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Am I Underweight? Calculator

Find out if you're underweight from your BMI, with your healthy weight range and kg to reach it.

Related tools

All Weight & body tools →
Am I Overweight? CalculatorFind out if you're overweight from your BMI, with your healthy weight range and kg to reach it.Biological Age CalculatorEstimate your biological age from lifestyle factors — fitness, diet, sleep, stress and more. Educational, not medical.Body Frame Size CalculatorSmall, medium or large frame from your height-to-wrist ratio, with a frame-adjusted weight range.Bra Size CalculatorBand and cup size (US, UK and EU) from your underbust and bust measurements.Child BMI Percentile CalculatorBMI-for-age percentile on the CDC reference, ages 2-20 — the percentile, not the BMI, is what counts for a child.Height Percentile CalculatorWhere your height sits against the US adult reference, with the z-score and standard-deviation band.Keto CalculatorCompute your keto macros (fat, protein, net carbs) from BMR/TDEE and your keto diet type.One-Mile Walk Test Calculator (1 mile = 1.6 km, Rockport)Estimate your VO₂max and cardio-fitness rating from the Rockport one-mile walk test. Enter the distance in miles or kilometres (1 mile = 1.609344 km), your weight in kg or lb, the walk time and your ending heart rate, and read your speed in both mph and km/h.

Am I Underweight? Calculator works straight from this page — free, instant, nothing to install. Its place is under Weight & body; Am I Overweight? Calculator and Biological Age Calculator answer the questions closest to this one.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What is Am I Underweight? Calculator?

Find out if you're underweight from your BMI, with your healthy weight range and kg to reach it.

When would I actually use this?

Getting a reference point before a check-up, setting a realistic target, and understanding why two people of the same weight can be in very different shape — and, the same act from the other side, putting a day's caffeine, salt, sugar or alcohol beside the ceiling a health body publishes for it.

What is the most common mistake?

Reading BMI as a measure of health. It cannot tell muscle from fat, which is why athletes routinely score as overweight and why it says little about an individual.

How is Am I Underweight? Calculator different from Am I Overweight? Calculator?

They sit next to each other but answer different questions: Am I Overweight? Calculator is the one to open when you need it to find out if you're overweight from your BMI, with your healthy weight range and kg to reach it. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

Biological Age Calculator is the closest one after this: Estimate your biological age from lifestyle factors — fitness, diet, sleep, stress and more. Educational, not medical.

What else is worth having open alongside it?

Body Frame Size Calculator and Bra Size Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

The formulas are the published ones (Quetelet for BMI, Mifflin-St Jeor or Harris-Benedict for energy expenditure) and the tool names which it used. None of this is a diagnosis or a substitute for a clinician.

Further reading

All guides →
ExplainerAm I Underweight? What the Question Is Really AskingThe 18.5 cut-off is a population screening line, not a diagnosis. Here is where it comes from, what the mortality data below it actually show, and the three things it cannot see: body composition, recent change — which is far more informative than the level — and the fact that the same line does not fit every age or every population.ExplainerAm I Overweight? Why One Number Cannot SayBMI reads muscular bodies as heavy and cannot see where fat sits at all — which is exactly why the waist measures exist. Here are the published waist and waist-to-height thresholds, a worked case where BMI and waist give opposite answers, an honest account of the contested mortality curves, and the 2025 shift toward defining obesity by adiposity and organ function rather than by a ratio.ExplainerBody Frame Size, and What Ideal-Weight Formulas AssumeFrame size exists because height alone does not fix skeletal mass, and the published methods are crude proxies with real cut-offs. The bigger story is the ideal-weight formulas: four of them, all built for drug dosing rather than health, disagreeing by up to 27 lb at the same height.ComparisonBiological Age vs Chronological Age: What Each One Actually MeasuresChronological age is exact. Biological age is not one thing but three — epigenetic clocks, blood-marker scores and lifestyle quizzes — and only the first two have published validation. Here is how to tell which one you are looking at.ExplainerHow Tall Will a Child Be? The Arithmetic Is Easy; the Error Bar Is the StoryAverage the parents' heights, shift by a sex constant, and you have a target height. The honest part is what comes next: the published prediction interval is about ±4 inches, wide enough to cover most of the adult population. Here is the formula, the interval, why bone age does better, and why children keep out-growing the target.GuideBra Sizing Is Two Measurements and Four Incompatible SystemsThe band and the cup are computed differently in the UK, the US, the metric EU, the French and Spanish markets and Italy. One body, its four labels, and the arithmetic behind each.