Am I Underweight

The end of the scale nobody writes about

Go looking for advice about weight and you will drown in it. Diets, deficits, before-and-afters, an entire industry pointed in one direction. Almost all of it assumes you want less of yourself.

Then there is this end of the scale, where the advice thins out to almost nothing, and where what does exist tends to arrive wrapped in a compliment. Underweight is treated as the nice problem. The one people say they wish they had. If you have ever tried to explain to someone that you are struggling to gain weight, you will know exactly how that conversation goes, and how quickly it stops being a conversation.

So let us take it seriously, because it deserves to be taken seriously and because the assumption underneath the compliment is wrong. Being underweight is not the safe side of the scale. It carries real risks, some of them substantial, and it is far more likely than being overweight to be a signal of something else going on that needs looking at.

What the tool did, and what it left out

The arithmetic is BMI: your weight in kilograms divided by your height in metres squared, checked against one line.

Take someone 5 feet 7 inches tall weighing 50 kg. That height is 67 inches, which is about 1.70 m, so 1.70 squared is roughly 2.90:

BMI = 50 ÷ 2.90
BMI ≈ 17.3

Below 18.5, so the tool says underweight. Above it, and the tool says not underweight. That is the entire logic: one line, two answers.

Two honest notes about our own tool while we are here.

First, that binary is coarser than it should be. Someone at 18.6 is told only that they are "not underweight", with no hint that they are a hair above the line and drifting in the wrong direction, which is information they could use. And someone at 14 gets exactly the same word as someone at 18.4, which is not remotely the same situation.

Second, it does not tell you what a healthier weight would look like, which is the obvious next question and one we should be answering on the same screen. We have flagged both. In the meantime, our Healthy Weight Calculator will give you the range for your height, and for the example above that runs from roughly 54 kg to roughly 72 kg. So our 50 kg person is about 3.6 kg below the bottom of the range, which is a far more useful thing to know than the word underweight.

One more limit: this is for adults. Children and teenagers are assessed against growth charts for their age and sex, and a BMI of 17 means something entirely different at fourteen than at forty. Use our Child BMI Calculator for anyone under 18.

Why being underweight is not the good problem to have

Here is what sits behind that line, and why the compliment is misplaced.

Your body needs a certain amount of tissue and a certain amount of stored energy to run properly, and when it does not have them it starts making economies. The economies are not random and they are not trivial.

• Bone. Low body weight is a well-established risk factor for reduced bone density, and bone you fail to build in your twenties is not straightforwardly recoverable later. This is the one that quietly matters most, because it does not announce itself until something breaks decades on.
• Immune function. Mounting an immune response is metabolically expensive. Bodies short of resources do it less well, which shows up as infections that arrive more easily and linger longer.
• Hormones and fertility. Reproduction is the first thing a body under strain will economise on, since from its point of view this is a poor moment to start. In women this commonly shows up as periods becoming irregular or stopping; in men, as reduced testosterone. Both are the body making a sensible decision about a situation you would rather it were not in.
• Reserve. This is the one people never think about. Being ill costs energy, and having some in the bank matters. Someone with reserves who catches something nasty has a buffer. Someone without one does not. This is why low body weight is a genuine concern in older adults, where the phrase "he has nothing to fall back on" is not a figure of speech.
• Muscle. If your weight is low because you are not eating enough, some of what is missing is muscle, and muscle is strength, stability and, later in life, independence.

None of this means a naturally slim person in good health is in danger. Plenty of people sit near the bottom of the range, eat well, are strong, and are entirely fine. The risks above attach to being genuinely under-resourced, not to having a small frame. But they are the reason the line exists, and the reason a yes from this tool is worth taking as seriously as a yes from the other one.

The question the tool cannot ask: why?

This is the most important section on the page, so we will be direct.

A BMI below 18.5 is not really the finding. The finding is whatever put it there, and the possibilities are not equivalent.

