A 31 inch waist is well below the average waist size for American men and women. But does this necessarily mean that having a 31 inch waistline is healthy?
Health isn't measured in inches, but the size of your waist can still be a good indicator of your risk for various chronic diseases. With that in mind, a 31″ waist isn't excessively large for most people, but it could be an indication that you're not at your optimal weight and body fat level.
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Is a 31 inch waist big for a woman?
Is a 31 inch waist big for a woman or not? No, a 31 inch waist is not big for a woman. However, it's also not small for a female. Indeed, according to some research, 31 inches is the cut-off point for women to be considered low risk for health problems like type 2 diabetes and heart disease. 
The one exception to this is if 31 inches when doubled exceeds your height in inches. This would mean that if you have a 31 in waist and are 5′1″ or shorter, then your waist is too big. The simple reason for this is that scientists often recommend keeping your waist size to less than half of your height to avoid health problems. 
Of course, it's possible to have a 31 inch waist size and be unhealthy because the size of your waist is simply an indicator of your health status rather than a guarantee of it. So you can't live a sedentary lifestyle and devour junk food while expecting to remain healthy—even if your waistline isn't excessively large.
Is a 31 inch waist small for a man?
Is a 31 inch waist small for a man or not? Yes, a 31 inch waist is definitely small for males of almost any height and build. This is especially true when you consider that the normal waist size for a man is almost 10 inches bigger than our 31 inch example.
Just how small is a 31 inch waistline?
To put things into perspective, a 31″ waist is a normal size for a 13 to 14-year-old boy. So you definitely have a slim stomach if your waist measures 31 inches in circumference.
Indeed, it's quite rare these days for anyone other than young men or men with very small builds to have a 31 inch stomach. But if you have a naturally slim waist or a low body fat level, then it's definitely possible to achieve a 31 in waist (or a very similar size) even if you're not that young anymore.
What does a 31 inch waist look like?
What does a 31 inch waist look like exactly? The appearance of a 31 inch waist primarily depends on your body fat level, height, and gender.
Let me explain.
If two people of the same height, build, and gender both have a 31 in waist, the person with the lower body fat level and better muscle tone will look like they have a smaller waist even though it's the exact same size as the other person's.
Similarly, if a person with a long torso and a person with a short torso both have a 31 inch waistline, the person with the longer torso will look like they have the smaller waist at a glance. This is because when you're tall or have a long torso, a given waist size makes up a smaller portion of the overall surface area of your torso and body.
Also, since men tend to have more muscular and defined abs than women, it's likely—but not a guarantee—that given an equivalent waist measurement, the man's waist will appear slimmer and more toned than the woman's. This is especially true if the man is taller than the woman (at an equivalent height, the differences will be less drastic).
How large is a 31.5 inch waist?
A 31.5 inch waist isn't large for a man or a woman unless the person has a really small build (for example, if you're less than 5 feet tall).
As a woman, you should factor in your hip size when assessing how your waist looks because some research shows that having a waist-to-hip ratio (WHR) that's close to 0.7 can make you look more attractive. 
In other words, you don't always need to lose belly fat if you want to look great; building your glutes via strength training can make your body more curvaceous and attractive (you can, of course, still look great with a higher WHR).
For a man, a 31.5 inch waist is definitely on the small side and is an indicator—not an absolute guarantee—that your body is in good health.
Are there any celebrities with a 31 inch waist?
Celebrities play a major part in today's culture. But not everything that you read about them is true.
If you want to amuse yourself, then do an internet search for "31 inch waist celebrities". You'll find a big list of celebs who supposedly have a 31 inch waist. Yet, these results are inaccurate because the list is autogenerated by a computer.
In other words, the data wasn't created by a human and isn't accurate. Therefore, you shouldn't trust it.
While there are bound to be some actors and media personalities with a 31 in waist, the majority of celebrities have bigger waistlines and much more normal bodes than you might think.
The point is that it's no use speculating about who might have a 31″ waist. After all, much of the "celebrity measurement" data that you see online—outside of fitness celebrities—is just someone's best guess.
Conclusion: Is a 31 inch waist good or not?
It is often said that your health isn't measured by inches on the tape or numbers on the scale. This is because it's possible to have a healthy BMI and waist size and still have health problems due to other factors.
Yet, the size of your stomach has been shown to be a reliable indicator of your health (more so than BMI in some cases).
For a man, a 31 inch waist is smaller than average and a strong sign that your weight and proportions are close to optimal.
For a woman, a 31 inch waist isn't big, but unless you're tall, it isn't tiny either. But unless your waist size, when doubled, exceeds your height, it's unlikely that your 31″ waist is a cause for concern.
- BBC News. (2012, June 25). More than half of British women’s waists ‘too big’. https://www.bbc.co.uk/news/health-18570445
- Mendes, E. L., Andaki, A. C. R., Brito, C. J., Guedes, J. M., Santos, M. P. M., & Mota, J. (2016). Waist circumference to height ratio predicts inflammatory risk in children. Annals of Human Biology, 44(4), 303–308. https://doi.org/10.1080/03014460.2016.1253771
- del Zotto, M., Framorando, D., & Pegna, A. J. (2018). Waist‐to‐hip ratio affects female body attractiveness and modulates early brain responses. European Journal of Neuroscience, 52(11), 4490–4498. https://doi.org/10.1111/ejn.14209