Your scale can only print one of two different words
When a scale or a chart calls you obese, it is almost always applying one rule: BMI of 30 or higher. That rule never looks at how much of you is fat. This page runs the BMI line and the body-fat line side by side on the same body, splits your weight into fat mass and fat-free mass, and tells you how many kilograms sit between you and each line — plus where the two lines have drifted apart at your age.
Obese by body fat, not by BMI. Body fat 30.1% (estimated from BMI) against the 25% obesity line for men, and BMI 30.0 against the line at 30.
Your 95.0 kg splits into 28.6 kg of fat and 66.4 kg of everything else — a fat mass index of 9.03 and a fat-free mass index of 20.95.
At 45, the body-fat obesity line corresponds to a BMI of 25.71, which is 4.29 points below the BMI-30 line. The two scales disagree by that much before your own body is even involved.
The BMI-30 line is -0.1 kg above you at this height; one BMI point costs 3.17 kg here.
Closing the body-fat gap with fat alone — lean mass untouched — is 6.5 kg, landing you at 88.5 kg and a BMI of 27.9.
One kilogram of body weight moves your BMI by 0.316 points here, so a two-kilogram morning swing is worth 0.63 BMI points — enough to change the word on a scale without changing your body.
Deurenberg (1991): BF% = 1.20·BMI + 0.23·age − 10.8·sex − 5.4
Fat mass index = BMI × BF% ÷ 100 · Fat-free mass index = BMI − FMIThe BMI classes are the World Health Organization adult categories and the body-fat obesity line is the obesity category of the ACE chart as it is commonly reproduced — 25% for men, 32% for women. Everything else on this page — every threshold table, the age drift, the FMI grid, the cross-classification and every kilogram figure in the answer card — is arithmetic this page performs on those published numbers. None of it is copied from anywhere, and none of it is a clinical diagnosis.
Two scales, two definitions, one word
The word obese gets used for two different measurements, and the mismatch is the reason so many people get a number they do not recognise. One definition is a ratio of weight to height. The other is a share of body mass. They are not the same quantity, they do not move together, and a device that reports one of them cannot report the other without guessing.
The practical consequence is that a single body can be called obese by one rule and not by the other, and neither rule is lying. They are answering different questions. The rest of this page is about how far apart those answers actually sit.
BMI 30 is not one body-fat number
Take the BMI-30 line and push it through the Deurenberg equation, which estimates body fat from BMI, age and sex. The line stops being a line: the body fat it corresponds to depends entirely on who is standing on the scale. Every figure below is that equation evaluated at BMI 30, computed here.
A 20-year-old man at BMI 30 is estimated at 24.4% body fat; a 70-year-old man at the same BMI is estimated at 35.9%. That is 11.5 percentage points of difference produced by age alone, with the height-to-weight ratio held perfectly constant. The BMI line is fixed; the body it describes is not.
Run it the other way: where the body-fat line sits in BMI units
Inverting the same equation answers a sharper question — at what BMI does body fat reach the obesity category at all? The answer moves by 0.1917 BMI points for every year of age, which is 1.92 points per decade, because the age term (0.23) is divided by the BMI coefficient (1.20).
Past about age 32 for women and age 49 for men, the body-fat obesity line sits at a BMI below 25 — inside what BMI calls normal. Two scales, same body, opposite words, and the disagreement grows by 1.92 BMI points every ten years of age.
Body composition: what the number on the scale is made of
Body weight is the sum of fat mass and fat-free mass, and fat-free mass is itself a bundle of muscle, bone, organs and water. Two people of identical height and weight can differ by twenty kilograms of fat. This is the part a scale cannot see, and it is why the same weight can mean two entirely different bodies.
The clean way to compare bodies of different sizes is to divide each component by height squared, exactly the way BMI divides total weight. Fat mass index and fat-free mass index fall out of numbers you already have: FMI is BMI multiplied by body fat share, and FFMI is whatever is left. The grid below is FMI computed across BMI and body fat percentage.
Read the grid sideways and the problem with BMI becomes obvious: an FMI of 7.50 appears at BMI 30 with 25% body fat and again at BMI 25 with 30% body fat and again at BMI 22 with 34% body fat. The same absolute fat load, three different BMI labels. BMI cannot separate those, because it never measures the split.
That is also why a scale that estimates body fat from weight alone is not really measuring composition — it is re-labelling the number it already had. A genuinely independent reading has to come from something weight does not determine: circumferences, skinfolds, or an electrical signal through the body.