If you have always been this size, eat well, have energy, and your weight is simply where it has always sat, this is probably your build. Worth a look at your diet and some resistance training, but not an alarm.

If your weight fell and you did not intend it to, that is a different matter entirely, and it is the one that matters most. Unintentional weight loss is one of the more meaningful signals in medicine, precisely because it is non-specific: overactive thyroid, coeliac disease, inflammatory bowel disease, undiagnosed diabetes, chronic infection, depression, and less common but more serious things all announce themselves this way. None of them is fixed by eating more, and all of them are better found early. If your weight has dropped without you trying, please take that to a doctor rather than to a calculator. That is the single most useful sentence on this page.

And if the number below the line is one you have been working toward, or if reading it produced relief rather than concern, or if eating is something you find you have to manage rather than simply do, then the honest thing to say is that this is worth talking to someone about. Not because there is anything wrong with you, but because that particular relationship with a number is a heavy thing to carry alone, and it responds well to help. A doctor is a reasonable place to start, and so is anybody you trust.

If you are trying to fix it

Assuming the cause is straightforward and you simply want to be bigger, some things that are actually true rather than merely encouraging.

It is genuinely hard, and not because you lack discipline. There is a well-known study in which volunteers were deliberately overfed by 1,000 calories a day for eight weeks, and the fat they gained varied by a factor of ten between individuals. The people who barely gained were not cheating. Their bodies had quietly ramped up involuntary movement, fidgeting and shifting about, and burned the surplus off without consulting them. If you have been told you must be eating less than you think, that study is your defence. Our Weight Gain Calculator covers it properly.

Eat on a schedule, not by appetite. Appetite is precisely the thing that is regulating against you. Waiting until you are hungry is waiting for a signal that is not coming often enough.

Volume is the enemy, so use density. Nuts, nut butters, oats, olive oil, whole milk, dried fruit, cheese. Getting the calories in is a logistics problem before it is a willpower problem.

Drink some of them. Liquids do not fill you up the way solids do, which is usually a problem and here is an advantage.

Lift weights. This decides what the extra becomes. Without resistance training a surplus has one destination, and it is not the one you wanted. With it, you are building something worth having.

And be patient with the timeline. Our Calorie Surplus Calculator will show you what a sensible surplus looks like, and it is smaller and slower than you would like.

Questions people ask

What BMI counts as underweight?

Below 18.5 for adults. Between 18.5 and 24.9 is the healthy range.

I have always been slim and I feel fine. Is that a problem?

Probably not. If your weight has been stable for years, you eat well and you have energy, a low BMI may just be your build. The risks attach to being under-resourced, not to having a small frame. It is still worth some resistance training.

Is being underweight worse than being overweight?

They are different risks rather than ranked ones, and it depends heavily on why you are there and by how much. The honest point is that underweight is not the safe end, which is how it is usually treated.

How fast should I gain?

Slowly. Around a quarter to half a kilogram a week is sensible, and faster mostly arrives as fat. Our Calorie Surplus Calculator explains why a bigger surplus does not build more muscle.

When should I see a doctor?

If your weight dropped without you meaning it to, if you are losing weight now, if you are tired or unwell alongside it, or if eating is something you struggle with. Unintentional weight loss in particular is worth investigating rather than feeding.

References

Where the figures come from. The adult BMI categories, including the underweight threshold of 18.5, are the standard classification published by the CDC, following the World Health Organization. The overfeeding study showing a ten-fold variation in fat gain, and the role of involuntary movement in resisting it, is Levine, Eberhardt and Jensen's work at the Mayo Clinic, published in Science.

1. Centers for Disease Control and Prevention. About Body Mass Index (BMI). https://www.cdc.gov/bmi/about/
2. Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999;283(5399):212-214. https://pubmed.ncbi.nlm.nih.gov/9880251/
3. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet. 2004;363(9403):157-163. https://doi.org/10.1016/S0140-6736(03)15268-3