How to Calculate BMI Manually: Formula, Examples, and Charts

To calculate BMI manually, divide your weight by your height squared: in metric units, BMI equals weight in kilograms divided by height in meters squared, and in imperial units, BMI equals weight in pounds divided by height in inches squared, multiplied by 703. Check any result instantly with our free BMI calculator, and read on for worked examples in both systems and the category chart.

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The BMI Formula in Metric Units

The metric BMI formula is the simplest version: BMI equals weight in kilograms divided by height in meters squared, written as kg/m2. Squaring the height is the step people get wrong, because it means multiplying height by itself, not doubling it, so a height of 1.75 meters squares to 3.0625, not 3.5. Take a person weighing 80 kilograms at 1.75 meters: divide 80 by 3.0625 and the BMI is 26.1, which falls in the overweight category explained below. If your height is in centimeters, convert first by dividing by 100, since 175 centimeters is 1.75 meters, and entering 175 directly into the formula would give an absurdly small answer. The metric formula is the definition of BMI, and every other version, including the imperial one, is just the same ratio with unit conversions folded in, so when in doubt, convert your measurements to kilograms and meters and use this form. Weigh yourself in the morning, after using the bathroom and before eating, for the most consistent readings, because a single day food and water weight can move the result by several tenths.

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The BMI Formula in Imperial Units

The imperial BMI formula is BMI equals 703 times weight in pounds divided by height in inches squared, written as 703 times lb/in2. The 703 is not a magic number: it is the conversion factor that turns pounds per square inch into kilograms per square meter, rounded from 703.069, and using 703 keeps hand calculations within a tenth of the true value. Take a person weighing 175 pounds at 69 inches tall: square 69 to get 4,761, divide 175 by 4,761 to get 0.03676, and multiply by 703 to get 25.8, again in the overweight range. The common imperial mistake is using feet instead of inches, so convert first: 5 feet 9 inches is 69 inches, not 5.9, and entering 5.9 would give a wildly wrong answer. If your scale reads in ounces rather than pounds, our free kilograms to ounces converter runs the conversion both ways, and a kitchen scale reading can be converted to pounds by dividing ounces by 16. For the height side, measure without shoes against a wall, since an inch of shoe or slouch changes the squared term more than most people expect.

The Adult BMI Category Chart

Adults are classified by the same cutoffs worldwide: under 18.5 is underweight, 18.5 to 24.9 is the healthy range, 25.0 to 29.9 is overweight, and 30.0 and above is obesity, with obesity split into class 1 from 30 to 34.9, class 2 from 35 to 39.9, and class 3 at 40 and above. These cutoffs come from population studies linking BMI ranges to disease risk, not from any single ideal body, and the healthy range is broad on purpose: a BMI of 19 and a BMI of 24 are both healthy by this chart but describe quite different bodies. A useful way to use the chart is to find the weight range for your height rather than a single target: at 1.75 meters, the healthy range spans from 56.7 kilograms to 76.3 kilograms, which is 125 to 168 pounds, so a person at 80 kilograms needs to lose about 4 kilograms to enter the range, not reach one magic number. Children and teenagers use a different system, BMI-for-age percentiles, because their healthy BMI changes as they grow, so never apply the adult chart to a child. If you track exercise alongside weight, our free minutes to hours converter totals weekly training time the same honest way.

Worked Examples You Can Check by Hand

Three quick examples show the formula working across the chart. Example one, metric: 62 kilograms at 1.68 meters. Square 1.68 to get 2.8224, divide 62 by 2.8224 to get 21.97, which rounds to 22.0, comfortably healthy. Example two, imperial: 200 pounds at 72 inches. Square 72 to get 5,184, divide 200 by 5,184 to get 0.03858, multiply by 703 to get 27.1, overweight. Example three, near a boundary: 95 kilograms at 1.80 meters. Square 1.80 to get 3.24, divide 95 by 3.24 to get 29.3, overweight but close to the obesity cutoff at 30, so a 3-kilogram change would cross it. Notice how sensitive the result is near boundaries: a single kilogram or a single centimeter can move a borderline BMI across a category, which is why a one-time measurement should never be treated as a diagnosis. Repeat the measurement a week apart, at the same time of day, and average the two, before drawing any conclusion from a borderline figure. Our free BMI calculator reproduces all three examples exactly, which makes it a good check on your hand arithmetic.

What BMI Gets Wrong

BMI is a screening tool, not a body-composition measurement, and its famous failures all come from one fact: it cannot tell muscle from fat. A muscular athlete can score 28, officially overweight, while carrying less body fat than a sedentary person scoring 22, because muscle is denser than fat and the formula only sees total weight. Older adults face the opposite problem, since muscle loss with age can keep BMI steady while body fat rises, hiding a real change in composition. BMI also ignores where fat sits, and abdominal fat carries more health risk than fat on the hips and thighs at the same BMI, which is why doctors pair BMI with waist circumference: above 40 inches for men and 35 inches for women signals added risk regardless of BMI. Pregnancy, amputation, and wheelchair use all make standard BMI misleading, and different populations show risk at different cutoffs, with some Asian populations facing elevated risk from a BMI of 23. The honest use of BMI is as a first screen that prompts better measurements, such as waist size, body-fat percentage, or a clinical assessment, not as a final judgment on a body.

Tracking BMI Changes Honestly

A single BMI number matters less than its direction over months, and tracking it honestly requires controlling the variables that move it daily. Weigh yourself at the same time, on the same scale, on a hard floor, wearing similar clothing, because a carpeted floor can under-read by several pounds and a post-meal weigh-in can add two. Record height once a year, since adults lose a little height with age and teenagers gain it quickly, and a stale height makes every later BMI wrong in the same direction. Plot the results monthly rather than daily, because daily weight swings of a kilogram or more from water, food, and glycogen are noise, while the monthly trend is signal. If your BMI is falling, check whether the loss is fat or muscle by tracking strength and waist size alongside it, since a falling BMI from muscle loss is not a health win. For a fuller health picture, our BMI basics guide explains how the index is used in clinics, and our age calculation guide covers the other number that frames every health target. A BMI tracked this way becomes a useful gauge, not a daily judgment. Set a review rhythm that matches the measurement: check the trend on the first day of each month, compare it with the previous three months rather than the previous week, and only change diet or training when two consecutive monthly readings point the same way, because a single month can still be skewed by travel, illness, or a scale battery running low. Patience is the method, and the trend is the verdict.