BMI at 165 cm and 50 kg — 18.4 (Underweight)

At 165 cm (5 ft 5 in) and 50 kg (110 lb), BMI is 18.4, which falls in the Underweight category under the World Health Organization (WHO) classification. The calculation is shown in full below, together with the healthy weight range for this height — 50.4–67.8 kg — and a table of BMI values across body weights at 165 cm so you can see exactly where this reading sits.

BMI is a screening tool: weight in kilograms divided by height in metres squared. It is useful because it is simple and standardised, and limited for exactly the same reason — it sees only height and weight, so it cannot tell muscle from fat and it says nothing about where fat is carried. Treat 18.4 as one signal among several, alongside waist circumference, body-fat estimates and the rest of your health picture.

Calculate your own BMI

Run your own numbers with the BMI calculator — enter your height and weight and it gives your BMI and WHO category immediately. If you want to know how much of your weight is fat rather than muscle, the body-fat calculator estimates body-fat percentage from waist, neck and hip measurements.

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The calculation, step by step

BMI = weight (kg) ÷ height (m)². The World Health Organization classifies the result as underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25 to 29.9, and obese at 30 and above, with obesity further split into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and above) (WHO BMI classification).

For this page:

  • Height: 165 cm = 1.65 m
  • Height squared: 1.65² = 2.7225
  • Weight: 50 kg (110 lb)
  • BMI = 50 ÷ 2.7225 = 18.4

That puts this reading in the Underweight category. The healthy weight range for a height of 165 cm is 50.4–67.8 kg (18.5–24.9 × 2.7225), and the obese threshold of BMI 30 is reached at 81.7 kg. This reading is 0.4 kg below the bottom of the healthy range for this height. Gaining that much — slowly, with food and strength training rather than in a rush — would bring BMI to 18.5, the lower edge of healthy.

BMI at 165 cm across body weights

Weight BMI WHO category
50 kg 18.4 Underweight
60 kg 22.0 Healthy Weight
70 kg 25.7 Overweight
80 kg 29.4 Overweight
90 kg 33.1 Obese
100 kg 36.7 Obese
110 kg 40.4 Obese
120 kg 44.1 Obese
130 kg 47.8 Obese
140 kg 51.4 Obese

The table shows how sensitive BMI is to weight at this height: each 10 kg changes BMI by about 3.7 points, and each single kilogram changes it by about 0.37 points. That is why small, sustained changes matter — losing 5 kg moves BMI by roughly 1.8 points, which is often enough to move from one category to the next, and why a slow drift of a kilogram a year adds up to a full category change over a decade. It also shows why height matters so much: the same 50 kg at a taller height would produce a lower BMI, and at a shorter height a higher one, because height is squared in the formula.

What the number means in energy terms

BMI tells you where a weight sits, not how many calories a body uses. As an illustration, the Mifflin-St Jeor equation (Mifflin et al., American Journal of Clinical Nutrition, 1990) estimates basal metabolic rate — the energy used at complete rest — from sex, weight, height and age. For a 40-year-old at 165 cm and 50 kg it gives about 1336 kcal a day for a man and about 1170 kcal a day for a woman. Multiply by 1.2 for a sedentary lifestyle and maintenance comes to roughly 1604 kcal a day (men) and 1404 kcal a day (women). These are estimates for illustration, not personal targets: real energy needs vary with muscle mass, activity, health and medication, and any calorie target should stay above the safety floors of 1,200 kcal a day for women and 1,500 kcal a day for men unless a clinician supervises a lower intake.

That energy context matters because BMI categories are reached through energy balance over time, not through any single meal. A sustained deficit of 300–500 kcal a day below maintenance typically produces weight loss of 0.5–1% of body weight a week; a sustained surplus of 200–300 kcal a day typically produces gradual gain. Either way, the rate matters more than the speed: faster changes are harder to sustain and more likely to cost muscle along with fat, or to overshoot into the next category.

What a BMI of 18.4 means

A BMI of 18.4 sits below the healthy range, which means that at 165 cm this weight is lower than the range associated with the lowest population-level health risk. That does not automatically mean someone is unwell — naturally slight builds, endurance athletes with very low body fat, and people recovering from illness can all land here — but it is a prompt to look at the wider picture: energy levels, strength, immune health, menstrual regularity where relevant, and whether weight has been falling without trying.

If weight has dropped without trying, or if appetite, digestion or mood have changed, that deserves a conversation with a clinician sooner rather than later, because unintentional weight loss can have causes that a BMI number alone cannot identify. If the goal is to gain weight, the healthier route is a modest calorie surplus — roughly 200–300 kcal a day above maintenance — built on energy-dense but nutritious foods (nuts, olive oil, whole grains, dairy or fortified alternatives, oily fish) plus resistance training two to four times a week, so the weight gained is more muscle than fat. Protein of roughly 1.6–2.2 g per kilogram of body weight a day supports that goal; at 50 kg that is about 80–110 g a day. Never chase rapid weight gain with very large surpluses or with ultra-processed food alone, and never use appetite suppressants or laxatives to manage weight.

