When a woman’s BMI is too low
A BMI below 18.5 can mean too little energy and nutrients for the body’s needs. In women that can cause irregular or missing periods, reduced fertility, loss of bone density and tiredness. Sometimes a low BMI is simply a slim build, but if it comes with these symptoms, or with unintended weight loss, it is worth seeing a doctor.
Setting a realistic weight goal
You do not need to reach the middle of the healthy range to benefit. Losing 5–10% of your current weight lowers blood pressure, improves blood sugar and eases joint pain. For a woman of 85 kg, that is 4–8.5 kg. The calculator shows how far you are from the nearest edge of the healthy range — a useful long-term marker rather than a short-term target.
Adult BMI categories
From the age of 20, everyone uses the same World Health Organization categories, whatever their sex or age:
| BMI (kg/m²) | Adult category (WHO) |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40 or more | Obesity class III |
How to measure for an accurate BMI
- Height: stand barefoot with heels, back and head against a wall; rest a flat object on your head and mark the wall.
- Weight: weigh first thing in the morning, after using the toilet, in light clothing, on a hard floor.
- Consistency: use the same scale at the same time of day when you recheck — daily weight can move by 1–2 kg with food and fluid.