Health & Fitness Calculators
Health calculators built on established formulas: BMI classified with WHO categories, BMR from the Mifflin-St Jeor equation, TDEE from activity multipliers, body fat from the US Navy circumference method, and pregnancy due dates from Naegele's Rule. They give estimates for general information, not diagnoses.
Key Metrics Explained📊
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Basal Metabolic Rate (BMR): the energy your body needs at complete rest, calculated from sex, age, weight and height. A 30-year-old man of 80 kg and 180 cm has a BMR of 10 × 80 + 6.25 × 180 - 5 × 30 + 5 = 1,780 kcal per day.
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Total Daily Energy Expenditure (TDEE): BMR multiplied by an activity factor, 1.2 for sedentary up to 1.9 for extremely active. For the same man, 1.2 gives 2,136 kcal and 1.55 (moderately active) gives 2,759 kcal.
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Body fat by the US Navy method: an estimate from neck, waist and (for women) hip circumference. It typically lands within about 3 to 4 percentage points of reference methods, enough to track changes over time without lab equipment.
Where the Formulas Come From
The Mifflin-St Jeor equation (1990) was derived from about 500 healthy adults and is now the most widely recommended equation for resting energy expenditure. A 2005 systematic review in the Journal of the American Dietetic Association (Frankenfield and colleagues) found it the most reliable of the common equations, within 10% of measured resting metabolic rate for most people, though not for everyone. It replaced the Harris-Benedict equation (1919), which came from a sample of about 240 people. BMI is classified with the WHO categories: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or more is obesity. BMI says nothing about muscle, fat distribution or health on its own, so treat it as a screening number.
Why Precision Matters in Health Tracking
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Measure consistently: weigh and measure at the same time of day under the same conditions, for example in the morning before eating, so results are comparable between sessions.
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Know the limits: BMI cannot separate muscle from fat, so it overstates risk for very muscular people. The Navy body fat method helps, but a very muscular neck or waist can skew it too.
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Compare with published ranges from WHO, NIH or CDC, and treat the result as a starting point for a conversation with a healthcare provider, not as a replacement for blood tests or an examination.
Questions and Answers
Why is the Mifflin-St Jeor formula preferred over Harris-Benedict for BMR?
Mifflin-St Jeor predicts measured resting metabolic rate better than the older equations in most comparisons. The Harris-Benedict equation dates from 1919 and a sample of about 240 people, while Mifflin-St Jeor (1990) used about 500 healthy adults measured with modern methods. The 2005 review by Frankenfield and colleagues in the Journal of the American Dietetic Association rated Mifflin-St Jeor the most reliable common equation, within 10% of measured values for most people. No equation is exact for an individual, so use the result as a starting point and adjust it against your real weight change over 2 to 4 weeks.
How does BMI differ from body fat percentage in practical health assessment?
BMI is weight in kilograms divided by height in meters squared, a screening number that relates to body fat across a population but does not measure fat or muscle. A 100 kg, 180 cm athlete has a BMI of 30.9, in the obesity range, despite possibly low body fat. Body fat percentage, for example from the US Navy method, estimates how much of your weight is fat, and it suits tracking changes during training or dieting. Use both: BMI for a quick screen and body fat or waist measurements for detail.
What is the margin of error for the US Navy Body Fat method?
The Navy method typically lands within about 3 to 4 percentage points of reference methods for most people, and the error can be larger for very lean, very muscular or very obese people. DXA scanning is a common reference method but needs a clinic and is not perfect itself. The Navy method works best for tracking direction over time: measure the same way every few weeks and watch the trend instead of a single number.
How is the Estimated Date of Delivery (EDD) calculated?
The due date comes from Naegele's Rule: EDD = first day of the last menstrual period (LMP) + 280 days, which equals 40 weeks. The rule assumes a 28-day cycle with ovulation on day 14. A longer or shorter cycle moves ovulation, so the true date shifts by the difference from 28 days. Early ultrasound dating is more accurate than LMP, and ACOG guidance lets an early ultrasound replace the LMP date when the two differ by more than about a week. Only about 4 to 5% of babies are born on their due date, and most arrive within two weeks on either side.
What is TDEE and why does it matter more than BMR alone for nutrition planning?
TDEE is your estimated daily calorie burn including activity, and it is the number you plan eating around. It equals BMR times an activity factor: sedentary 1.2, lightly active 1.375, moderately active 1.55, very active 1.725, extremely active 1.9. A man with a BMR of 1,780 kcal burns about 2,759 kcal at 1.55. Planning a deficit from BMR alone ignores activity and gives a deficit much larger than intended, which is hard to sustain and can cost muscle. These factors are averages, so check your result against your weight trend.
Should these results be used for self-diagnosis or to replace medical advice?
No. The calculators give reference values for personal awareness and planning, and they do not diagnose anything. Results for BMI, body fat or calorie needs should be read together with blood tests, medical history and a physical examination by a licensed healthcare provider. See a doctor if you have symptoms, are pregnant, are under 18, or plan a large change in diet or weight.