The Complete Guide to Health & Fitness Metrics

Body Mass Index, or BMI, is one of the most widely used screening tools in health and medicine. It is calculated by dividing a person's weight in kilograms by the square of their height in metres (kg/m²). The resulting number places individuals into categories: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above).

While BMI is a useful initial indicator of whether someone's weight may be putting their health at risk, it has well-documented limitations. It does not directly measure body fat and can misclassify muscular individuals as overweight. It also does not account for age, sex, ethnicity, or where fat is distributed on the body. Despite these limitations, BMI remains a valuable population-level metric used by healthcare providers around the world.

For the most accurate health picture, BMI should be considered alongside other measurements such as waist circumference, body fat percentage, and blood pressure. Our free BMI Calculator gives you an instant reading along with an interpretation of what your result means.

Your Basal Metabolic Rate (BMR) is the number of calories your body needs to maintain basic physiological functions — breathing, circulation, cell production — while at complete rest. It represents the minimum energy your body requires just to stay alive and accounts for approximately 60–75% of total daily calorie expenditure for most people.

Total Daily Energy Expenditure (TDEE) builds on BMR by factoring in physical activity. It is calculated by multiplying your BMR by an activity multiplier that ranges from sedentary (desk job, little or no exercise) to extra active (very hard exercise, physical job). Understanding your TDEE is fundamental to any weight management goal — whether you want to lose, maintain, or gain weight, you must first know how many calories your body actually burns each day.

The Mifflin-St Jeor equation is widely considered the most accurate formula for estimating BMR. Use our BMR Calculator and TDEE Calculator to find your personalised energy needs.

Body fat percentage is the proportion of your total body weight that is made up of fat mass. Unlike BMI, it directly measures body composition and provides a much clearer picture of health and fitness. Essential fat — the fat required for normal physiological function — makes up approximately 2–5% of body weight in men and 10–13% in women. Athletes typically have 6–13% (men) or 14–20% (women), while healthy fitness ranges extend to 24% for men and 31% for women.

Excess body fat, particularly visceral fat around the abdominal organs, is strongly associated with increased risk of type 2 diabetes, cardiovascular disease, metabolic syndrome, and certain cancers. Monitoring body fat percentage over time is therefore one of the most meaningful ways to track the impact of diet and exercise changes.

Common estimation methods include skinfold calipers, bioelectrical impedance analysis (BIA), and the U.S. Navy and U.S. Army circumference methods. Our Body Fat Calculator uses the Navy method for a convenient, equipment-free estimate.

Macronutrients — protein, carbohydrates, and fat — are the three primary categories of nutrients that supply your body with energy. Each plays a distinct and essential role. Protein (4 calories per gram) is critical for building and repairing muscle tissue, producing enzymes and hormones, and supporting immune function. Carbohydrates (4 calories per gram) are the body's preferred fuel source, especially during high-intensity exercise. Dietary fat (9 calories per gram) is essential for hormone production, vitamin absorption, and brain health.

The ideal macronutrient ratio varies depending on your goals. Someone focused on fat loss might follow a higher-protein, moderate-fat, lower-carbohydrate approach. An endurance athlete may need a higher proportion of carbohydrates to fuel long training sessions. Someone looking to build muscle will prioritise adequate protein intake — typically 1.6–2.2 grams per kilogram of bodyweight — while maintaining a calorie surplus.

Use our Macro Calculator and Protein Calculator to dial in your nutrition targets based on your specific body stats and goals.

Pregnancy is a profound physiological journey that spans approximately 40 weeks from the first day of the last menstrual period (LMP). Knowing your estimated due date helps you and your healthcare provider plan antenatal care, schedule key screenings, and prepare for birth. The most common method for estimating due date is Naegele's Rule: add one year, subtract three months, and add seven days to the first day of your LMP.

Healthy weight gain during pregnancy is important for both mother and baby. The Institute of Medicine (IOM) recommends different ranges depending on pre-pregnancy BMI — underweight women may need to gain more, while those who are overweight should gain less. Excessive weight gain increases the risk of gestational diabetes, hypertension, and caesarean delivery, while insufficient gain can lead to low birth weight.

Our pregnancy tools — including the Due Date Calculator, Ovulation Calculator, and Pregnancy Weight Gain Calculator — are designed to give expectant parents clear, evidence-based information at every stage.

Your heart rate during exercise is one of the most reliable indicators of training intensity. Maximum heart rate (MHR) is most commonly estimated using the formula 220 minus your age. Training zones — defined as percentages of MHR — dictate which energy systems your body uses and what physiological adaptations occur. Zone 2 training (60–70% MHR) builds aerobic base and metabolic efficiency. Zone 4–5 training (85–100% MHR) builds speed, power, and VO2 max.

Understanding your target heart rate zones enables smarter, more purposeful training. Use our Target Heart Rate Calculator to find the exact beats-per-minute ranges for each training zone.