A 12 inch neck is definitely below the average neck size for women and men. But is that necessarily a bad thing?
It depends. If your 12 inch neck is indicative of you being underweight, for example, then having a 12" neck obviously isn't ideal. On the flip side, if you're measuring your child's neck and it's 12 inches, then, depending on their age, that could also be an indicator that they're possibly underweight or overweight.
This guide discusses the pros and cons of having a 12 in neck and then explains why strengthening your neck with specific exercises might be a good idea for some people.
Related Neck Guides:
Is a 12 inch neck big or not?
No, in general, a 12 inch neck is not big for an adult man or woman. On the other hand, a 12 inch neck might be large for a child and an indicator that they're overweight. But for a teenager or an adult, no, a 12" neck definitely isn't big and is, in fact, on the small side for both males and females.
For men, a 12 inch neck is around 3 inches smaller than average. For women, it's around an inch smaller than normal.
Based on some North American data, a 12 inch neck is normal for teenagers aged 11-14. However, depending on the build and body fat level of the teen, their neck may be larger or smaller than this.
Why do some people only have a 12 in neck?
It's quite rare for an adult man to have a 12 inch neck unless he's naturally very skinny or is losing muscle due to old age.
On the other hand, a 12 inch neck or a 12.5 inch neck is fairly normal for women who have more petite builds and less body fat. Sure, it's a bit smaller than average, but some women just have genetically slim necks.
As you can see, the size of your neck can tell you quite a lot about yourself. This is one reason why neck circumference has often been touted as a potential replacement for BMI (it's certainly a more convenient measurement to take). 
Of course, having a 12" neck is no guarantee of anything. If you're a man, for example, then you might have a perfectly normal body weight but a much smaller neck than average.
Should you increase the size of your 12" neck?
There are a few reasons why you might want to increase the size of your neck. Let's start with the most practical reason: Neck strength.
Research shows that regular neck strengthening exercises can improve the quality of life in people with neck pain.  Extrapolating from this data, I think it's plausible to say that strengthening your neck muscles makes your neck more resistant to damage and injuries.
So, if you were unfortunate enough to be in a car accident, having a strong neck could prevent you from getting a concussion. This is one reason why contact sports players often do neck strengthening exercises—it reduces their chance of getting injured when they take a blow to the neck/head.
On the other side of the coin, thickening up your neck could make you look more masculine, which might be beneficial for your self-confidence if you're a man who's worried about being too skinny.
Indeed, many men lift weights for their main muscle groups and then forget all about the neck. So if you want a complete physique, then it's likely worth doing some neck-specific exercises, even if it's just with the weight of your head.
The verdict: Is it bad to only have a 12 inch neck?
Having a 12 inch neck or a 12.5 inch neck is only bad if it's indicative of you being unhealthy in some way ( such as being underweight).
From an aesthetic standpoint, you could definitely argue that a 12" neck is very skinny for a man and that he would look better if he thickened up his neck.
But on the flip side, you definitely shouldn't derive your self-worth from the size of your neck, or from any other muscle group/body part, for that matter.
- Park, M. (2010, July 6). Can neck measure indicate body fat better than BMI? - CNN.com. CNN. https://edition.cnn.com/2010/HEALTH/07/06/bmi.neck.fitness/index.html
- Salo, P. K., Häkkinen, A. H., Kautiainen, H., & Ylinen, J. J. (2010). Effect of neck strength training on health-related quality of life in females with chronic neck pain: a randomized controlled 1-year follow-up study. Health and Quality of Life Outcomes, 8(1), 48. https://doi.org/10.1186/1477-7525-8-48