Health calculators
Free, private, and browser-based health tools. No accounts, no tracking, no data collection. Each calculator uses a published formula and explains what the result actually means.
Health calculators are useful for one thing: turning a vague question like "am I drinking enough water?", "how many calories do I need?", or "how much sleep should I aim for?" into a concrete starting number. What they can not do is diagnose you, replace a professional, or account for the hundreds of individual factors that make you you. Treat every output on this page as a ballpark to think from, not a prescription.
What each tool does — and its limits
BMI (Body Mass Index)
A simple weight-for-height ratio (kg divided by height in metres squared) that places you in one of five bands: underweight, healthy, overweight, obese, and severely obese. Its huge limitation: it doesn't know how much of your weight is muscle. A rugby player and a sedentary office worker at the same BMI can have completely different health profiles. Use BMI as a rough screening tool, not a diagnosis.
Body Fat Percentage
The US Navy method uses waist, neck, and (for women) hip measurements to estimate body fat percentage. It's more personal than BMI because it responds to body composition, but it's still an estimate — DEXA scans and hydrostatic weighing are the gold standard for accuracy.
Calorie needs
The daily calorie calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR) — the calories your body burns at rest — then multiplies by a published activity factor to give total daily energy expenditure (TDEE). Aim to eat around TDEE to maintain weight, roughly 500 kcal below TDEE for slow-and-sustainable fat loss, or 300-500 kcal above for lean muscle gain.
Water intake
Recommended daily water is a function of body weight and activity. The "eight glasses a day" rule is a very rough approximation; the calculator gives you a more personalised starting point. Remember that food (fruit, vegetables, soup) and other drinks contribute to your total fluid intake.
Sleep cycles
The average adult sleep cycle is around 90 minutes. Waking at the end of a cycle — rather than in the middle — leaves you feeling more rested. The sleep calculator gives you bedtime or wake-time options that align with 90-minute cycles from the moment you fall asleep.
Running pace
Convert between pace (min/km, min/mile) and speed (km/h, mph), and predict finishing times for common race distances. Useful for setting a target pace on your next long run or working backwards from a goal marathon time.
Frequently asked questions
Are health calculators from CalcOrchard a substitute for a doctor?
Absolutely not. Body Mass Index, body fat percentage, calorie targets, and hydration estimates are population-level rules of thumb. They tell you where you sit relative to a statistical band — not whether you personally are healthy, whether you should change your diet, or whether a symptom needs attention. If you have any health concerns, speak to a GP or qualified clinician.
Which formulas do you use for calories and body fat?
The calorie calculator uses the Mifflin-St Jeor equation for Basal Metabolic Rate (widely regarded as the most accurate for the general adult population) combined with a standard activity multiplier. The body fat calculator uses the US Navy circumference method. Both formulas are published, peer-reviewed, and reproducible from your inputs.
Is my data private when I use these tools?
Yes. Every health calculator on CalcOrchard runs entirely in your browser. Your weight, height, age, gender, waist measurement, or any other input is never sent to our servers, stored in a database, or shared. There are no accounts, no cookies for personalisation, and no tracking of what you calculate.
Why doesn't a low BMI mean I'm healthy?
BMI ignores muscle mass and body composition. A muscular athlete can have a 'high' BMI without any excess body fat, and someone in a 'normal' BMI band can still have poor metabolic health. Treat BMI as one signal among many — waist-to-hip ratio, resting heart rate, blood pressure, and blood markers all say more about actual health than BMI alone.
Medical disclaimer: The tools and articles on CalcOrchard are for general educational purposes only and are not medical advice. They are not designed to diagnose, treat, cure, or prevent any condition. Always consult a qualified healthcare professional before making changes to your diet, exercise, sleep, or any other aspect of your health.