BMI Calculator — and What It Can’t Tell You
Here’s your BMI, and here’s the part most BMI calculators leave out: it cannot tell fat from muscle. Which is awkward, because when you’re losing weight on a GLP-1, that’s the only distinction that really matters.
Your BMI
Your BMI
29.0
weight in kg divided by height in metres squared
Standard category
Overweight
a population screening band — not a verdict on your health
The thing BMI can’t tell you. As the CDC puts it, BMI “does not distinguish between fat, muscle, and bone mass.” That is exactly the distinction that matters while you’re losing weight on a GLP-1 — two people at the same BMI can carry very different amounts of muscle, and a falling BMI can’t tell you whether what you lost was fat or the muscle you were trying to keep.
BMI is a screening measure, not a diagnosis, and clinical guidance says judgement is needed where it may not reflect body fat well — including in people with high muscularity. Use it as one rough data point among several, and take any question about what it means for you to your healthcare provider.
Why we built this one reluctantly
BMI is genuinely useful for what it was designed to do — screening populations. It’s cheap, quick and needs nothing but a tape and a scale. What it was never designed to do is tell one person whether the weight they just lost was fat or muscle, and on a GLP-1 that’s the question worth asking.
Two people at an identical BMI can carry very different amounts of muscle. Someone who has lifted for years can land in the “overweight” band while carrying less fat than someone in the “healthy” one. And as you lose weight, your BMI falls whether what left was fat or the muscle you were trying to keep — it drops either way, and looks like progress either way.
So use the number as one rough data point, then use something that can actually see the difference. A waist-to-height ratio isn’t inflated by muscle, and your strength numbers tell you directly whether the muscle is still there.
A number that can’t see your muscle is a poor way to judge whether you’re keeping it. Mira gives you one that can: it coaches short strength sessions through your phone and tracks your reps, load and form, so “stronger” becomes something you can actually watch.
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