Health calculators
Everyday wellness numbers — BMI, daily calorie needs, hydration, and more — explained with the formula behind them, not just a bare result. These are general educational estimates built on standard published equations; they don't replace advice from a doctor or dietitian who knows your full medical picture.
BMI calculator
Body Mass Index from your weight and height.
How this calculator works
Body Mass Index is a simple ratio of weight to height, used as a population-level screening tool rather than a diagnosis. It doesn't distinguish between muscle and fat, which is why it can misclassify very muscular or very lean individuals.
BMI = weight (kg) / height (m)²
- Under 18.5 — underweight
- 18.5–24.9 — normal weight
- 25–29.9 — overweight
- 30 and above — obese
Because BMI doesn't account for muscle mass, bone density, or fat distribution, healthcare providers typically use it alongside other measurements, not as a standalone diagnosis.
Daily calorie needs (TDEE)
Estimated calories per day using the Mifflin-St Jeor formula.
How this calculator works
Your Total Daily Energy Expenditure (TDEE) is the number of calories your body burns in a day, including both resting metabolism and physical activity. It's calculated in two steps: first estimate your Basal Metabolic Rate (BMR) — calories burned at complete rest — then multiply by an activity factor to account for movement and exercise.
BMR (men) = 10×weight(kg) + 6.25×height(cm) − 5×age + 5
BMR (women) = 10×weight(kg) + 6.25×height(cm) − 5×age − 161
TDEE = BMR × activity multiplier
The Mifflin-St Jeor equation used here is generally considered more accurate than the older Harris-Benedict formula, but individual metabolism still varies — treat the result as a starting estimate to adjust based on real-world weight trends over 2–3 weeks.
Water intake calculator
A general daily hydration target based on weight and activity.
How this calculator works
This estimate uses a common rule of thumb: roughly 33 mL of water per kilogram of body weight per day for baseline hydration, plus a small amount of extra water per minute of exercise to replace what's lost through sweat.
Liters/day = (weight in kg × 0.033) + (exercise minutes × 0.012)
This is a general guideline, not a strict requirement — actual needs shift with climate, altitude, sweat rate, and overall health, and roughly 20% of daily water intake typically comes from food rather than drinks alone.
Weight loss / weight gain calorie target
A daily calorie target based on your goal and desired weekly rate of change.
How this calculator works
Weight change is fundamentally about calorie balance: eating less than you burn creates a deficit that leads to weight loss, while eating more creates a surplus that leads to weight gain. This calculator first estimates your maintenance calories (TDEE), then adjusts by an amount tied to your target weekly rate of change, using the common approximation that roughly 7,700 kcal equals 1 kg of body weight.
Weekly calorie adjustment = target kg/week × 7,700
Daily calorie target = TDEE ± (weekly adjustment / 7)
Very aggressive deficits (more than about 1 kg/week for most people) are generally discouraged, since they're harder to sustain and can increase muscle loss — a moderate, consistent deficit tends to produce more durable results.
Pregnancy due date calculator
Estimate your due date using the first day of your last menstrual period (Naegele's rule).
How this calculator works
Naegele's rule is the standard clinical shorthand for a first-pass due date estimate: it assumes a 28-day cycle and adds 280 days (40 weeks) to the first day of your last menstrual period, since pregnancy is conventionally counted from that date rather than from conception.
Estimated due date = first day of last period + 280 days
This is a population-average estimate; only about 5% of babies are actually born on their exact due date. Cycle length varies between individuals, so an ultrasound dating scan in early pregnancy is generally more precise than this calculation alone.
Health calculators on this page are general estimates only and do not account for individual medical conditions. Speak with a healthcare provider for personalized guidance.
Frequently asked questions
How is the calorie target calculated?
We estimate your maintenance calories (TDEE) using the Mifflin-St Jeor formula, then add or subtract calories based on your goal — roughly 7,700 kcal per kg of body weight change.
How accurate is the due date calculator?
It uses Naegele's rule (280 days from your last period), the same starting estimate doctors typically use before an ultrasound. Actual due dates vary — always confirm with your healthcare provider.
Is BMI a reliable measure of health?
BMI is a useful population-level screening tool, but it has known limitations for individuals — it can't distinguish muscle from fat, so athletes and very muscular people are often misclassified as "overweight" despite low body fat. It's best used alongside other measurements like waist circumference or body composition testing, not on its own.
Why do TDEE calculators give different results for the same person?
Different formulas (Mifflin-St Jeor, Harris-Benedict, Katch-McArdle) use slightly different assumptions and variables. Mifflin-St Jeor, used here, is generally considered the most accurate for the general population, but activity level self-reporting is often the biggest source of variation between calculators.
How much water is actually too much?
While rare, drinking excessive amounts of water in a short period can dilute sodium levels in the blood (hyponatremia), which is a genuine medical risk, particularly for endurance athletes. The estimates here reflect normal daily needs, not a maximum — spreading intake throughout the day is generally safer than drinking large volumes at once.
Should I eat back the calories I burn from exercise?
The activity multiplier in the TDEE calculation already accounts for your typical weekly exercise level, so for consistent routines you generally shouldn't need to add calories back on top. It becomes more relevant for occasional, unusually intense workouts well beyond your normal activity level.
Why does the calculator ask for sex when estimating calorie needs?
On average, men tend to carry more lean muscle mass than women at the same weight and height, and muscle burns more calories at rest than fat tissue. The Mifflin-St Jeor formula includes a sex-based adjustment to account for this typical difference in body composition.
Are these calculators safe to use during pregnancy or with a medical condition?
These tools provide general population estimates and are not tailored to individual medical conditions, pregnancy, or nursing. If you have a health condition, are pregnant, or are under medical supervision for weight or nutrition, always follow your healthcare provider's specific guidance instead.