BMI Mistakes and Limits: Athletes, Age, Pregnancy, Units

By Nesim Razon, independent Android developer · 2026-11-10

BMI is quick, which is exactly why it gets misused. People read too much into it, apply it where it does not fit, or type the wrong units and trust a number that is simply wrong. This article lists the usual traps and how to avoid them. It is general information, not medical advice.

Mistake one: treating BMI as a health verdict

BMI only compares weight with height. It says nothing about your fitness, your blood pressure, your sleep or what you eat. Two people with the same BMI can have very different health. Doctors use the number to start a conversation, not to end it.

Equally, a result in the normal band is not a guarantee of good health, and a result outside it is not a diagnosis. If something about your health worries you, the number is a reason to talk to a doctor, who can look at much more than height and weight.

Athletes and very muscular people

Muscle weighs more than the same volume of fat, and the BMI formula cannot tell them apart. A person who lifts weights or plays a strength sport can reach the overweight or even obesity band while having low body fat. The formula is not wrong about the arithmetic; it just answers a question about total weight that does not fit this person.

If you train hard and your result looks odd, other measures such as waist circumference or a body composition assessment by a professional give a better picture.

Older adults

With age, people tend to lose muscle and can become slightly shorter. Both change the formula: less muscle means the same weight carries more fat, and a smaller height number raises BMI. The standard bands were written for adults in general, and how well they fit older people is something health professionals discuss. The sensible step is to ask a doctor how to read a result for an older person rather than relying on the table alone.

Pregnancy

BMI is not meant to be used during pregnancy, because the weight now includes the baby, fluids and other changes. Doctors and midwives usually look at the BMI from before pregnancy and follow weight gain with their own guidance. If you are pregnant, ignore the app result and follow the advice of your healthcare team.

Children and teenagers

The adult bands do not apply under 18. A child is growing, and what is normal changes with age and differs between boys and girls. For young people, a BMI value is compared with percentile curves for their age and sex, and a doctor interprets it. If you want to check a child, take the measurements to a doctor or nurse instead of using the adult table.

Unit mistakes that wreck the result

• Pounds typed into a kilogram field. One kilogram is about 2.2 pounds. A person of 154 lb and 175 cm has a BMI of 22.9, but typing 154 as kilograms gives 50.3, which looks like severe obesity.
• Decimal feet instead of feet and inches. 5.9 ft does not mean 5 ft 9 in. It means 5.9 x 12 = 70.8 inches, which is 5 ft 10.8 in, about 180 cm instead of 175 cm. For a 70 kg person that moves BMI from 22.8 to 21.7.
• Metres and centimetres mixed up. 1.75 belongs in a metre formula, 175 in a centimetre field. A height of 1.75 typed as centimetres gives an absurd result.
• Using 703 with kilograms. The factor 703 belongs only to the pounds and inches formula.
• Rounding too early. Keep the height squared and the division unrounded, and round only the final BMI.

The BMI Calculator app has a metric mode (cm and kg) and an imperial mode (feet, inches and pounds). It converts with the exact definitions, 2.54 cm per inch and 0.45359237 kg per pound, and keeps your values when you switch units. It also only accepts realistic adult ranges, so a typing slip is more likely to be caught.

Measuring well

A few habits make the inputs more reliable, and none of them is complicated. Weigh yourself on the same scale, on a hard flat floor, at roughly the same time of day, for example in the morning after using the toilet and before eating, wearing little. Body weight naturally moves by a kilogram or two during a day because of food, drink and water.

Measure height barefoot, with heels, back and head against a wall, looking straight ahead, and have someone mark the top of your head with a flat object. Heights can be slightly larger in the morning than in the evening. If you are near a category boundary, a one centimetre error can move your result across it.

A quick sanity check before you trust a result

Before you read anything into a number, ask whether it is plausible. A BMI below about 10 or above about 60 is almost certainly a typing error, not a person. A fast way to check: multiply your height in metres squared by 22, and you get the weight that would give a BMI of 22. For 1.75 m that is 3.0625 x 22 = 67.4 kg. If your weight is far from that and your result is wildly different from what you expected, recheck the units first.

Also check which unit mode the app is in. If you switch from centimetres and kilograms to feet and pounds, the fields change meaning even though the layout looks similar.

Other measures worth knowing about

Because BMI ignores body shape, many health professionals add other simple measures. Waist circumference shows how much fat sits around the abdomen. A commonly cited rule is to keep your waist below half of your height, so for 175 cm that is under 87.5 cm. Blood pressure, blood sugar and cholesterol results say much more about health than weight alone, and only a doctor can interpret them for you. The point is not to collect numbers for their own sake, but to see that BMI is one small window, not the whole room.

Where the app fits in

The BMI Calculator app is meant to do the arithmetic cleanly: it shows your BMI to one decimal place, the WHO adult category, and the weight range that matches the normal band for your height. It does not know your age, muscle mass or medical history, and it is not a medical device. Use it as a starting point, and take any worry about weight or health to a doctor.