Standard BMI Categories for Women
BMI (Body Mass Index) is calculated as weight in kilograms divided by height in metres squared (kg/m²). The WHO categories apply universally to adult women regardless of age:
| BMI Range | Category | Health Implications |
|---|---|---|
| Below 18.5 | Underweight | Risk of malnutrition, osteoporosis, anaemia |
| 18.5–24.9 | Normal weight ✓ | Lowest overall health risk |
| 25.0–29.9 | Overweight | Moderate increased risk |
| 30.0–34.9 | Obese Class I | High risk — lifestyle changes recommended |
| 35.0+ | Obese Class II–III | Very high risk — medical support recommended |
Why BMI Becomes Less Reliable After 40
While the BMI categories remain the same, the body composition behind any given BMI number changes significantly with age. Two women aged 25 and 50 can have identical BMIs of 24.0, but the 50-year-old is likely to have 8–12% more body fat and 4–6 kg less muscle mass. This means her cardiometabolic risk is higher than her BMI alone suggests.
After age 35, women lose approximately 1% of muscle mass per year without resistance exercise. This muscle loss — sarcopenia — reduces the resting metabolic rate by an estimated 2–4% per decade, making weight gain more likely at the same calorie intake.
The Waist Circumference Test — More Important Than BMI After 40
Waist circumference is a better predictor of cardiometabolic risk than BMI for women over 40, because it directly measures abdominal fat accumulation — the most dangerous fat depot. Measure at the narrowest point of the torso, approximately 2.5cm above the navel, after exhaling normally.
High risk: Waist circumference above 88cm (34.6 inches)
These thresholds apply regardless of overall BMI. A woman with a "normal" BMI of 23.0 but a waist of 85cm has a meaningfully elevated cardiovascular risk.
What Actually Matters: Body Composition
For women over 40, the most meaningful health targets are not about a specific number on the scale or BMI calculator — they are about:
- Preserving or building muscle mass through resistance training
- Reducing visceral (abdominal) fat through diet and exercise
- Maintaining bone density through weight-bearing exercise and calcium/vitamin D
- Keeping waist circumference below 80cm
- Normal blood markers: blood pressure, blood sugar, and cholesterol
Realistic Weight Goals for Women Over 40
Rather than targeting a specific "ideal" weight from a chart, focus on a healthy BMI range (18.5–24.9) and prioritise waist circumference. A safe weight loss rate of 0.25–0.5 kg per week preserves muscle better than rapid weight loss. Women over 40 who combine moderate calorie restriction with resistance training lose comparable amounts of fat to younger women, while better maintaining metabolic rate.