BMI and calories: what the number shows, and what it does not

What BMI actually is

BMI, body mass index, is weight in kilograms divided by height in metres squared. At 72 kg and 175 cm the sum is 72 / (1.75 × 1.75) = 23.5. That is the same figure the BMI calculator on this site shows, with the same WHO cut-offs: under 18.5 underweight, 18.5-25 normal weight, 25-30 overweight, 30 and above obesity.

The number was built for groups, not for one body. Sundhed.dk says so in as many words: BMI is a decent tool for finding overweight in a population, and a worse tool for judging a person. In Denmark's 2023 national health profile, 52.9 percent of adults had a BMI of 25 or more. That is a fact about Denmark. It is almost nothing about your Tuesday.

What the number cannot see

BMI does not tell fat from muscle. Someone with a lot of muscle can land above 25 without carrying much fat. A still person with the same BMI can have it the other way round. The number does not know waist, blood pressure, sleep, medication, or whether the milk in the fridge was drunk.

It also does not fit children, pregnancy, or bodies the formula was not written for. The WHO cut-offs on this site are for adults. The ideal-weight calculator takes the same height and shows the range for BMI 18.5-25, not one magic kilogram. A range is more honest than a target that pretends to know you.

Calories are a different question

BMI asks: how does weight sit against height, today. Calories ask: how much energy came in, and how much went out, this week. You can have a BMI in the normal band and eat past your need every night. You can have a BMI over 25 and sit still at maintenance. The two numbers are not in a fight. They do not measure the same thing.

Daily need on this site is Henry (2005) for resting burn, times an activity factor from the Nordic Nutrition Recommendations 2023. A 34-year-old woman at 72 kg and 175 cm, low-active, lands somewhere around 1,900 kcal to hold weight. A slow-loss goal is a 300 kcal step under that, not a new BMI. The figure is an estimate. It is not a diet, and it is not a diagnosis.

The receipt already knows the names

What kills most calorie counters is not the maths. It is the paperwork. You have to remember lunch, guess a sauce, look up a Danish product name in a box that does not know it. Thursday you quit. The number was never the hard part.

A Nemlig cart is already a list. A Coop receipt too. The names exist. CaloriSpy reads them, matches them to the catalog, and puts the food in the Food Bank. At night you tick what you actually ate. About fifteen seconds. Buying milk is not drinking it, so the check-in stays. The blank page is what goes.

That is why BMI and calories belong to two different habits. BMI you take when the scale has moved, same calculator, same height, same honesty about what the number is not. Calories are the week that passed, and they live in the shopping.

How to use both numbers

Use the BMI calculator when you want the screening number. Use the calorie calculator when you want a starting point for the day. Do not invent one from the other. A BMI of 27 does not become a calorie target by multiplying with a round figure you saw on a forum.

If you want to see what you actually eat, the receipt is the input. Connect the shop you already use. Let the Food Bank fill. Check in at night. Skip an evening and no streak breaks, because there is no streak.

The number on the scale moves slowly. The number in the shopping moves tomorrow. They should not be chased on the same screen, three times a day.

What we are not claiming

This is not medical advice. It is not a stand-in for a talk with your doctor, and it is not a promise of weight loss. The calculators are estimates. Catalog figures are a guide. A line we cannot stand behind waits rather than becoming a made-up number.

The BMI cut-offs are WHO's, the same ones Patienthåndbogen on sundhed.dk uses. Resting burn is Henry (2005). Activity factors are NNR 2023. 52.9 percent at BMI 25 or above is from the 2023 national health profile, cited on sundhed.dk. None of those sources know your body. A receipt does not either. It just knows the names.