What Is BMI? How to Read Your Number
Key takeaways
- BMI (body mass index) is your weight in kilograms divided by your height in meters squared — a 10-second screening number for weight-related health risk.
- The categories: under 18.5 (underweight), 18.5–24.9 (healthy), 25–29.9 (overweight), 30+ (obese).
- Example: 70 kg at 1.75 m tall → 70 ÷ 3.0625 ≈ 22.9, in the healthy range.
- BMI can’t tell muscle from fat, so athletes and regular lifters should also check body fat percentage.
What is BMI? Body mass index is a simple ratio of your weight to your height, used worldwide as a first-pass screen for whether your weight puts you at higher risk of conditions like heart disease and type 2 diabetes. It isn’t a diagnosis — it’s a starting number. Run yours in seconds with our free BMI calculator.
This article is for general information only and isn’t medical advice — talk to your doctor before changing your diet or exercise routine.
How do you calculate BMI?
The formula: BMI = weight (kg) ÷ height (m)². In imperial units: BMI = 703 × weight (lb) ÷ height (in)².
A worked example: you weigh 70 kg and stand 1.75 m tall. Square the height: 1.75 × 1.75 = 3.0625. Divide: 70 ÷ 3.0625 = 22.9. That’s your BMI — one number, no lab work required.
What do the BMI categories mean?
| BMI | Category | What it signals |
|---|---|---|
| Below 18.5 | Underweight | Possible nutrient deficiencies; worth discussing with a doctor |
| 18.5 – 24.9 | Healthy weight | Lowest statistical risk band for weight-related disease |
| 25 – 29.9 | Overweight | Risk begins rising; lifestyle changes usually recommended |
| 30 and above | Obese | Significantly higher risk; see a doctor for a plan |
These cutoffs come from population studies linking BMI bands to disease rates — they’re statistical, not personal verdicts. Where you sit within a band matters less than the trend over time and the company it keeps (blood pressure, cholesterol, blood sugar).
What is a healthy BMI range?
The 18.5–24.9 band is considered the healthy range for most adults. If you’re above it, you don’t need to land at 22 overnight — even a 5–10% weight loss measurably improves blood pressure, blood sugar, and joint load.
To translate a target BMI into a target weight: weight = BMI × height². At 1.75 m, the healthy band spans 18.5 × 3.0625 ≈ 56.7 kg to 24.9 × 3.0625 ≈ 76.3 kg. Our healthy weight calculator and ideal weight calculator do this math for your height.
What does BMI miss?
BMI’s biggest blind spot: it can’t tell muscle from fat. A rugby player at 95 kg and 1.85 m has a BMI of 27.8 — “overweight” — while carrying 12% body fat. Conversely, someone with a “healthy” BMI of 23 but very little muscle and high body fat (“skinny fat”) can still carry metabolic risk.
That’s why BMI is a screening tool, not a final answer. If you lift weights, play sports, or your BMI says one thing while the mirror says another, check your body fat percentage — it measures what BMI can’t see.
Does BMI work for kids and older adults?
Children and teens use BMI-for-age percentiles, not adult cutoffs, because healthy BMI shifts as kids grow — a pediatrician plots it on a growth chart. Older adults (65+) may be healthiest slightly above 24.9, since a little extra reserve protects against frailty and bone loss.
Pregnancy also changes the picture: pre-pregnancy BMI guides recommended weight gain, but BMI itself isn’t tracked the same way during pregnancy. When in doubt, these are exactly the conversations to have with your doctor.
How should you use your BMI number?
Treat BMI as a traffic light, not a grade. Green (18.5–24.9): keep doing what you’re doing. Yellow (25–29.9): a good nudge to tighten up food and activity — our healthy living basics guide lays out the full system. Red (30+ or under 18.5): book a doctor’s visit and make a plan together.
Recheck every few months, not every morning — weight fluctuates 1–2 kg day to day with water and food. And pair BMI with at least one other measure (waist circumference or body fat percentage) for the full picture. Browse all our health calculators to track yours.
FAQs
What is a good BMI for my age?
For most adults 20–65, the healthy range is the same: 18.5–24.9, regardless of age. Children use BMI-for-age percentiles instead, and adults over 65 may be healthiest slightly above 25. Age matters less than the overall trend and your other health markers like blood pressure and cholesterol.
Is BMI accurate?
BMI is accurate as a population screening tool but imperfect for individuals — it can’t distinguish muscle from fat, so athletes often score as overweight while being very lean. Pair it with waist circumference or body fat percentage for a personal answer. For most non-athletic adults, though, it points in the right direction.
What BMI is considered obese?
A BMI of 30 or higher is classified as obese (30–34.9 class I, 35–39.9 class II, 40+ class III). These bands mark where population-level risk of heart disease, diabetes, and joint problems rises steeply. If you’re at 30+, a doctor can help build a safe, sustainable plan — you don’t have to figure it out alone.
How do I lower my BMI?
Lowering BMI means losing weight: eat about 500 kcal below your daily expenditure for roughly 0.45 kg of loss per week, keep protein high (1.6–2.2 g/kg), and add both cardio and strength training. Aim for 0.5–1% of body weight per week — faster usually means lost muscle and a rebound.
Can you be healthy with a high BMI?
Sometimes — muscular athletes can have BMIs over 25 with excellent metabolic health, which is exactly why body fat percentage matters. But for non-athletes, a BMI over 30 with a large waist circumference reliably signals elevated risk. The number is a prompt to investigate, not a final verdict.