BMI sounds technical but the actual math is grade-school arithmetic. This guide shows you the formula in both metric and imperial, worked examples for the most-Googled heights and weights, what each category really means, and the limits of BMI as a single-number health measure. By the end you will be able to calculate it on the back of a napkin, and know when the result is meaningful versus when it is misleading.
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Body Mass Index with healthy weight range
What BMI actually measures
BMI (Body Mass Index) is a single number that compares your weight to your height squared. It was developed in the 1830s by Adolphe Quetelet as a population statistic, and adopted by physiologist Ancel Keys in the 1970s as an individual screening tool. It is widely used because it is cheap and easy — you only need a scale and a tape measure. It is imperfect because it does not measure body fat directly.
The metric formula
BMI = weight (kg) / height² (m²)
Take your weight in kilograms. Take your height in metres, square it. Divide the first number by the second.
Worked example: 70 kg, 1.75 m
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 / 3.0625 = 22.86
- Category: Normal weight (18.5 – 24.9)
The imperial formula
BMI = (weight (lb) × 703) / height² (in²)
The 703 is the conversion factor that gets you from pounds and inches to the metric base. Multiply your pounds by 703, divide by your height in inches squared.
Worked example: 160 lb, 5'9" (69 inches)
- Weight × 703: 160 × 703 = 112,480
- Height squared: 69 × 69 = 4,761
- BMI: 112,480 / 4,761 = 23.6
- Category: Normal weight
BMI categories
| BMI range | Associated health risk | |
|---|---|---|
| Underweight | Below 18.5 | Increased (nutrient deficiency) |
| Normal weight | 18.5 – 24.9 | Lowest |
| Overweight | 25 – 29.9 | Slightly increased |
| Obese (Class I) | 30 – 34.9 | Significantly increased |
| Obese (Class II) | 35 – 39.9 | High |
| Obese (Class III) | 40 and above | Highest |
These ranges apply to adults 20 and over. Children and teens use percentile-based BMI-for-age charts because their growing bodies do not fit adult thresholds.
BMI by common heights (healthy weight range)
| Healthy weight (lb) | Healthy weight (kg) | |
|---|---|---|
| 5'0" (152 cm) | 97 – 127 lb | 44 – 58 kg |
| 5'2" (157 cm) | 104 – 135 lb | 47 – 61 kg |
| 5'4" (163 cm) | 110 – 144 lb | 50 – 65 kg |
| 5'6" (168 cm) | 118 – 154 lb | 53 – 70 kg |
| 5'8" (173 cm) | 125 – 163 lb | 57 – 74 kg |
| 5'10" (178 cm) | 132 – 173 lb | 60 – 78 kg |
| 6'0" (183 cm) | 140 – 183 lb | 63 – 83 kg |
| 6'2" (188 cm) | 148 – 193 lb | 67 – 88 kg |
The mental shortcut
Quick approximation if you cannot reach a calculator:
- Metric: pretend your height in cm minus 100 is your healthy weight in kg. 175 cm → 75 kg as a rough midpoint of the healthy range.
- Imperial: 100 lb at 5 ft, add 5 lb per inch. 5'10" ≈ 100 + (10 × 5) = 150 lb midpoint. (This is rougher.)
These are not precise enough for clinical use but they get you within a few BMI points.
Why your BMI might not mean what you think
BMI is a screening tool, not a diagnosis
Where BMI works well
- Population studies (individual differences average out)
- Tracking your own changes over time at constant body composition
- Quick screening combined with waist measurement and lifestyle context
Where BMI fails
- Athletes with high muscle mass — often classified overweight or obese despite low body fat
- Older adults with muscle loss — BMI can look fine while body fat is high
- Pregnant or breastfeeding women — not designed for them
- Very short or very tall stature — formula's accuracy decreases at extremes
- People of certain ethnicities — Asian populations show increased health risks at lower BMI thresholds (some guidelines use 23 instead of 25 as the overweight cutoff)
Better measures to combine with BMI
BMI alone is okay. BMI plus one of these is much better:
Waist circumference
Wrap a tape measure around your waist at the navel. Over 40 inches (102 cm) for men or 35 inches (88 cm) for women is associated with elevated metabolic risk regardless of BMI. The best single at-home complement.
Waist-to-height ratio
Divide waist circumference by height. Aim for under 0.5 (i.e., keep your waist less than half your height). Some researchers consider this a better single number than BMI.
Body fat percentage
Direct measurement via skinfold calipers, bioelectrical impedance scale, or DEXA scan. Healthy ranges: 10-20% for men, 18-28% for women, varying with age. DEXA is the gold standard but costs $50-150 per scan.
BMI Prime
BMI Prime is BMI ÷ 25. It expresses your BMI as a multiple of the upper healthy threshold:
- Under 0.74 = underweight
- 0.74 – 1.00 = healthy
- 1.00 – 1.20 = overweight
- Over 1.20 = obese (Class I or above)
Useful because it is dimensionless (no units, no rounding differences between metric and imperial) and because "20% above healthy" is more intuitive than "BMI of 30".
Common questions
What is the ideal BMI?
For most adults, a BMI between 20 and 22 shows the lowest all-cause mortality in large studies. The full healthy range (18.5 – 24.9) is fine; individual targets within that range depend on body composition and family history.
Can I be healthy with a BMI of 28?
Possibly. "Metabolically healthy overweight" exists — people with elevated BMI but normal blood pressure, blood sugar, and cholesterol. Long-term data suggest it is less stable than it sounds (many such people develop metabolic disease within 10-15 years), but it is not automatically a problem in the short term.
What about kids?
Children and teens 2-19 use BMI-for-age percentiles from the CDC or WHO, not adult ranges. A 10-year-old with a BMI of 20 could be underweight, healthy, or overweight depending on age and sex — it is the percentile that matters.
How often should I check my BMI?
Monthly at most. Daily weight fluctuations of 1-3 lb are normal water-weight changes, not real BMI shifts. Use a 7-day average if you weigh yourself often.
Does muscle weigh more than fat?
Per pound, no — a pound is a pound. Per volume, yes — muscle is denser than fat. A muscular 200-lb athlete looks much thinner than an out-of-shape 200-lb sedentary person at the same height and BMI. This is one of the main limits of BMI.