BMR Calculator

What BMR Actually Means?

BMR is the number of calories your body burns doing absolutely nothing — in a temperature-controlled room, lying still, 12+ hours after your last meal. It accounts for approximately 60–75% of total daily energy expenditure for sedentary people. Breathing, heartbeat, brain activity, cell repair, and organ function all consume this energy continuously.

What's the difference between Mifflin-St Jeor and Harris-Benedict?

Mifflin-St Jeor (1990): Male: (10×weight) + (6.25×height) − (5×age) + 5. Female: same minus 161. This is considered the most accurate for most people in studies comparing to measured metabolic rate. Harris-Benedict (revised 1984) tends to overestimate by 5% on average, particularly for sedentary individuals.

What should I know about BMR Is Not TDEE?

BMR is the foundation — to get your Total Daily Energy Expenditure (TDEE), multiply BMR by an activity multiplier (1.2 for sedentary to 1.9 for very active). Eating at BMR level would put most people in a significant deficit. Use the TDEE calculator for your actual calorie needs.

What Affects BMR?

Muscle mass increases BMR — each kilogram of muscle burns approximately 13 kcal/day at rest. Age decreases BMR by roughly 1–2% per decade after 30, partly because of muscle loss. Fasting, very low calorie diets, and extreme weight loss can reduce BMR by 15–40% through adaptive thermogenesis — the metabolic adaptation studied in post-diet 'rebound' research.

What's the difference between BMR and RMR?

Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR) are often used interchangeably, but they are measured under slightly different conditions. BMR is measured in a strict laboratory setting: after 8 hours of sleep, a 12-hour fast, complete physical rest, and a controlled room temperature, with the person lying still and awake. RMR is measured under more relaxed, realistic conditions — no overnight stay required, a shorter fast, and the person simply resting quietly rather than meeting all of BMR's strict criteria. Because of this, RMR readings are typically 5-10% higher than true BMR, since the body is never quite as fully 'at rest' as the laboratory BMR protocol demands. In everyday use, including on this calculator, the terms are used loosely to mean the same thing: the energy your body burns at rest to keep the heart beating, lungs breathing, brain functioning, and cells repairing themselves. The formulas used to estimate it here (Mifflin-St Jeor and similar equations) are technically estimating RMR from easily measured variables like weight, height, age and sex, since true BMR can only be measured directly with specialised equipment such as indirect calorimetry.

What do I need to know about How BMR Changes With Age and Body Composition?

BMR declines by roughly 1-2% per decade after early adulthood, mainly because of a gradual loss of lean muscle mass (sarcopenia) and, to a lesser extent, changes in hormone levels. Muscle tissue is metabolically expensive to maintain — even at rest it burns considerably more energy per kilogram than fat tissue — so someone who loses muscle while gaining fat can see their BMR drop even if their total body weight stays the same. This is a major reason many people find they need to eat less, or exercise more, to maintain the same weight in their 40s and 50s as they could in their 20s. Resistance training is the most effective tool for countering this decline, because building or preserving muscle mass directly raises resting energy expenditure; this is one reason strength training is now recommended by health bodies at every adult age, not just for younger athletes. Other factors affecting BMR include thyroid function (an underactive thyroid lowers it, an overactive one raises it), pregnancy and breastfeeding (which raise it), and extended very-low-calorie dieting (which can lower it as the body adapts to conserve energy) — this adaptive drop is a well-documented reason weight loss often slows well before someone reaches their target weight.

What should I know about BMR and Thyroid Function?

The thyroid gland is one of the most powerful regulators of basal metabolic rate, because thyroid hormones (T3 and T4) directly control how fast cells throughout the body use energy, even at rest. An underactive thyroid (hypothyroidism) slows metabolism measurably, which is why unexplained weight gain, fatigue and feeling persistently cold are classic symptoms — the BMR genuinely drops below what a formula based on age, sex, height and weight alone would predict. Conversely, an overactive thyroid (hyperthyroidism) raises BMR, often causing unintended weight loss, a faster resting heart rate and feeling persistently warm or sweaty, despite no change in diet or activity. This is why standard BMR formulas (Mifflin-St Jeor, Harris-Benedict) are reasonable population-level estimates but can be significantly inaccurate for someone with an undiagnosed or poorly controlled thyroid condition — the formula has no way to account for a hormonal factor that directly changes cellular energy use. Anyone experiencing unexplained, persistent weight change alongside fatigue, temperature sensitivity or heart rate changes should raise this with a GP, since a simple blood test can check thyroid function.

Worked Example: Calculating BMR With the Mifflin-St Jeor Equation

Applying the Mifflin-St Jeor equation step by step shows exactly how each input — weight, height, age, and sex — contributes to the final BMR figure, and why two people of very similar size can still end up with noticeably different results.

Worked example: for a 35-year-old man weighing 70kg and 170cm tall, the Mifflin-St Jeor formula is (10 × 70) + (6.25 × 170) − (5 × 35) + 5. That's 700 + 1,062.5 − 175 + 5, giving a BMR of approximately 1,592.5 kcal per day. For a woman of the same age, weight and height, the formula subtracts 161 instead of adding 5, giving 700 + 1,062.5 − 175 − 161 = 1,426.5 kcal per day — around 166 kcal lower, reflecting the average difference in body composition (typically more lean mass relative to body weight in men) that the sex term is designed to approximate. This BMR figure is then multiplied by an activity factor (commonly 1.2 for sedentary up to around 1.9 for very active) to estimate total daily energy expenditure, which is a separate step covered in more detail by a dedicated TDEE calculation.

Why Do Online BMR Results From Different Calculators Sometimes Disagree?

Two BMR calculators given identical weight, height, age and sex inputs can still return different results if they use different underlying formulas — Mifflin-St Jeor and Harris-Benedict, both referenced above, are not the only equations in use, and some tools still default to the older, generally less accurate Harris-Benedict version without clearly labelling which formula is being applied. Differences of 50-150 kcal between calculators using different formulas for the same person are common and are not a sign that either calculator is "wrong" as such, just that they are answering the question using different validated equations with different historical accuracy records. Checking which formula a given calculator states it uses is the most reliable way to understand why results vary between tools, rather than assuming one figure is simply more trustworthy than another without knowing the method behind it.

Can Crash Dieting Actually Lower BMR Over Time?

Prolonged, severe calorie restriction can lead to a genuine reduction in BMR beyond what would be predicted purely from the resulting weight loss, a phenomenon often referred to as adaptive thermogenesis or "metabolic adaptation." This happens partly because the body responds to a sustained large energy deficit by reducing energy expenditure in ways not fully explained by the loss of body mass alone, including changes in thyroid hormone output and nervous system activity. This is one of the key reasons very aggressive, rapid weight-loss diets often become progressively harder to sustain and are more prone to weight regain once normal eating resumes, compared with a more moderate, sustained calorie deficit — the body's compensatory response to extreme restriction tends to be more pronounced than its response to a smaller, more gradual one.