BMI calculator for hair loss
Check whether your weight could be part of your shedding, or whether it's more likely something else. Nothing you enter is saved or sent anywhere.
For adults 20 and over who aren't pregnant. Categories follow the CDC[1].
Why your weight can affect your hair
Hair is not essential for survival, so when your body is short on energy, protein or iron, it cuts back on hair growth first. That can push more hairs into the shedding phase, usually showing up two to four months after the change.
Most of my own shedding came from hormones, heavy periods and low iron, not my weight. But during my vegan phase, my nails peeled, my hair got drier and the shedding sped up. It taught me how quickly my hair reacted when my body wasn't getting what it needed.
Being underweight
For adults, a BMI under 18.5 is classed as underweight[1]. Being underweight often goes with eating less protein and fewer minerals overall, and hair needs both. If your BMI is in this range and you're shedding, it's worth looking at what and how much you're eating with your doctor or a dietitian.
Losing weight quickly
Fast weight loss is one of the best-known triggers of temporary shedding. A 2021 meta-analysis found around 57% of people reported excessive hair loss within a year of weight-loss surgery[2]. The same can happen with crash diets or very low-calorie eating. The good news is that this kind of shedding usually settles once eating and weight stabilise.
Low iron and protein, at any weight
Low iron stores are linked with shedding in women[3], and a ferritin of 40 micrograms per litre or lower may point to deficiency in people with hair loss[4]. This can happen at any BMI, especially with heavy periods. A healthy weight doesn't always mean healthy iron.
If your BMI is in the healthy range
Then your weight is less likely to be the main reason for your hair loss, and it's worth looking at the other common roots: hormones, iron, thyroid, gut and stress. The 2-minute self-check can help you work out where to start.
If your BMI is above 25
Weight itself isn't usually what makes hair shed, but a higher weight can go along with insulin resistance, which is linked to PCOS-related hair thinning. If that sounds like you, ask your doctor about checking fasting insulin or HbA1c. Please don't crash diet to fix it: fast weight loss can trigger shedding of its own.
What BMI can't tell you
BMI is a quick screening number, not a diagnosis. It doesn't measure muscle, body fat or nutrition, so use it as one clue alongside how you eat and how you feel.
If food feels hard right now. If you're restricting food, worried about your eating, or feel anxious about your weight, you deserve support. Please talk to your GP, or contact an eating disorder helpline in your country. You don't have to sort out your hair and your relationship with food on your own.
Good to know
Can being underweight cause hair loss?
It can contribute. When the body doesn't get enough energy, protein or iron, hair growth is one of the first things it cuts back, and more hairs can shift into the shedding phase. A BMI under 18.5 is classed as underweight for adults.
Can losing weight quickly cause hair loss?
Yes. Rapid weight loss is a well-known trigger for temporary shedding called telogen effluvium. A 2021 meta-analysis found about 57% of people reported excessive hair loss in the year after weight-loss surgery. Shedding usually shows up two to four months after the change.
If my BMI is healthy, can weight still be the cause?
It's less likely to be the main driver, but a healthy BMI doesn't rule out low protein, low iron or recent fast weight loss. If your BMI is in the healthy range, look at the other common roots: hormones, iron, thyroid, gut and stress.
Is BMI accurate for women?
BMI is a quick screening number, not a diagnosis. It doesn't measure body fat, muscle or nutrition directly, so use it as one clue alongside how you eat and how you feel, and talk to your doctor.
Personal experience and general education only, not medical, nutritional or diagnostic advice. Results vary. Please talk to your own qualified practitioner before changing anything. "Apple" is a pen name.
Sources
- Centers for Disease Control and Prevention. Adult BMI categories. Source
- Zhang W, et al. Hair loss after metabolic and bariatric surgery: a systematic review and meta-analysis. Obes Surg. 2021. Source
- Trost LB, Bergfeld WF, Calogeras E. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss. J Am Acad Dermatol. 2006;54(5):824-844. Source
- Serum ferritin levels: a clinical guide in patients with hair loss. Cutis. 2023. Source