# Guide to the health calculators

The health calculators cover body weight, sleep and breathing. One works out your body mass index, one suggests bedtimes or wake-up times from 90-minute sleep cycles, and one picks a breathing exercise and guides you through it.

## Health

### [BMI Calculator](https://www.calculator.land/calculators/bmi/)

Works out body mass index for adults from height and weight, in metric or imperial units, and places it in the WHO weight categories, with a chart showing where it falls. It also gives the weight range that matches a BMI of 18.5 to 24.9 at your height, and adds notes based on the ethnic background and optional age you enter, such as the lower cut-offs of 23 and 27.5 for a South Asian or Chinese background.

### [Breathing Exercises Calculator](https://www.calculator.land/calculators/breathing-exercises/)

Picks one of twelve breathing techniques, such as 4-7-8 or box breathing, to match the situation and goal you choose, and works out how many breathing cycles fit the minutes you set, or suggests a length if you leave the field empty. It then paces you through the exercise with an animated wave and a timer for each inhale, hold and exhale.

### [Sleep Calculator](https://www.calculator.land/calculators/how-much-sleep-do-you-need/)

Suggests bedtimes for a chosen wake-up time, or wake-up times for a chosen bedtime, by counting whole 90-minute sleep cycles plus the minutes it takes you to fall asleep. Your age group and activity level narrow the choices, a button starts from the current time, and a chart shows the typical mix of sleep stages in each cycle. For newborns and infants it shows a note instead of times.