BMI Calculator
Free BMI calculator with metric or imperial units. Get your body mass index, CDC weight category, your healthy weight range, and pounds to lose or gain.
BMI Calculator
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For adults 20 and over. BMI is not a reliable screen during pregnancy.
Estimates only. BMI is a screening tool, not a diagnosis. It does not distinguish fat from muscle, and it does not account for fat distribution, age, or ethnicity. Talk to a healthcare provider for personalized targets, especially if you are pregnant, nursing, an older adult, or a competitive athlete.
What BMI measures (and what it doesn't)
Body mass index is a weight-to-height ratio. Two numbers, one bucket. That is the whole tool. The CDC uses it as a population-level screen for under-, healthy-, or overweight status, and the formula has not changed in over a century: weight divided by height squared, with a unit conversion factor of 703 baked in for imperial.
What BMI does not do is just as important. It does not measure body fat, it cannot tell muscle from fat, it ignores where fat sits on your body, and it treats a 25-year-old and a 75-year-old the same. A defensive lineman and a sedentary office worker with the same height and weight will get the same number. So use BMI the way clinicians do: as a starting screen, not a diagnosis or a goal.
BMI categories explained
The CDC adult categories run as follows. Underweight is below 18.5. Healthy weight is 18.5 to 24.9. Overweight is 25.0 to 29.9. Obesity covers everything 30 and above, split into three sub-classes: Class I (30 to 34.9), Class II (35 to 39.9), and Class III, also called severe obesity (40 and above). The same ranges apply to adult men and women.
The calculator above prints your category as a single badge, with the obesity sub-class noted just below for accuracy. The healthy range also shows up as a weight band for your specific height, so you can see exactly how far you are from the edges of that band.
Using BMI as a starting point before a 75-day challenge
Treat your BMI as the baseline weigh-in the night before Day 1, not the number you are chasing. Pair it with progress photos, body measurements (waist, hips, chest), and a couple of subjective markers like energy levels and sleep quality. The number moves slowly even with tight discipline, so weekly check-ins beat daily ones for keeping perspective.
If you want a free way to log this alongside your daily checklists, Reset75 has a built-in Weigh-In task and a statistics dashboard that plots the trend for you, no account required. The weight tracker and progress photo compare tools pair cleanly with your BMI baseline, and the 75-day challenge calculator can map out the dates.
Limitations of BMI (read this before you panic)
BMI overstates risk in muscular athletes, underestimates it in older adults with sarcopenia, and varies across ethnic groups. South Asian populations tend to carry higher health risk at lower BMI values, which is why some health bodies use a lower overweight cutoff (23 instead of 25) for those populations. Pregnancy also breaks the screen entirely, since the weight gain is mostly not fat.
A healthy BMI does not guarantee good health, and an overweight BMI does not guarantee disease. Waist circumference, body fat percentage, blood pressure, blood glucose, lipid panels, and how you actually feel day to day all matter more than a single number. If your BMI lands far outside the healthy band in either direction, talk to a doctor or a registered dietitian for a personalized plan rather than a generic deficit.
Frequently Asked Questions
Common questions about bmi calculator
What is BMI and what does it measure?
BMI (body mass index) is a weight-to-height ratio used as a screening tool for under-, healthy-, or overweight status in adults. It does not measure body fat directly, and it does not tell you anything about fat distribution, muscle mass, or overall health on its own. The CDC and WHO use it as a population-level indicator, not a diagnosis.
How is BMI calculated?
In metric units, BMI = weight in kilograms divided by height in meters squared. In imperial units, BMI = 703 times weight in pounds, divided by height in inches squared. So a 5 ft 9 in (69 in), 170 lb adult has a BMI of 703 x 170 / (69 x 69), or roughly 25.1.
What is a healthy BMI range?
The CDC defines a healthy adult BMI as 18.5 to 24.9. Below 18.5 is classed as underweight, 25.0 to 29.9 is overweight, and 30 and above falls into one of three obesity classes. The same ranges apply to adult men and women.
Is BMI accurate for women, men, athletes, and older adults?
BMI is a decent population screen but a poor individual measurement. Muscular athletes can land in the overweight or obese range despite low body fat. Older adults can land in the healthy range while carrying very little muscle (sarcopenia). It also varies in meaning across ethnicities, with some health bodies using lower cutoffs for South Asian populations.
What should my BMI be before starting a 75-day challenge?
There is no required starting BMI. Use it as a baseline, not a goal. Log your number the night before Day 1, recheck it weekly, and pair it with progress photos and body measurements. A doctor or registered dietitian can give you a personalized target weight if you want one. The Reset75 app has a daily Weigh-In task that makes this easy to log.
How much weight do I need to lose to reach a healthy BMI?
The healthy upper edge for your height is the weight at BMI 24.9. In imperial: (24.9 x height in inches squared) / 703. In metric: 24.9 x (height in meters squared). The calculator above does this for you and prints the exact pounds or kilos between your current weight and that upper edge.
What's the difference between BMI and body fat percentage?
BMI uses only weight and height, so it cannot tell muscle apart from fat. Body fat percentage measures how much of your weight is fat tissue, using skinfold calipers, bioelectrical impedance scales, or a DEXA scan. Body fat is a better measure of body composition, but BMI is the faster screen.
Can I use BMI to track progress during a 75-day challenge?
Yes, with some caveats. Weigh in on the same scale, at the same time of day (mornings, after the bathroom, before water and food), and recalculate weekly. Pair the BMI trend with the progress photo tool and the weight tracker so you can see body composition shifts the scale alone misses.