Healthy Weight Calculator

This calculator does one arithmetic step: it takes the WHO's normal BMI band of 18.5 to 25 kg/m² and rearranges the formula to give you weight instead of an index.¹ Everything interesting about the answer is in that band, not in the maths.

The band is wide on purpose. When the Global BMI Mortality Collaboration pooled 239 studies covering roughly ten million people, the relationship between BMI and mortality was not a sharp V with a single lowest point — it was a broad, flat trough between about 20 and 25, with risk climbing steadily above it.² Within that trough, the evidence does not distinguish one weight from another.

This is the most useful finding in the weight-loss literature and the one least reflected in how people set goals. Most of the measurable health improvement from weight loss arrives early — in the first 5 to 10% — and further loss adds progressively less.

Wing and colleagues found that a 5–10% reduction produced clinically meaningful improvements in blood pressure, triglycerides, HDL cholesterol and glycaemic control in overweight and obese adults with type 2 diabetes.⁴ For someone at 100 kg, that is five to ten kilograms — a target that is achievable and maintainable, unlike the thirty that a BMI table might imply.

It is worth knowing in advance that the body does not treat weight loss neutrally. Sumithran and colleagues measured appetite hormones in people ten weeks after a supervised diet and again a year later, and found the changes persisted: ghrelin elevated, leptin and peptide YY suppressed, and subjective hunger higher than before the diet — a full year on.³

That is not a reason for pessimism, and it is certainly not evidence that maintenance is impossible. It is a reason to plan for it: the pull back toward the old weight is physiological, it lasts, and people who expect it do considerably better than people who interpret it as a personal failure.

• Build the habit before the deficit. Whatever you are doing to lose the weight should be something you could keep doing at maintenance calories.
• Keep protein and resistance training in. Both protect lean mass, and lean mass is what keeps your TDEE from falling further than the weight change alone requires.
• Recalculate as you go. A smaller body needs fewer calories. The Calorie Calculator target that worked at the start will be too high after 5 kg.
• Weigh the trend, not the day. Weekly averages; daily numbers are mostly water.

• If you carry a lot of muscle. BMI cannot tell muscle from fat, so the range reads low for trained lifters and many athletes. Check body fat percentage before acting on it.
• If you are over 65. A little more reserve appears protective in older age, and the bigger risk is usually muscle loss rather than excess weight.
• If you are pregnant. Not interpretable. Use pre-pregnancy weight with the Pregnancy Weight Gain Calculator.
• If you are under 20. Children and teenagers are assessed against age- and sex-specific percentiles, not adult thresholds.
• If your ancestry is East or South Asian. Cardiometabolic risk rises at lower BMI values in many Asian populations, and some national guidelines use thresholds below 25.