Health Calculators

Why Use a Health Calculator?

Health metrics like body weight, calorie needs, and sleep timing look simple but involve variables that interact in non-obvious ways. A 35-year-old woman who exercises five days a week needs roughly 500–700 more calories per day than the same woman who is sedentary — a difference that determines whether she loses, maintains, or gains weight over months. Our health calculators translate established medical formulas into instant, actionable numbers so you can make better decisions without guesswork.

Every calculator on this page is based on formulas endorsed by health organizations including the World Health Organization (WHO), the National Sleep Foundation (NSF), and the Academy of Nutrition and Dietetics. All calculations happen in your browser — no data is collected or stored.

What Each Calculator Is For

BMI Calculator

Calculates your Body Mass Index from your height and weight using the standard WHO formula: BMI = weight (kg) ÷ height (m)². Shows your BMI score, weight category (Underweight, Normal, Overweight, Obese), and the healthy weight range for your height. Supports both imperial (pounds and inches) and metric (kilograms and centimetres) units. BMI is a useful population-level screening tool but does not directly measure body fat or account for muscle mass.

Calorie Calculator (TDEE)

Estimates your Total Daily Energy Expenditure (TDEE) — the number of calories your body needs each day based on your age, sex, height, weight, and activity level. Uses three validated formulas: Mifflin-St Jeor (recommended for most people), Harris-Benedict (original and revised), and Katch-McArdle (most accurate for lean individuals who know their body fat percentage). Also provides calorie targets for weight loss, maintenance, and weight gain.

Sleep Calculator

Recommends bedtimes or wake-up times aligned with natural 90-minute sleep cycles. Waking at the end of a cycle rather than mid-cycle reduces sleep inertia — the groggy feeling caused by interrupting deep sleep. Enter your target wake-up time and the calculator shows the optimal bedtimes for 4, 5, or 6 complete cycles, including 14 minutes to fall asleep.

Period Calculator

Predicts your next period date, ovulation window, and fertile days based on the first day of your last period and your average cycle length. Shows the next five cycles in a schedule and identifies your current phase (menstrual, follicular, ovulation, or luteal). Most accurate for cycles that are consistent within ±2–3 days month to month.

Body Fat Calculator

Estimates body fat percentage, fat mass, and lean mass using the U.S. Navy circumference method — a validated field measurement technique accurate to within ±3–4 percentage points of DEXA scan results when measurements are taken carefully. Requires neck and waist measurements for men, and neck, waist, and hip measurements for women. Also shows BMI for comparison and how much fat loss would be needed to reach the fitness range.

Key Health Concepts

BMI categories and their limitations

The WHO defines four BMI ranges for adults: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). BMI is widely used because it requires only height and weight, making it practical at population scale. Its main limitation is that it treats muscle and fat as equivalent — a highly muscular person can have a "overweight" BMI despite low body fat. For a more complete picture, use BMI alongside body fat percentage and waist circumference.

BMR vs. TDEE — and why it matters for weight

Basal Metabolic Rate (BMR) is the energy your body uses at complete rest just to maintain vital functions — breathing, circulation, cell repair. TDEE multiplies BMR by an activity factor (1.2 for sedentary to 1.9 for very active). Weight is stable when you eat at TDEE, decreases when you eat below it, and increases when you eat above it. A sustainable calorie deficit of 500 kcal/day produces approximately 0.5 kg (1.1 lb) of fat loss per week, because 1 kg of body fat contains roughly 7,700 kcal.

How sleep cycles affect recovery

Sleep consists of repeated 90-minute cycles, each containing four stages: N1 (light sleep), N2 (consolidated sleep), N3 (deep slow-wave sleep critical for physical recovery), and REM (rapid eye movement sleep critical for memory consolidation). N3 dominates early cycles; REM dominates later ones. This is why sleeping 7.5 hours (5 complete cycles) often leaves people feeling better than sleeping 8 hours but waking mid-cycle. The National Sleep Foundation recommends 7–9 hours for adults aged 18–64.

Body fat classification by sex

Body fat standards differ by sex because women naturally carry more essential fat (around 10–13%) for hormonal and reproductive functions. The American Council on Exercise (ACE) classifies men as: Essential fat 2–5%, Athletes 6–13%, Fitness 14–17%, Average 18–24%, Obese 25%+. For women: Essential fat 10–13%, Athletes 14–20%, Fitness 21–24%, Average 25–31%, Obese 32%+. Body fat percentage is a more accurate health indicator than BMI for active and muscular individuals.

Frequently Asked Questions

Are these health calculators accurate?

Our calculators use the same validated formulas applied in clinical and research settings. Accuracy depends on the accuracy of your inputs — height, weight, and circumference measurements should be taken carefully. Results are population-level estimates; individual variation exists. For specific medical concerns, consult a qualified healthcare provider.

Which BMR formula should I use in the calorie calculator?

The Mifflin-St Jeor formula is recommended for most people — it is the most extensively validated for the general population and is endorsed by the Academy of Nutrition and Dietetics. If you know your body fat percentage accurately (from a DEXA scan or Navy method), use Katch-McArdle for a more precise result, since it directly accounts for lean body mass rather than estimating it from height and weight.

How is ovulation related to cycle length?

Ovulation typically occurs 14 days before the start of the next period, regardless of total cycle length. For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, ovulation is around day 18. The fertile window includes the 5 days before ovulation and the day of ovulation itself, because sperm can survive in the reproductive tract for up to 5 days.

What is a healthy body fat percentage for women?

For women, the fitness range (21–24% body fat by ACE standards) is associated with good health outcomes. Athletes typically carry 14–20%. Above 32% is classified as obese. These ranges apply to adult women generally — individual health context, age, and muscle mass can shift what is optimal for a specific person.

Should I base my diet on BMR or TDEE?

Always use TDEE, not BMR, for diet planning. BMR is your calorie need at complete rest — eating at BMR means a severe deficit for most people, which leads to muscle loss and metabolic adaptation. TDEE accounts for your actual activity and is the correct baseline from which to calculate a calorie surplus or deficit.