General

Where would you say the lines are between the categories “thin,” “normal,” “chubby,” and “fat?” I consider numbers (such as BMI or weight in pounds) to be poor markers since between two people with the same weight, one could be fat and the other not depending on things like height, muscle mass, etc, so I think it would have to be judged by a set of qualitative criteria, but I don’t know what they should be.

The main reason I ask this is, of course, because I want to know when I cross into chubby territory (or if I already have!) so that I can celebrate appropriately!

A wonderful classification of body sizes. I had made one by myself but it isn't as detailed as yours.

It's really subjective I think but I'll just share my thoughts.

thin - When their thighs are really small. I look at thigh girth as probably the biggest sign someone is thin. Seeing their thighs as small as their knee kinda makes me double-take at how thin someone is.

normal - I'd use the term "average" before normal. Generally the way I consider someone average is if I didn't pay any mind to their size.

chubby - If you're softer, but can still hide it. Someone who is chubby can look fat, but they have the ability to still hide it with looser tops, layers, etc. I consider myself chubby.

fat - You're fat when you're chubby, but can't hide it anymore. Whether it's layered clothes or loose tops, if your tricks are no longer hiding your rolls, you're fat.

My classification is based on BMI. Yes, I know BMI is not very accurate but it's easier to calculate than fat percentage.

I Skinny/clinically undeweight

BMI < 18
What most people would considerate "too thin".

II Thin / slim

BMI 18 to 21
What media defines as a "perfect" body. Female celebrities would face criticism if they are not within this range

III Average

BMI 22 to 24
The most common weight range. Women at this stage are often labelled as "curvy" if they had a low wais/hip ratio

IV Plump/thick

BMI 25 to 30
Clinically defined as overweight even though they often have no health problems. Recently has become acceptable (but not desiderable) due to 'body positivity' Most plus-size models fit here.

V Chubby/ fatty

BMI 30 to 39

Clinically defined as obese (grade I and II) People would call anybody at this weight as "fat' but I won't unless they have a big belly.

VI Trully fat/BBW

BMI 40 to 60

Fat. My favourite type

VII Super fat/ SSBBW

BMI over 60
Very fat. Very rare outside the U.S. Major health problems

IX USSBBW

BMI usually over 90

Inmobile or near inmobile. Serious health problems.
Not my cup of tea

I am aware of that. I only use BMI as a reference. Two people of the same height and weight do have different bodies depending of many factors. In fact, many studies show that waist/height ratio is a better indicator of health and percentage of body fat.

Looking at the waist-to-height ratio it would be like:

Thin : WHR < 0,42
Average: 0,42 to 0,5
Plump: 0,5 to 0,55
Chubby: 0,56 to 0,6
Fat: 0,61 to 0,7
SSBBW: above 0,7

but these figures are based on the measurements of female celebrities and Plus-Size models who often have a hourglass/pear figure . For men and apple-shaped the limits would be higher.

This topic comes up again and again, from time to time, so let's present my usual definitions:

I wouldn't use weight alone, not even BMI, as people have different body proportions, fitness levels etc. regardless of weight.

I would use relative categories, mostly focused on their effects on daily life.

For 1 to 10 (I intentionally designed this system so that almost everyone fits into 1-6, ideal if you want to create fantasy scenarios with dice). I made this list with women in mind, but you can adjust them to men if you wanted

1: skin and bones, even those who prefer thin would call her too thin. Unhealthy and dangerous. Likely has an eating disorder.

2: thinner than average, but not clinically so.

3: most people would consider this to be normal, and strive to reach this weight if they differ from it.

4: chubby, considered above average weight by most people, but she can still hide the extra weight by carefully chosen clothing and camera angles.

5: definitely fat, and can no longer hide it. Still, the extra weight is not too limiting in most daily tasks and activities. If really attractive and has a great personality, might still be found desirable by those who aren't specifically into fatties (but they still see her weight as a major negative point)

6: obese, the weight starts to impose limits on many activities. People who aren't into fatties start feeling at least somewhat disgusted. Folds appear on different parts of the body regardless of posture, belly can firmly rest in the laps when sitting, some chairs become too tight, some difficulties when picking up things from the floor, basically the biggest size one can get without having either a severe eating disorder or being into feederism, and the biggest size most people will regularly see outside reality TV, or the Internet.

7. fatter than most things are designed for, problems with fitting into small cars, can't fit into some restaurant booths, and even the largest clothes in supersize clothing stores start becoming too small. I consider the border between 6 and 7 to be the border between BBW and SSBBW.

8. still being able to care for themselves, but only with great difficulties. Standing up, or going up a single flight of stairs, or walking more than a few dozen steps, or just standing more then a few minutes are all very exhausting. Basically the biggest size one can have while being able to live completely independently.

9. requires a caretaker for everyday life, but is not completely bedridden. Still, getting up is difficult and might require help.

10. immobility.