BMR Calculator by Age — How Metabolism Changes Over Time

What BMR Measures

Basal Metabolic Rate (BMR) is an estimate of how many calories your body would burn in a 24-hour period if you stayed completely at rest — awake, lying still, comfortable room temperature, in a post-absorptive state (not digesting a recent meal). BMR represents the energy cost of keeping all your vital systems running: the heart, lungs, brain, liver, kidneys, and every other organ and tissue performing the thousands of chemical reactions required simply to stay alive.

BMR is the single biggest component of total daily energy expenditure for most adults, typically accounting for 60 to 75 percent of total calories burned each day. The rest comes from non-exercise activity thermogenesis, purposeful exercise, and the thermic effect of food. Knowing your BMR helps you design realistic calorie targets for weight change, maintenance, or athletic performance fueling.

The Two BMR Formulas

This calculator presents two of the most widely used BMR estimation formulas side by side so you can compare. Neither is perfect — both estimate within a typical population error band of about ±5 to 10 percent for individuals where no measured metabolic data is available.

1. Mifflin-St Jeor Equation (Published 1990)

The Mifflin-St Jeor equation was published in 1990 by Mifflin et al. at Oregon State University after reviewing 114 studies and validating against a large sample of measured indirect calorimetry results. Multiple independent reviews, including a 2005 analysis in the Journal of the Academy of Nutrition and Dietetics, have concluded Mifflin-St Jeor is generally the most accurate population-level formula for most modern adults.

2. Harris-Benedict Equation (Revised 1984 by Roza & Shizgal)

The original Harris-Benedict equation dates to 1918-1919, developed by James Arthur Harris and Francis Gano Benedict using calorimetry data from 239 men and women. A 1984 revision by Roza and Shizgal updated the constants using more modern measurement techniques on a larger sample. The revised Harris-Benedict formula remains common in many evidence-based and older software systems, though it typically produces results slightly different from Mifflin-St Jeor depending on age and body size.

Worked Example

For a 30-year-old male, 175 cm, 70 kg. Mifflin-St Jeor: (10 × 70) + (6.25 × 175) − (5 × 30) + 5 = 700 + 1093.75 − 150 + 5 = 1648.75 kcal/day ≈ 1649 kcal/day. Revised Harris-Benedict: 88.362 + (13.397 × 70) + (4.799 × 175) − (5.677 × 30) = 88.362 + 937.79 + 839.825 − 170.31 ≈ 1696 kcal/day. The two formulas give answers about 47 kcal apart for this example — a difference of roughly 3 percent.

FAQ-Style Explanations

Why are the two formulas different? They were derived on different populations at different times using different measurement equipment. Harris-Benedict was developed on a small, mostly European-origin sample in the early 1900s. Mifflin-St Jeor was developed in 1990 using a larger and more diverse sample. The differences reflect the underlying data more than any deep disagreement about human metabolism.

Which one should I use? For most adults, use the Mifflin-St Jeor number as your primary estimate. If you already know from experience how many calories you need to maintain your current weight, compare it to both formulas and pick whichever is closer to your personal reality. If you are very muscular, very elderly, or have a medical condition affecting metabolism, both formulas will be less accurate and a measured resting metabolic rate test would give you a better answer.

Does BMR decrease with age? Yes, on average. After roughly age 20, BMR declines approximately 2 to 3 percent per decade in most people. This decline is largely driven by sarcopenia — the gradual loss of muscle mass that occurs with aging when strength training is not maintained. Regular resistance training and adequate protein intake can slow this decline substantially and preserve metabolic rate into older age.

Known Limitations

• Estimates only. Real resting metabolic rate can only be measured directly via indirect calorimetry or a doubly labeled water study.
• Do not account for body composition. Two people with identical height, weight, age, and sex can have very different BMR if one has much more muscle than the other.
• Do not account for thyroid function, regular health products, fever, pregnancy, lactation, or metabolic conditions.
• Less accurate at the extremes of body size (very high or very low BMI) and for people of non-European ethnic backgrounds where the original validation samples were thin.
• BMR is not a target to eat at — it is the minimum. Almost everyone needs significantly more calories than BMR in daily life.

Why BMR Declines With Age

Basal Metabolic Rate tends to fall by roughly 1 to 2 percent per decade from early adulthood onward, but age itself is only part of the story. The decline is driven mainly by two forces working together: the gradual loss of skeletal muscle (sarcopenia) and shifting hormone levels. Because the Mifflin-St Jeor formula includes an explicit age term that subtracts 5 calories per year of age, the calculated BMR drops automatically as you get older. But the larger, slower effect comes from losing metabolically active lean mass — muscle burns more calories at rest than fat, so as muscle fades, resting metabolism fades with it. The good news is that the muscle-loss portion of this decline is largely preventable with resistance training, which is why active older adults often maintain BMR far above the typical curve.

BMR Across the Decades

The table below shows approximate BMR for a 175 cm (5 ft 9 in) adult of average build at different ages, using the Mifflin-St Jeor formula. These are illustrative reference points, not predictions for any individual — actual BMR depends heavily on weight, lean mass, and activity history. Notice how the age term and the assumed gradual weight and muscle shifts combine to lower BMR over time, and how resistance training can keep an active adult well above the sedentary curve at the same age.

Muscle Loss and Hormone Changes

Sarcopenia — the gradual loss of skeletal muscle — begins as early as the 30s and accelerates after 60 in people who do not train. Since muscle is the most metabolically active tissue at rest, losing it directly lowers BMR. Hormone shifts compound the effect: declining testosterone in men and shifting estrogen around menopause in women both contribute to a reduction in lean mass and a redistribution of fat. The age term in the Mifflin-St Jeor formula captures part of this statistically, but the real-world driver is body composition. Two 55-year-olds of the same weight can have very different BMRs depending on how much muscle each carries.

Slowing the Decline With Strength Training

• Resistance training two to four times per week is the most effective way to preserve and build muscle, which is the largest controllable factor in BMR.
• Adequate protein — 1.2 to 1.6 grams per kilogram of body weight, rising for those who train — supports lean mass and the thermic effect of food.
• Stay active throughout the day. Steps and light movement add to total expenditure and help maintain muscle.
• Prioritize sleep and avoid extreme diets, both of which protect resting metabolism from the adaptation that lowers BMR over time.