If you have 18 inch thighs, then your legs are approximately 3-4 inches smaller than the average thigh size. This is for both men and women.
Typically, the only adults in the general population who have 18 inch legs are elderly men and women. This is because you usually lose muscle mass when you age, which is a process that's known as sarcopenia.
Of course, those with eating disorders may have much thinner thighs than usual. But you may also have 18″ thighs if you have a naturally slim build and a top-heavy fat storage distribution.
How Do Your Thighs Compare?
What do 18 inch thighs look like?
18 inch thighs look small and skinny on just about any kind of physique. Of course, the taller you are, the thinner that your 18 inch thighs will look. This is because your leg mass is spread over a larger surface area when your thigh bones are long.
This isn't to say that 18 in thighs will look bulky on people with short legs because they most certainly won't. It's just to say that tall people tend to have thinner-looking legs even if they have an equivalent amount of thigh muscle and fat to a shorter person.
If you're a man with 18 inch quads, then your legs will look very straight, and you won't have much of what bodybuilders would call a quad sweep, which is another way of saying that your thighs won't have any real muscle bulk to them.
Whether or not this look is desirable depends on your goal physique. Having 18 inch thighs only becomes dangerous when it's indicative of you being underweight, which is when your BMI is below 18.5. 
How big is an 18 inch thigh circumference for women?
Despite what you may see on social media or read in magazines, most women don't have an 18 inch thigh circumference. The average American female actually has around 22 inch thighs.
As such, 18 inch and 18.5 inch thighs aren't very big at all for a woman, especially a woman under 50.
If you're a senior lady, on the other hand, then it's normal to have only 18″ thighs because of age-related muscle loss, something that you can mitigate to a large extent by performing resistance training. 
While it's true that some women naturally have thinner thighs than others without being underweight, having 18 inch legs could also be a sign that you have a low BMI.
How about for men?
Every adolescent male has 18 inch thighs at some point during his physical development. However, for an adult male, 18 inch quads are definitely on the small side.
If you have 18 inch legs as a man, then it could be a sign that you're not consuming enough calories or getting sufficient amounts of protein in your diet. If this is the case for you, then try to get a good serving of protein (eggs, dairy, meat, fish, nuts, beans) with every meal that you consume.
On the other hand, there are some men who would be expected to have 18 inch thighs. For example, if you have a really small build, let's say you're 5'3″, then it's conceivable that you'd have 18″ thighs.
Additionally, elderly men, especially those aged 80 and older, tend to have much less thigh mass than younger and middle-aged men.
How can you build up your 18 inch thighs?
Lower body resistance training is the best way to boost your leg strength and increase the size of your thighs. Bodyweight squats are a great place to start, but if you want to gain considerable amounts of muscle mass, then you can always lift leg weights at a gym.
Both men and women can benefit from strengthening their legs with weights because resistance training increases your bone density and resting metabolic rate while simultaneously reducing your fat mass. 
If you don't want to make your thighs too big (which is hard to do, believe me), then you can stick to light weights and/or bodyweight movements.
Just make sure to eat enough protein to make the best use of your resistance training efforts.
Healthy fats are also important because they regulate your hormone production, which in turn gives you more energy. 
Many people report improved body confidence once they start lifting weights, so don't worry about putting on some muscle mass (and even a bit of fat) because you might be surprised at just how fit and healthy it makes you look!
Conclusion: Is it bad to only have 18 inch legs?
It depends. If you have 18 inch thighs and you're underweight, then the size of your legs is obviously indicative of a low BMI, which is definitely bad.
On the other hand, if you have a healthy body weight but just so happen to have thin thighs, then that's nothing that you should be too concerned about.
You can always add muscle to your legs using the tips above if you so desire.
It's important to note as well that just because your legs are bigger than 18 inches, that doesn't mean that they're too big. It's much better for your health, for example, to store fat on your thighs than it is to store it on your stomach.
- Underweight | Office on Women’s Health. (2019, March 27). Office on Women’s Health. https://www.womenshealth.gov/healthy-weight/underweight
- Porter, M. M. (2001). The Effects of Strength Training on Sarcopenia. Canadian Journal of Applied Physiology, 26(1), 123–141. https://doi.org/10.1139/h01-009
- Westcott, W. L. (2012). Resistance Training is Medicine. Current Sports Medicine Reports, 11(4), 209–216. https://doi.org/10.1249/jsr.0b013e31825dabb8
- Herrera, K. (2020, July 22). How Dietary Fat Benefits Hormones - Women's International Pharmacy. Women’s International Pharmacy. https://www.womensinternational.com/blog/dietary-fat-benefits-hormones/