Health tools

What BMI cannot tell you

• It cannot tell muscle from fat. Very muscular people — athletes, manual workers, regular strength trainers — often read as "overweight" or "obese" while carrying little body fat.
• It says nothing about where fat is carried, and fat around the middle matters more for risk. NICE suggests also checking whether your waist is less than half your height.
• It is not valid in pregnancy. NICE's overweight and obesity guideline explicitly does not cover pregnancy.
• It is not valid for children and teenagers aged 2 to 17, who need age- and sex-adjusted centiles rather than these adult bands. Use the NHS children's calculator instead.
• It becomes less reliable in older adults, and in people who have lost muscle through illness.
• Risk rises at a lower BMI for some family backgrounds, which is what the checkbox above is for.

What the NHS says

• The Eatwell Guide advice is 6 to 8 cups or glasses of fluid a day, on top of the water you get from food.
• Water, lower-fat milk, sugar-free drinks, tea and coffee all count. Fruit juice and smoothies count too, but the NHS advises limiting them to a combined 150ml a day because of the sugar.
• You need more in hot weather, when exercising, and if you have a fever, vomiting or diarrhoea.
• Some people are told to limit fluid — for example with heart failure or kidney disease. If that is you, follow what your clinician told you, not a general figure from a website.

A better check than a number

The NHS signs of dehydration are dark yellow, strong-smelling urine, weeing less often than usual, feeling thirsty, dizzy or tired, and a dry mouth, lips and tongue. Pale straw-coloured urine, and passing it regularly through the day, means you are probably drinking enough.