BMI Calculator (Body Mass Index)

Your doctor checks your BMI at every physical. It's in every fitness app and weight loss program. Yet most people couldn't tell you what it actually measures or why a number between 18.5 and 24.9 is called "normal." The BMI calculator gives you your number in seconds and — more importantly — gives you the context to interpret it sensibly.

How BMI Is Calculated in Imperial Units

BMI = (weight in lbs / height in inches²) × 703

For a 160-lb person at 5'6" (66 inches): BMI = (160 / 66²) × 703 = (160 / 4,356) × 703 = 25.8 — just into the overweight range. The same person at 5'8" (68 inches): BMI = (160 / 4,624) × 703 = 24.3 — normal. Two inches of height makes the difference between two categories. This illustrates both how the formula works and how sensitive BMI is to height errors — always use your measured height, not your estimated height. Use this online calculator for any weight and height combination.

BMI Weight Categories and Health Implications

• Below 18.5 (Underweight): risk of malnutrition, bone loss, weakened immunity; in young women, may indicate eating disorder — discuss with a doctor
• 18.5–24.9 (Normal weight): lowest population-level risk for weight-related disease; doesn't mean risk-free, but weight isn't the primary concern
• 25.0–29.9 (Overweight): moderately elevated risk; particularly concerning if combined with large waist circumference, high blood pressure, or abnormal blood sugar
• 30.0–34.9 (Obese Class I): substantially elevated risk for type 2 diabetes, hypertension, sleep apnea, osteoarthritis
• 35.0–39.9 (Obese Class II): high risk; most clinical guidelines recommend intensive lifestyle intervention or medical treatment
• 40.0+ (Obese Class III): very high risk; bariatric surgery considered for appropriate candidates at this level

A More Complete Picture: What to Measure Beyond BMI

If your BMI falls outside the normal range, these additional measurements add essential context before drawing health conclusions:

• Waist circumference: the most accessible measure of abdominal fat, which is the metabolically active fat that drives cardiovascular and diabetes risk. Target: under 35 inches for women, under 40 inches for men.
• Waist-to-height ratio: divide your waist circumference by your height (both in the same unit). Below 0.5 is the general health target across ages and ethnicities — more reliable than BMI for metabolic risk prediction.
• Blood tests: fasting glucose, HbA1c, lipid panel — these directly measure the metabolic consequences of excess weight and provide actual health data rather than a proxy.

The BMR calculator and health fitness calculators build on BMI with metabolic rate and caloric needs estimates.

How Much Do You Need to Change to Shift BMI Category?

For someone at 5'6" (66 inches), the weight thresholds for each BMI category: normal weight requires 115–154 lbs; overweight starts at 155 lbs; obesity starts at 186 lbs. To move from BMI 27 (overweight) to BMI 24.9 (top of normal): weight loss of approximately 13 lbs (about 6 kg). Each pound of weight change moves BMI by approximately 0.16 points at this height. Small changes are meaningful — even 5–10% of body weight lost produces significant improvements in blood pressure, blood glucose, and lipid levels, even if BMI category doesn't change.