BMR Calculator

Basal Metabolic Rate represents the minimum calories your body requires to sustain life at complete rest. Think of it as your body's baseline operating cost—the energy needed just to keep you alive if you did absolutely nothing all day.

The Components of Your Daily Calorie Burn

Your total daily calorie expenditure splits into three main components:

Basal Metabolic Rate (60-75% of total): The largest portion goes to basic functions—heart beating, lungs breathing, brain operating, kidneys filtering, liver processing, cells dividing and repairing. A 30-year-old man weighing 180 pounds at 5'10" burns roughly 1,850 calories daily just existing. His heart pumps blood 100,000 times per day. His brain, despite being 2% of body weight, consumes 20% of his BMR. His liver processes nutrients continuously. All this happens whether he moves or not.

Physical Activity (15-30% of total): Everything from walking to your car to intense gym sessions. A sedentary person might add only 300-400 calories daily from activity. Someone moderately active adds 800-1,200 calories. Elite athletes training 2-3 hours daily can add 1,500-2,500 calories from exercise alone.

Thermic Effect of Food (8-10% of total): Your body burns calories digesting and processing food. Protein has the highest thermic effect—you burn 25-30% of protein calories just digesting it. Eating 100 grams of protein (400 calories) costs 100-120 calories to process. Carbs cost 5-10% to digest. Fats cost only 0-3%. This is why high-protein diets have a metabolic advantage.

For our 180-pound man with 1,850 BMR who's moderately active, his total looks like: 1,850 (BMR) + 900 (activity) + 230 (thermic effect) = 2,980 total daily calories burned.

Why BMR Varies So Much Between People

Two people with identical height, weight, age, and gender can have BMRs differing by 200-400 calories. Several factors explain this:

Muscle mass is the biggest variable. Muscle tissue burns 6-10 calories per pound daily at rest. Fat burns only 2-3 calories per pound. A 170-pound person with 140 pounds of lean mass (18% body fat) burns significantly more than someone 170 pounds with 120 pounds of lean mass (29% body fat). That 20-pound lean mass difference translates to 120-200 extra daily calories—over 800 calories weekly, equivalent to running 7-8 miles.

Genetics account for 20-30% variation. Some people are born with naturally higher metabolic rates. Studies on identical twins show genetic influence on BMR independent of body composition. If your parents have slower metabolisms, you likely do too. This isn't an excuse—you can still build muscle and optimize your BMR—but it explains why your friend eats 2,800 calories maintaining 160 pounds while you maintain the same weight at 2,200 calories.

Previous dieting history matters. Someone who chronically dieted for years can have a BMR 10-15% below predicted values. Their body adapted to conserve energy during repeated calorie restrictions. A person with a predicted 2,000 BMR who spent 5 years yo-yo dieting might actually have a 1,700-1,800 BMR. This is metabolic adaptation—partially reversible through reverse dieting and muscle building, but it takes time.

Thyroid function directly controls metabolism. Your thyroid gland produces hormones regulating metabolic rate. Hypothyroidism (underactive thyroid) can reduce BMR by 10-20%. Someone with a predicted 1,800 BMR might actually burn only 1,450-1,620 calories. Hyperthyroidism (overactive thyroid) does the opposite, raising BMR by 10-20%. If you're consistently not losing weight despite accurate calorie tracking and your BMR seems unusually low, get thyroid function tested.

The BMR Formulas: Which One and Why

Three main formulas calculate BMR, each with different origins and accuracy:

Mifflin-St Jeor (1990) is the most accurate for modern populations. Researchers measured metabolic rates in hundreds of subjects and developed equations accurate within 10% for most people. It accounts for the reality that Americans and Europeans today are heavier and carry different body compositions than populations from earlier studies. For a 30-year-old woman, 5'6", 145 pounds: BMR = (10 × 65.77kg) + (6.25 × 167.64cm) - (5 × 30) - 161 = 1,396 calories.

Harris-Benedict (1984 revision) updated the original 1919 formula. It tends to overestimate BMR by 5% for many people, particularly those who are overweight. Same woman using Harris-Benedict gets 1,419 calories—23 calories higher. Over a year, that's an 8,400-calorie difference (2.4 pounds). Not huge, but meaningful if you're tracking precisely.

Katch-McArdle uses lean body mass instead of total weight, making it most accurate if you know your body fat percentage. The formula: 370 + (21.6 × lean body mass in kg). Our 145-pound woman at 28% body fat has 104 pounds lean mass (47.6 kg), giving BMR = 370 + (21.6 × 47.6) = 1,398 calories. Almost identical to Mifflin-St Jeor. But if she's 145 pounds at 22% body fat (113 pounds lean = 51.5 kg), Katch-McArdle gives 1,482 calories—84 calories higher because she carries more metabolically active muscle.

Use Mifflin-St Jeor unless you have accurate body fat measurements from DEXA scan, Bod Pod, or hydrostatic weighing. Don't use body fat from home scales—they're too inaccurate (5-8% error margin) to make Katch-McArdle worthwhile.