Four ways to be called obese, and four ways not to be
The eight bodies below are computed, not surveyed. Each one has a height and a weight, which fixes BMI, and a set of circumferences, which the Navy equation turns into a body fat percentage. The Navy equation never sees body weight, so the two verdicts are genuinely independent — unlike a BMI-derived estimate, where the two answers are algebraically tied together.
The first row is the case that makes people distrust BMI: 98 kg at 180 cm, BMI 30.2, and a waist-to-neck relationship that puts body fat at 19.9% — under the 25% line. The second row is the opposite and arguably the more dangerous one: BMI 27.4, technically only overweight, with body fat at 26.7%. A scale that only knows BMI never flags that body at all.
How many kilograms the word costs
Because BMI divides by height squared, the kilograms behind one BMI point change with height. At 155 cm a point is 2.40 kg; at 190 cm it is 3.61 kg. That is why the same weight loss moves the scale's word further for a shorter person.
And here is the part that makes single readings unreliable. One kilogram of body weight is worth this much BMI, depending on height:
Two kilograms of water is worth 0.58 to 0.83 BMI points depending on height. Anyone sitting within about one BMI point of a class boundary — roughly 2.4 to 3.6 kg — will cross it and cross back on ordinary daily fluctuation. That is not a change in the body; it is a change in the reading.
Where this page's arithmetic stops being true
Everything here is arithmetic on two published equations, and it inherits their limits.
- When you choose the BMI-derived estimate, the two verdicts are not independent — the body fat number is computed from the BMI you just entered. The cross-classification only means something when body fat comes from an independent measurement.
- The Navy equation is a prediction equation developed on a specific population. It can be several points off for an individual body, and this page does not claim otherwise.
- The 25% and 32% body-fat lines are chart categories, not clinical diagnostic criteria. No organisation uses them to diagnose anything.
- The age drift of 0.1917 BMI points per year is a property of the Deurenberg equation, not a claim that what counts as obesity changes with age. It is an artefact of the model run honestly, and it is worth knowing that the model produces it.
- Fat mass index has no agreed cut-off anywhere. It is shown here because it separates fat load from height cleanly, not because anyone has defined where it becomes dangerous.
Questions this page gets asked
My scale says obese but I look lean. Is it broken?
Probably not. A scale that reports a category is almost always applying BMI, which cannot distinguish muscle from fat. The check is independent: measure waist and neck and run the Navy equation, or compare against body fat percentage charts. If both scales disagree strongly, the BMI one is the weaker claim.
What BMI counts as obese?
The World Health Organization adult categories put obesity at 30.0 and above, split into class I from 30.0 to 34.9, class II from 35.0 to 39.9 and class III at 40.0 and above. Overweight runs from 25.0 to 29.9 and normal from 18.5 to 24.9.
What body fat percentage counts as obese?
On the ACE chart as commonly reproduced, the obesity category starts at 25% for men and 32% for women. Those are chart bands, not diagnostic thresholds — no clinical body uses them on their own.
How much weight do I need to lose to stop being classed as obese?
Whatever it takes to bring BMI under 30.0, which the calculator above works out for your height. At 170 cm that is 86.6 kg; at 180 cm it is 97.0 kg. Anything within about three kilograms of the line is inside normal daily water swing, so treat the boundary as a zone rather than a point.
Is BMI or body fat the better number?
They measure different things and neither is a health verdict. BMI is cheap, repeatable and blind to composition. Body fat is closer to the thing people mean by obese, but harder to measure well. Fat mass index is the cleanest of the three for comparing bodies of different heights, and it has no agreed cut-off — which is both its weakness and its honesty.
Why does the answer change when I change my age?
Because the body-fat estimate carries an age term of 0.23 percentage points per year. Weight and height do not change, so BMI stays put, but the body fat implied by that BMI moves — and with it the BMI at which body fat would cross the obesity line. That drift is 1.92 BMI points per decade.
Related tools
body fat calculator · BMI calculator · body fat index vs BMI · FFMI calculator · FFMI vs BMI · Navy body fat calculator · BMI and smart scales · measure body fat · weight loss percentage · body recomposition · waist-to-height zones · charts by age and sex
Not medical advice. Every number on this page is arithmetic performed on published equations and published category boundaries, not a diagnosis and not a substitute for a clinical assessment. See our disclaimer.