💪 Health & Fitness
BMI, calorie, hydration, and fitness calculators — 26 free tools available. No signup required.
Calculators for energy needs, body composition, hydration, training and sleep — the numbers used to plan, not to diagnose.
Every one of these applies a published equation to figures you type, and each page names the equation, cites where it came from, and states the population it was derived from. That last part is the one usually omitted elsewhere and it is the one that determines whether a result applies to you.
None of this is medical advice, none of it is a diagnosis, and no calculator can account for your medical history. Anything that matters should be discussed with a clinician.
Where to start
- TDEE & Calorie Calculator — Mifflin-St Jeor with activity multipliers, and an explanation of why the multiplier is the weak link.
- Macro & Calorie Deficit Calculator — Splits a calorie target into protein, carbohydrate and fat with the reasoning shown.
- US Navy Body Fat Calculator — Uses the US Navy circumference method and states its real error margin.
- Daily Water Intake Calculator — Scales with body weight and activity rather than repeating the eight-glasses myth.
- Target Heart Rate Calculator — Training zones from measured or estimated maximum, with the estimate's spread made explicit.
- Running Pace & Split Calculator — Pace, time and distance in any direction, with split tables.
All health & fitness (26)
TDEE & Calorie Calculator
A TDEE calorie calculator using Mifflin-St Jeor for BMR, with targets for loss, maintenance or gain — and how wide…
Daily Water Intake Calculator
How much water to drink daily by weight, exercise and climate — and the part other calculators miss: the official…
Macro & Calorie Deficit Calculator
Daily protein, carb and fat targets from the Mifflin-St Jeor equation — and the only calculator that checks your…
US Navy Body Fat Calculator
Estimate body fat from a tape measure with the US Navy formula, cross-checked against the BMI method — and see the…
Ideal Body Weight (IBW) Calculator
An ideal body weight calculator comparing all four formulas plus adjusted body weight — and why they disagree by…
Target Heart Rate Calculator
Get your five training zones from a max heart rate formula that is actually validated — plus why 220 minus your…
Intermittent Fasting Tracker & Timer
Track your daily intermittent fast with live countdown timer, 16:8, 18:6, 20:4 protocol presets, and biological…
Sleep Cycle & Bedtime Calculator
A sleep calculator built on real 90–110 minute cycles, with the hours the AASM recommends by age and a flag on any…
Lean Body Mass (LBM) Calculator
A lean body mass calculator running three published equations, with the James formula dropped automatically past…
Calorie Burned Activity Calculator
A calorie burn calculator for 30+ activities using standard MET values — plus the number others omit: how much you…
Pregnancy Due Date Calculator
Calculate your estimated baby due date, current gestational age in weeks and days, and pregnancy trimester…
Blood Pressure Category Checker
A blood pressure checker that averages your readings the way the AHA asks, compares both arms, and sizes your…
Running Pace & Split Calculator
A running pace calculator for km and mile splits, treadmill speed, and race predictions that use Riegel's formula…
Alcohol Units, BAC Clearance & Calories Calculator
Calculate standard alcohol units, pure ethanol grams, drink calories, and estimated biological body clearance…
Workout Sweat Rate & Electrolyte Calculator
Calculate hourly fluid loss sweat rate (L/hr), percentage dehydration, and sodium electrolyte replacement…
Sleep Debt Deficit & Circadian Recovery Calculator
Calculate your cumulative weekly sleep debt deficit and generate an optimal circadian-friendly gradual recovery…
UV Index & Safe Vitamin D Sun Exposure Timer
Calculate safe sunlight exposure time in minutes for optimal Vitamin D synthesis (1,000–2,000 IU) based on UV…
Jet Lag Circadian Shift & Recovery Planner
Beat jet lag with a pre-flight sleep shift plan. Get day-by-day bedtime adjustments and light exposure timing for…
Waist-to-Hip Ratio Calculator
Calculate your waist-to-hip ratio in inches or centimeters and compare it to the WHO thresholds for increased…
One-Rep Max (1RM) Calculator
Estimate your one-rep max from any submaximal set using both the Epley and Brzycki formulas side by side, plus a…
VO2 Max Calculator
A VO2 max calculator using the Rockport 1-mile walk test or the Cooper 12-minute run test, both with published…
Reaction Time Test
Test your reaction time: wait for green, then click or press Space. Five rounds give your average, median, fastest…
BAC Calculator
Estimate blood alcohol concentration from drinks, weight, sex and time with the Widmark formula, plus a range and…
Army Body Composition Calculator (WHtR)
Check the Army waist-to-height ratio standard: enter height and waist to see your WHtR, whether it is under 0.55…
White Noise Generator
Play white, pink or brown noise generated live in your browser, with a tone control and sleep timer that fades…
Steps to Miles Converter
Convert steps to miles or kilometers — and miles to steps — using your height or your own measured step length.
BMI was designed for populations, not people
Body mass index divides weight by height squared. It was devised to describe populations and it does that job adequately. Applied to an individual it cannot distinguish muscle from fat, ignores where fat is distributed — which is the part most associated with metabolic risk — and systematically misclassifies muscular people as overweight and people with low muscle mass as healthy. It also behaves differently across ethnic groups, which is why several health bodies use adjusted thresholds. It is a rough screening number and nothing more; waist circumference and body fat percentage carry more information about an individual.
The activity multiplier is where the error lives
Resting metabolic rate equations such as Mifflin-St Jeor are reasonably accurate — typically within about 10% for most people. The large uncertainty enters when that figure is multiplied by an activity factor chosen from a short list of descriptions, because people consistently overestimate which bracket they are in and the brackets themselves are wide. The practical approach is to treat any TDEE figure as a starting estimate, hold it for two or three weeks, and adjust based on what actually happens to your weight. That measured feedback is worth more than any equation.
Hydration does not have a single number
The "eight glasses a day" figure has no clinical basis and is not the recommendation of any major health body. Actual requirements vary with body size, activity, ambient temperature, humidity and diet — food contributes a substantial share of total intake, which fixed-glass advice ignores entirely. Sweat rates differ several-fold between individuals doing identical exercise. Thirst is a reasonable guide for most people in ordinary conditions, and for sustained exercise in heat, measuring weight change across a session is far more informative than any formula.
Maximum heart rate estimates have a wide spread
The familiar 220 minus age has a standard deviation of roughly ten to twelve beats per minute, meaning a substantial fraction of people sit more than twenty beats from their predicted value. Training zones derived from an estimated maximum inherit all of that error, so a zone boundary calculated this way can easily be wrong by a zone. Newer formulas fit the data slightly better but do not fix the fundamental spread. If zone training matters to you, a measured maximum or a lactate threshold test is a different order of accuracy.
Frequently asked questions
Is any of this medical advice?
No. These are calculators that apply published equations to numbers you enter. They cannot account for your medical history, medication or individual circumstances. Consult a qualified clinician for anything that matters.
Is my health data stored?
No. Every calculation runs in your browser. Weight, height, age and everything else you enter stay on your device and are never transmitted.
Which equation does the calorie calculator use?
Mifflin-St Jeor for resting metabolic rate, which is the equation most commonly recommended for general use. The page names it, shows the formula and cites the source.
Why do two calculators give me different numbers?
Because they use different published equations, derived from different study populations. The spread between them is real and is a useful indication of how much uncertainty any single figure carries.
Is BMI useful at all?
As a population-level screening measure, yes. For an individual it is a crude indicator that cannot separate muscle from fat. Treat it as one input among several rather than a verdict.