How to use this result

Start by checking the inputs. Height is often misremembered — measure it in the morning, standing straight against a wall, without shoes — and weigh yourself in the morning, after using the bathroom, in light clothing, on a flat hard surface. A 2 cm error in height changes BMI by roughly 0.4 points at this weight, so sloppy inputs can move the category. Re-measure monthly rather than daily: daily weight fluctuates by 1–2 kg with food, water and glycogen, and that noise hides the trend.

Then add the two measurements BMI cannot give you. First, waist circumference: measure at the level of the navel after breathing out. A waist above 94 cm for men or 80 cm for women signals increased health risk even when BMI is in the healthy range, and a waist above 102 cm (men) or 88 cm (women) signals substantially increased risk. Second, a body-fat estimate — the body-fat calculator uses the U.S. Navy method (Hodgdon & Beckett, 1984), which is accurate to roughly ±3–4 percentage points. A person with a BMI of 18.4 who lifts weights and has a 34-inch waist is in a very different position from someone with the same BMI and a 44-inch waist, and only the waist and body-fat numbers reveal that.

If you decide to act on the number, pick one measurable change and track it for four weeks before judging it: a daily step target, two strength sessions a week, a protein target at each meal, or a consistent meal pattern. At 165 cm, a 5 kg change moves BMI by about 1.8 points, so progress shows up in the number fairly quickly — but the habits are the goal, and the BMI is only the scoreboard. If you have a medical condition, take medication that affects weight, are pregnant, or are under 18, do not use this page's figures as personal advice: BMI cut-offs for children and teenagers use age-specific charts, and pregnancy changes the interpretation entirely.

What BMI does NOT tell you

BMI cannot tell muscle from fat. A rugby player at 165 cm and 50 kg would score 18.4 and land in the same category, yet carry far less fat than a sedentary person at the same height and weight. It cannot tell where fat sits — fat around the waist carries more risk than fat on the hips and thighs at the same BMI. It does not adjust for age: older adults naturally lose muscle, so the same BMI can hide more fat at 70 than at 30, and some evidence suggests the healthiest BMI range shifts slightly upward in older age. It does not adjust for ethnicity either: at the same BMI, people of South Asian, Chinese and some other ancestries tend to carry more body fat and face higher metabolic risk, which is why some health bodies use lower BMI action points for those groups.

It also says nothing about fitness. Cardiorespiratory fitness predicts health outcomes more strongly than BMI in several large studies, and a fit person with a BMI of 18.4 can have better blood pressure, blood sugar and cholesterol than an unfit person with a BMI in the healthy range. None of that makes BMI useless — as a cheap, standardised screening tool it is good at flagging who should look closer — but it makes BMI a starting point, never a verdict. If this number worries you, the next step is not to argue with the formula but to gather the missing data (waist, body fat, blood pressure, blood markers, fitness) and, if needed, take it to a clinician.

Frequently asked questions

Is a BMI of 18.4 at 165 cm and 50 kg too low?

By the WHO classification it is underweight, 0.4 kg below the healthy range of 50.4–67.8 kg for 165 cm. Whether that matters for one person depends on build, muscle mass, health history and whether weight has changed recently — a clinician can judge that. Unintentional weight loss should always be checked.

What is the healthy weight range for 165 cm?

50.4–67.8 kg, from the WHO healthy BMI range of 18.5–24.9 applied to a height of 165 cm (1.65 m squared is 2.7225). Below 50.4 kg is underweight by BMI, 50.4–67.8 kg is healthy, 68.1 kg is where the overweight range starts (BMI 25), and 81.7 kg is where the obese range starts (BMI 30).

How is BMI calculated for 165 cm and 50 kg?

Divide weight in kilograms by height in metres squared: 50 ÷ (1.65 × 1.65) = 50 ÷ 2.7225 = 18.4. The WHO classification is underweight below 18.5, healthy 18.5–24.9, overweight 25–29.9, obese 30 and above. Try the BMI calculator to run any other height and weight through the same formula.

How much does 1 kg change BMI at 165 cm?

About 0.37 BMI points, because 1 kg divided by height squared (2.7225) is 0.37. A 5 kg change moves BMI by roughly 1.8 points and a 10 kg change by about 3.7 points at this height. That is why the same weight change matters more at shorter heights, where height squared is smaller.

Can I be healthy with a BMI of 18.4?

BMI alone cannot answer that. At 18.4 (Underweight), the useful follow-up checks are waist circumference, body-fat percentage, blood pressure, blood sugar and cholesterol, plus fitness and how you actually feel day to day. A clinician weighs those together. If several of those are in good shape, a single BMI number matters less; if several are off, the BMI is one more reason to act — but it is never a diagnosis by itself.

Does BMI work the same for athletes and older adults?

No. Athletes with a lot of muscle can score 18.4 or higher while carrying little fat, because BMI counts muscle and fat identically. Older adults can have a healthy-looking BMI while carrying more fat and less muscle than the number suggests. In both cases waist circumference and a body-fat estimate — for example from the body-fat calculator — are more informative than BMI alone.

Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.