Health Tools

What these health tools cover

Two everyday calculators: one for sleep timing and one for body mass index. Both are reference tools. They tell you what a standard formula or guideline says about the numbers you entered, which is a different thing from telling you about your health.

The sleep calculator works backwards and forwards from a fixed point. Give it a wake-up time and it returns the bedtimes that allow seven, eight or nine hours; give it a bedtime and it does the reverse. The BMI calculator takes adult height and weight, returns the index and the reference weight range for that height, and can add a waist-to-height reading.

Reading the results properly

BMI is a ratio of weight to height and nothing else. It does not distinguish muscle from fat, and it does not account for build or where weight is carried, which is why a lean, heavily built person can land in the same band as someone carrying excess weight. It is a screening number, used because it is cheap to calculate at population scale — not a diagnosis.

Waist-to-height is offered alongside it for exactly that reason. It captures something BMI cannot, namely where the weight sits, and the two readings together say more than either does alone.

The sleep figures are arithmetic around sleep-cycle lengths and an allowance for falling asleep. Actual sleep need varies between people and across a life, so treat the suggested times as a starting point to adjust from rather than a target to hit.

When to ask someone qualified

Neither tool is medical advice and neither can be. If a result concerns you, or you are making a decision about diet, training, medication or a persistent sleep problem, that belongs with a doctor or another qualified professional who can see the rest of the picture.

The BMI calculator is built for adults. Children and teenagers are assessed against age-and-sex percentile charts rather than the fixed adult bands, so the number this returns for them is not the number a clinician would use.