You can't dramatically change your BMR overnight, but strategic long-term actions make meaningful differences. Here's what actually works versus popular myths:

Build Muscle: The Only Permanent Solution

Muscle tissue burns 6-10 calories per pound daily at rest. Fat tissue burns only 2-3 calories per pound. Gaining 10 pounds of muscle increases your BMR by 60-100 calories daily. That might not sound like much, but over a year that's 22,000-36,500 extra calories burned—equivalent to 6-10 pounds of fat.

A 150-pound person at 30% body fat (45 pounds fat, 105 pounds lean) might have a BMR around 1,550 calories. Through 12 months of consistent strength training and adequate protein, they could gain 12 pounds of muscle, reaching 162 pounds at 20% body fat (32 pounds fat, 130 pounds lean). Their BMR now sits at 1,700-1,750 calories—150-200 calories higher permanently, as long as they maintain that muscle.

This requires patience and commitment. Building significant muscle takes 6-12 months of proper training (3-5 sessions weekly focusing on progressive overload) and nutrition (eating adequate calories with 0.7-1g protein per pound body weight). But it's the only sustainable way to meaningfully raise BMR.

Prevent Age-Related BMR Decline

BMR naturally drops 1-2% per decade after age 20, primarily from muscle loss (sarcopenia). A sedentary person loses 3-5 pounds of muscle per decade. Someone with a BMR of 1,900 at age 25 might have a BMR of 1,700-1,800 by age 55 if inactive—a 100-200 calorie drop allowing 10-20 pounds of fat gain at the same calorie intake.

But this decline isn't inevitable. A 60-year-old who's strength trained consistently for 30 years can maintain the same lean body mass—and therefore similar BMR—as a 30-year-old. They might even have more muscle if they were sedentary in their youth but started training seriously at 35.

The prescription is simple: strength train 2-3 times weekly, every week, for life. Not negotiable if you want to maintain metabolic rate as you age. Someone who starts strength training at age 45 and builds 15 pounds of muscle by age 50 can have a higher BMR at 50 than they did at 40—bucking the typical decline.

Avoid Metabolic Adaptation from Chronic Dieting

Prolonged calorie restriction causes metabolic adaptation—your BMR drops beyond what's expected from weight loss alone. Someone losing 30 pounds would expect a 150-200 calorie BMR drop from reduced body mass. But if they achieved this through severe restriction (1,000 calories daily for 6 months), their BMR might drop 300-400 calories due to adaptation—an extra 100-200 calorie penalty.

Minimize this through:

• Moderate deficits: 300-500 calories below TDEE, never below BMR
• Diet breaks: Every 8-12 weeks, eat at maintenance for 1-2 weeks to give your body a metabolic reset
• High protein: 0.8-1g per pound body weight prevents muscle loss
• Strength training: Signals your body to preserve muscle despite calorie deficit
• Slower loss: Aim for 0.5-1% of body weight weekly, not 2-3 pounds weekly regardless of starting weight

Someone who lost 40 pounds over 12 months with diet breaks, strength training, and moderate deficits might have a BMR only 200 calories lower than predicted. Someone who crashed to lose the same 40 pounds in 4 months through extreme restriction could have a BMR 400 calories lower—twice the metabolic damage.

What Doesn't Work: Metabolism Myths

Let's dispel common myths about boosting BMR:

Eating frequent small meals doesn't increase BMR. Your thermic effect is determined by total calories and macros, not meal frequency. Eating 2,000 calories as 6 small meals or 2 large meals produces identical thermic effect. The "stoke the metabolic fire" concept is false. Meal frequency is personal preference.

Caffeine's effect is minimal and temporary. Caffeine can increase metabolic rate by 3-5% for a few hours. Someone with 2,000 calorie daily burn might get 60-100 extra calories from coffee—equivalent to a 10-minute walk. It's not nothing, but building 5 pounds of muscle accomplishes more permanently.

Spicy foods (capsaicin) have negligible impact. Studies show hot peppers might increase metabolism by 50 calories daily if you eat them with every meal. Most people won't sustain that level of spice intake. Not a practical strategy.

Green tea extract and metabolism supplements are oversold. Green tea might increase calorie burn by 30-50 calories daily—the equivalent of a 5-minute walk. Metabolism supplements claiming to "boost fat burning 300%" often show 3% increases in controlled studies. Save your money.

Cold exposure has minor effects. Shivering burns calories, but the uncomfortable level of cold needed to sustain significant thermogenesis isn't practical for most people. Taking cold showers might burn an extra 10-20 calories. Again, building muscle is more effective.

Focus on what matters: build muscle through strength training, maintain muscle as you age, avoid metabolic damage through sensible dieting practices. These create meaningful, permanent BMR increases measured in hundreds of calories—not the 20-30 calorie effects from supplements and tricks.