Obesity Classes by BMI — Class 1, 2 and 3

Obesity is split into three classes by BMI: class 1 from 30.0 to 34.9, class 2 from 35.0 to 39.9, and class 3 at 40 and above. Those exact cutoffs come from Table IV-1 of the NHLBI clinical guidelines, which labels the third one extreme obesity, class III. Calculate your number and read on for what the split is for.

The table as the NHLBI published it

BMI (kg/m²)	Category
Below 18.5	Underweight
18.5 – 24.9	Normal
25.0 – 29.9	Overweight
30.0 – 34.9	Obesity, class I
35.0 – 39.9	Obesity, class II
40 and above	Extreme obesity, class III

Source: Table IV-1 of the Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults (NHLBI, 1998), itself adapted from a 1997 WHO consultation report. Current NIDDK materials use severe obesity for the 40-and-above band; morbid obesity is the wording that has fallen out of use, not a different range.

What those lines look like on a scale

Classes are easier to judge in pounds. At 5 ft 8 in (1.73 m), with BMI = 703 × lb ÷ 68²:

• class 1 starts at about 198 lb (90 kg)
• class 2 starts at about 231 lb (105 kg)
• class 3 starts at about 264 lb (120 kg)

So someone 225 lb at that height (102 kg) reads 34.2 — class 1, six pounds short of class 2. At 250 lb (113 kg) the same person reads 38.0, and at 270 lb (122 kg), 41.0. Every line moves up or down with height, which is the whole point of using an index instead of a weight.

Why anyone splits obesity into three parts

The classes exist because a single label covering everything from 30 to 55 is not useful to anyone. The NIDDK reports that 42.4% of US adults have obesity while 9.2% have severe obesity, so the category is enormous and internally very different. Sorting it lets population statistics track the tail separately, and lets clinical protocols key on a number.

They also track risk in aggregate. The NIDDK lists type 2 diabetes, high blood pressure, heart disease, stroke and sleep apnea among the conditions tied to excess weight, and the association strengthens as BMI rises. Aggregate is the operative word: it describes groups.

A class boundary is not a physiological boundary

Nothing happens to a body at 34.9 that fails to happen at 35.0. The numbers are round because humans chose round numbers, and the cutoffs were drawn to make counting and comparison possible. Two people at 32 can carry entirely different risks depending on where the fat sits, what their labs say, and what they were doing five years ago.

That is why a class is a starting point for a clinical conversation rather than a verdict. See what the band below looks like at overweight BMI, how the index connects to the heart at BMI and heart disease risk, and run the number in kilograms with the metric calculator.

This content is informational and does not replace evaluation by a health professional.