and older. Children and teens are assessed with BMI-for-age percentiles from the CDC growth charts, because a normal BMI changes a lot between ages 2 and 19.
Healthy weight range for your height
Turn the formula around and you get the weight range that corresponds to a BMI of 18.5-24.9 at your height: multiply your height in inches squared by 18.5 and by 24.9, then divide by 703. At 69 inches that is 125 to 169 lb. At 5 ft 4 in (64 inches) it is 108 to 145 lb, and at 6 ft (72 inches) it is 136 to 184 lb. The calculator prints your range in both pounds and kilograms and tells you how far you are from the nearest boundary.
Those boundaries are population statistics, not personal targets. Someone who is 2 lb over the line is not meaningfully different from someone 2 lb under it. Use the range as a rough frame of reference and think about the direction you are heading over months, rather than a single number on a single day.
What BMI misses
BMI is a quick screening number, and it has well-known blind spots. It cannot tell muscle from fat, so a muscular athlete can land in the overweight or even obese category with very little body fat, while an inactive person can have a normal BMI and a high body-fat percentage. It ignores where fat is stored: abdominal fat carries more cardiovascular and metabolic risk than fat on the hips, so waist circumference (over 40 inches for men or 35 inches for women is the usual flag) adds information BMI cannot. Pregnancy makes BMI meaningless, and in adults over 65 a somewhat higher BMI is not necessarily harmful.
Risk at a given BMI also varies by ancestry. Because people of Asian descent tend to develop diabetes and heart disease at lower BMIs, the WHO and many US clinicians use lower action points for them: 23 for overweight and 27.5 for obesity. If your doctor uses those thresholds, the categories in the table above will not match. Treat any BMI as a starting point for a conversation about health, not as a verdict, and rely on a clinician for anything that matters.