A BMI of 18 is classified as underweight. But is this necessarily a bad thing?
While an 18 BMI is unlikely to be ideal for most people, it’s certainly possible to still be healthy while having a low body mass index like 18.
After all, we all have different bone structures and body compositions, which can greatly affect our body weight and BMI.
See How Your BMI Compares:
- Healthy BMI for women
- Body mass index of 11
- Body mass index of 12
- Body mass index of 13
- Body mass index of 14
- Body mass index of 15
- Body mass index of 16
- Body mass index of 17
- Body mass index of 19
Is a BMI of 18 good for women?
A BMI of 18 is considered slightly underweight for women because it falls below the 18.5 BMI cut-off point. But what does this mean for your health?
Well, it depends on your body composition, something that BMI can’t measure.
If you have minimal muscle mass but quite a lot of body fat, then it’s likely that your score of BMI 18 is suboptimal.
This is because low levels of muscle mass, especially in extreme cases, can lead to a weakened immune system and a lack of functional strength, which is a particular problem among elderly individuals.
On the other hand, if you’re quite lean for a woman and feel that you’re stronger than average for a female, then a body mass index of 18 might be okay. It’s hard to say for certain because body mass index isn’t good for determining your health status.
Is a BMI of 18 okay for men?
While most men have a body mass index score that’s around 10 points higher than our 18 BMI case study (which isn’t a good thing), some naturally skinny males may have a BMI of 18.
Any man who has an 18 body mass index likely doesn’t have much muscle mass, something that can be problematic (and harder to correct) as a man gets older.
Some endurance athletes, who need to maintain a low body weight to excel in their sport, may also have a BMI of 18.
Again, since a BMI score doesn’t tell you about a person’s body composition, we can’t categorically say that BMI 18 is too low for a man.
Still, a low BMI is probably less than ideal because if it gets any lower, then you could run into health problems like malnutrition.
What does an 18 BMI look like?
Since body mass index doesn’t measure your body fat or muscle mass, it’s impossible to say what an 18 BMI looks like. In other words, a BMI of 18 will look different on different people.
Let me explain.
If a man and a woman both have a BMI of 18, then the man is likely (but not guaranteed) to have a leaner physique with more muscle mass and less body fat because men typically carry more lean mass than women.
On the other hand, someone with a BMI of eighteen could have relatively little muscle mass and quite a bit of body fat.
Common 18 BMI scores
When you use an online BMI calculator, you often get a precise BMI score with decimals. And while a few tenths of a BMI point don’t count for that much, we wanted to discuss who is likely to have each specific BMI score.
A BMI of 18.1 is in the underweight category. If you have an 18.1 BMI, then you probably have relatively little body fat and muscle mass and potentially also a slim build.
If you have a BMI of 18.2, then you’re classed as underweight. Individuals with an 18.2 BMI often lack muscle mass and strength and may also have minimal body fat.
A BMI of 18.3 is still considered underweight and may be an indication that an individual is consuming insufficient calories and isn’t getting the nutrients that their body needs.
An 18.4 BMI is ever so slightly in the underweight category for women and men. With a BMI of 18.4, a person will look much slimmer than average but, depending on their build and body composition, perhaps not extremely skinny.
An 18.5 BMI is the established cut-off point for the underweight category. So if you have a BMI of 18.5, then you’re on the cusp of being underweight and should try to gain a small amount of weight safely.
A BMI of 18.6 is technically in the normal category, although it is also very close to the underweight cut-off point of 18.5. You might have an 18.6 BMI if you don’t have much muscle mass, which can be a problem if you don’t get enough protein in your diet.
An 18.7 BMI is indicative of a healthy body weight. Obviously, a BMI of 18.7 is only a tiny bit bigger than a BMI of 18.4, which is considered underweight. This is why you shouldn’t rely on body mass index as a diagnostic tool. 
If you have a BMI of 18.8, then you have a healthy body weight. Although this may well be the case, many individuals with an 18.8 BMI could benefit from increasing their muscle strength.
A BMI of 18.9 is the final body mass index score in the 18 BMI category and is nearly half a BMI point into the healthy weight classification.
The verdict on having a body mass index of 18
People who are naturally skinny due to their genetics—bone structure and body composition—may have a BMI of 18.
As a general rule of thumb, when it comes to low BMI scores, those who have more muscle mass and less body fat are typically healthier than those with minimal muscle mass but a fair amount of body fat.
- Weir, C. B., & Jan, A. (2022). BMI Classification Percentile And Cut Off Points. In StatPearls. StatPearls Publishing.
- Nuttall F. Q. (2015). Body Mass Index: Obesity, BMI, and Health: A Critical Review. Nutrition today, 50(3), 117–128. https://doi.org/10.1097/NT.0000000000000092