Every HealthTools calculator

10 calculators covering body composition, energy balance, training intensity, hydration and pregnancy dating. Every entry below states the formula, the study or guideline it came from, and the conditions under which it stops being reliable.

A tool that hands you “18.4% body fat” and nothing else is asking you to trust an anonymous script. It is more useful once you can see the figure came from the Hodgdon and Beckett circumference equations, that those were validated on US Navy personnel, and that they carry roughly 3–4% error against a DEXA scan. Where the research genuinely disagrees (ideal body weight has at least four competing equations still in clinical use) we run all of them and show the spread rather than quietly picking a favourite.

Body Composition 3 calculators Nutrition & Energy 3 calculators Fitness & Training 3 calculators Women's Health 1 calculator Wellness In development

Body Composition

Fat, lean mass and the ratios that predict health risk.

BMI Calculator

Body mass index with WHO and Asian-Pacific thresholds

BMI = weight (kg) ÷ height (m)²

Where it breaks down. BMI does not distinguish muscle from fat and misclassifies very muscular or very sedentary bodies.

Source: WHO Expert Consultation, Lancet 2004; WHO Technical Report Series 894 (2000)

Open calculator →

Body Fat Calculator

Body fat percentage from tape measurements

Men: %BF = 495 ÷ (1.0324 − 0.19077·log₁₀(waist − neck) + 0.15456·log₁₀(height)) − 450

Where it breaks down. Circumference equations carry roughly ±3–4% error against DEXA and were validated on military populations.

Source: Hodgdon & Beckett, Naval Health Research Center Report 84-29 (1984)

Open calculator →

Ideal Weight Calculator

Four clinical formulas, side by side

Devine (men): 50 kg + 2.3 kg per inch over 5 ft

Where it breaks down. These equations were built for drug dosing, not aesthetics, and ignore frame size and muscle mass.

Source: Devine, Drug Intell Clin Pharm (1974); Robinson et al. (1983); Miller et al. (1983)

Open calculator →

Nutrition & Energy

Energy needs, macronutrient targets and hydration.

TDEE Calculator

Daily calorie burn from BMR and activity

TDEE = BMR × activity multiplier (1.2 sedentary → 1.9 very active)

Where it breaks down. Predictive equations sit within about ±10% of measured expenditure for most people; track real intake to calibrate.

Source: Mifflin et al., Am J Clin Nutr (1990); Roza & Shizgal, Am J Clin Nutr (1984); Katch & McArdle (1996)

Open calculator →

Macro Calculator

Protein, carbs and fat targets by goal

Protein 1.6–2.2 g/kg · Fat ≥ 0.6 g/kg · Carbohydrate fills the remainder

Where it breaks down. Ranges suit healthy adults. Kidney, liver or metabolic conditions need clinician-set targets.

Source: Morton et al., Br J Sports Med (2018); Jäger et al., JISSN (2017)

Open calculator →

Water Intake Calculator

Daily fluid target adjusted for training and climate

Baseline ≈ 35 ml per kg body weight, +500–750 ml per hour of exercise

Where it breaks down. Heart failure, kidney disease and some medications require fluid restriction. Follow medical advice over any calculator.

Source: EFSA Panel on Dietetic Products, EFSA Journal (2010); ACSM Position Stand (2007)

Open calculator →

Fitness & Training

Heart rate zones, strength standards and workout output.

Heart Rate Zone Calculator

Five training zones from max or resting heart rate

Max HR = 208 − (0.7 × age) · Karvonen: target = ((maxHR − restHR) × %) + restHR

Where it breaks down. Age-predicted max heart rate varies by roughly ±10 bpm between individuals. A lab or field test beats any formula.

Source: Tanaka et al., J Am Coll Cardiol (2001); Karvonen et al., Ann Med Exp Biol Fenn (1957)

Open calculator →

One-Rep Max Calculator

Estimated 1RM plus a full percentage table

Epley: 1RM = w × (1 + reps ÷ 30) · Brzycki: 1RM = w ÷ (1.0278 − 0.0278 × reps)

Where it breaks down. Accuracy drops above about 10 reps, and estimates differ by lift. They track best on squat, bench and deadlift.

Source: Epley, Boyd (1985); Brzycki, JOPERD (1993); Wathen (1994); Lombardi (1989)

Open calculator →

Calories Burned Calculator

Energy cost of activity from MET values

kcal = MET × 3.5 × weight (kg) ÷ 200 × minutes

Where it breaks down. MET values are population averages and ignore fitness level, technique and terrain. Treat output as a ballpark.

Source: Ainsworth et al., Compendium of Physical Activities, Med Sci Sports Exerc (2011)

Open calculator →

Women's Health

Cycle tracking, ovulation windows and pregnancy dating.

Pregnancy Due Date Calculator

Estimated due date with full trimester timeline

Naegele's rule: LMP + 280 days, adjusted for cycle length other than 28 days

Where it breaks down. Only about 4% of births land exactly on the estimated date. First-trimester ultrasound dating is more accurate than any formula.

Source: ACOG Committee Opinion 700, Obstet Gynecol (2017)

Open calculator →

Wellness

Sleep timing, recovery load and daily habit targets.

This section is still being built. We publish a calculator once we have an equation we can cite and defend. The tools for this section are still in research, and we would rather hold them back than ship a scoring system we invented ourselves.

See what is planned

Why a figure drifts, and what to do next

Knowing your TDEE is 2,410 kcal is the easy part. Why that figure drops when you diet, why two equations disagree by 200 kcal, and why waist circumference predicts risk better than weight does all take explanation rather than arithmetic. The guides cover that ground.

If you spot an error, an outdated citation or a formula we have applied wrongly, tell us. Corrections are logged and the page is updated. Our editorial policy sets out how we source and review, and the medical disclaimer marks the boundary.

Read the guides Report a correction