Four essential health calculators — beautifully simple, clinically accurate, completely free. Understand your body in seconds.
Health information should be clear, calming, and trustworthy. We built every tool with that in mind.
Body Mass Index is a screening tool that uses height and weight to estimate body fat. While BMI has limitations — it doesn't account for muscle mass or where fat is stored — it remains a widely used starting point for health conversations.
A BMI between 18.5 and 24.9 is generally considered healthy for most adults. Our calculator shows you the full picture, not just a number.
Your Total Daily Energy Expenditure (TDEE) is the number of calories your body actually needs. Eating below this causes weight loss; above it causes gain. Neither is inherently bad — it depends entirely on your goals.
Our calculator factors in your age, sex, height, weight, and activity level using the Mifflin-St Jeor equation — the most accurate formula available outside a lab setting.
Sleep happens in 90-minute cycles moving through light sleep, deep sleep, and REM. Waking at the end of a complete cycle feels dramatically better than waking mid-cycle — even if you slept fewer total hours.
Our sleep calculator helps you time your sleep so you always wake up at the end of a cycle, making every morning feel more natural and refreshed.
The "8 glasses a day" rule is a myth. Your actual water needs depend on your body weight, activity level, climate, and diet. A 120-pound sedentary person needs far less than a 200-pound athlete in summer heat.
Our hydration calculator gives you a personalized target — not a one-size-fits-all number — so you stay properly hydrated without overdrinking.
About Our Health Calculators
Every VitalTally calculator is built on formulas used in clinical research and healthcare settings — not approximations or oversimplified rules of thumb.
Health data is among the most sensitive personal information there is. VitalTally is built so we never have access to yours.
All calculator logic runs as JavaScript directly on your device. When you enter your height, weight, age, or activity level, that data is processed locally and never transmitted to any server. There are no accounts, no data collection, and no third-party data sharing of your health inputs.
You can verify this yourself: open your browser's developer tools, go to the Network tab, and observe that no outgoing requests are made when you use any calculator.
Body Mass Index is one of the most searched health metrics — and one of the most misunderstood. The WHO adult classification thresholds are:
BMI cannot distinguish muscle from fat, doesn't account for fat distribution, and varies in accuracy by age, sex, and ethnicity. Our calculator explains these limitations alongside your result — because context matters more than a number.
Your daily calorie needs depend on two things: your Basal Metabolic Rate (BMR — calories burned at complete rest) and your activity level. Multiply them together and you get TDEE — Total Daily Energy Expenditure.
From your TDEE, goal-based targets follow directly:
All calorie estimates carry a 10–20% margin of error — individual metabolic rates vary based on genetics, hormones, and gut microbiome. Treat the result as a starting point, then adjust based on real-world results over 2–4 weeks.
Sleep progresses through repeating 90-minute cycles of light sleep, deep slow-wave sleep, and REM sleep. Waking at the end of a cycle — when sleep is lightest — leaves you alert. Waking mid-cycle causes sleep inertia: the groggy, disoriented feeling that persists for 30–60 minutes.
The National Sleep Foundation recommends:
Our sleep calculator adds 15 minutes of average sleep onset time to align your suggested schedule with natural cycle boundaries.
The "8 glasses a day" rule has little scientific support — it originated from a 1945 guideline that specified most water would come from food, a nuance lost in popular culture. The National Academies of Sciences recommends approximately 3.7 litres (125 oz) total daily water for men and 2.7 litres (91 oz) for women, including water from food (~20% of intake).
Individual needs vary significantly based on body weight, activity level, climate, and health status. A 90 kg person exercising in summer heat needs dramatically more than a 55 kg sedentary person in a cool office. Our calculator accounts for both weight and activity level to give you a personalized target rather than a generic number.
VitalTally calculators provide estimates for informational and educational purposes. Results are not medical advice and should not replace consultation with a qualified healthcare professional.
The formulas we use are clinically validated at the population level — they work well as starting points for the majority of healthy adults. They are less accurate for:
If you fall into any of these categories, please consult a healthcare provider for personalized guidance.
Getting the most from health calculators requires understanding what they measure and what they don't: