Calculate your Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE) — the number of calories your body burns at rest and at your activity level.
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — the energy required to maintain basic functions: heartbeat, breathing, temperature regulation, cell repair and organ function. It represents approximately 60–70% of total daily energy expenditure for most people.
The most commonly used formula is the Mifflin-St Jeor equation, considered the most accurate for most adults:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Total Daily Energy Expenditure (TDEE) multiplies your BMR by an activity factor to estimate total daily calorie burn including exercise and movement. TDEE is the number you actually need to know for weight management — eating at TDEE maintains weight; below it loses weight; above it gains weight.
A widely misunderstood finding from a 2021 Science study analysing 6,400+ people found that metabolic rate is stable between ages 20 and 60 when adjusted for body size. The metabolic slowdown most people experience in middle age is not primarily caused by age-related cellular changes — it is caused by muscle mass loss and reduced activity.
Muscle tissue burns approximately 3× more calories at rest than fat tissue. Losing 5kg of muscle (which can happen over a decade of inactivity) reduces BMR by roughly 150–200 calories/day — equivalent to significant weight gain per year if intake stays constant. Resistance training is the most effective metabolic preservation strategy.
A genuine 0.7% per year decline in metabolic rate does begin from age 60 onward. This is modest but compounds over time — an additional reason to maintain muscle mass through consistent resistance exercise as we age.
A deficit of 500 calories per day produces approximately 0.5kg of weight loss per week (1lb). Deficits below 1,200 calories (women) or 1,500 calories (men) are generally not recommended without medical supervision, as they risk muscle loss and nutrient deficiencies.
A modest surplus of 200–500 calories above TDEE, combined with resistance training and adequate protein (1.6–2.2g/kg/day), supports muscle gain while minimising fat gain.
TDEE calculations carry approximately ±15% error for individuals. Rather than treating the number as precise, use it as a starting point, track your weight for 2–3 weeks at that intake, and adjust based on what your body actually does.