BMR Calculator: Basal Metabolic Rate & TDEE

BMR Calculator: Basal Metabolic Rate and TDEE

BMR is what you'd burn lying still all day, about 60-75% of total expenditure; TDEE adds activity on top. Mifflin-St Jeor predicts BMR within ±10% from height and weight; Katch-McArdle tightens that to ±5% with a real body-fat number. The calculator runs both.

How to use this calculator

Enter age, biological sex, height, and weight. Morning measurements before eating are most consistent because hydration and food intake shift weight by 1-3 pounds across the day. If you have a body-fat percentage from a DEXA scan, calipers, or a smart scale, switch to Katch-McArdle and enter the value; that runs the calculation from lean body mass instead of total weight, which usually tightens the estimate.

Activity level:

Select from the dropdown. Most desk workers who exercise three to four times per week fall into Lightly Active; overestimating activity is the most common input error and inflates TDEE by 100-200 calories. When in doubt, choose one level lower than you think.
When to recalculate:

Update inputs when weight changes by 10-15 pounds, when activity level shifts (new job, injury, training change), or after 8-12 weeks in a deficit (metabolic adaptation can pull true BMR below the formula prediction).
Scope and limits:

Mifflin-St Jeor and Katch-McArdle formulas. TDEE = BMR × activity multiplier. Estimates within ±10% (Mifflin) or ±5% (Katch with measured body fat). Validated for adults 18-65 in typical body composition (15-30% body fat); accuracy decreases at extremes and for adolescents, pregnancy, post-bariatric patients, or uncontrolled thyroid conditions. All calculations run locally; no data is stored.

How each input affects BMR

Each Mifflin-St Jeor input contributes a fixed weighted slice of the answer; Katch-McArdle replaces total weight with lean body mass, which is why a single accurate body-fat number can move BMR by hundreds of calories.

Age:

Mifflin subtracts 5 calories per year of age; a 20-year age gap shifts BMR by ~100 calories. The textbook 2%-per-decade decline is being reassessed: Pontzer et al. (Science, 2021) found energy expenditure is roughly flat from 20 to 60 once body composition is held constant. Most apparent 'slowing metabolism' is muscle loss, not aging metabolism.
Sex (biological):

Mifflin gives men +5 and women −161 calories. Same height, weight, and age, the male prediction is ~166 calories higher because of typical lean-mass and hormonal differences. If you're on cross-sex hormone therapy, use the sex matching your current hormonal profile, or switch to Katch-McArdle, which is sex-agnostic.
Height:

6.25 calories per centimeter; about 16 calories per inch. Tall bodies simply have more tissue to maintain. Measure in the morning; you're slightly shorter by evening from spinal compression.
Weight:

Roughly 10 calories per kilogram in Mifflin (about 45 calories per 10 lb), the largest single driver. Weigh in the morning, after the bathroom, before eating, in minimal clothing, and use a 3-7 day average to filter water-weight noise.
Body fat % (Katch-McArdle):

Converts total weight to lean body mass (LBM = Weight × (1 − Body Fat %)). Only metabolically active tissue counts. Worked example: two people both at 180 lb, one at 15% body fat (153 lb lean / 69.4 kg) lands at BMR ≈ 1,870 cal; the other at 30% body fat (126 lb lean / 57.2 kg) lands at BMR ≈ 1,605 cal. Difference is about 265 cal/day. Measurement error matters: DEXA ±1-2%, hydrostatic ±2-3%, calipers ±3-4%, bioimpedance scales ±4-8%.
Activity level (for TDEE):

Multiplies BMR to estimate total daily burn. Sedentary 1.2 (desk job, under 5,000 steps/day, no exercise). Lightly Active 1.375 (5,000-7,500 steps or light exercise 1-3×/week). Moderately Active 1.55 (7,500-10,000 steps or moderate exercise 3-5×/week). Very Active 1.725 (10,000+ steps or hard exercise 6-7×/week). Athlete 1.9 (physical job plus daily training or 2×/day). Most desk-bound regulars overestimate by one tier; if you have a desk job and lift four times a week, you're Lightly Active, not Very Active.

The formulas

Two peer-reviewed equations cover the calculator. Each has a validated accuracy band and a specific use case.

Mifflin-St Jeor (1990):

$\text{BMR}_{\text{men}} = 10W + 6.25H - 5A + 5 \quad \text{Women: } \text{BMR} = 10W + 6.25H - 5A - 161$
W = weight in kg, H = height in cm, A = age in years. Validated against indirect calorimetry; predicts within ±10% in roughly 80% of adults in normal body composition. Best when you don't have a body-fat number.
Katch-McArdle (Cunningham 1991):

$\text{BMR} = 370 + 21.6 \times \text{LBM}$
LBM = Weight (kg) × (1 − Body Fat %). Predicts within ±5% when body fat is measured accurately. Best for athletes and anyone whose body composition departs from the population average.
TDEE:

$\text{TDEE} = \text{BMR} \times \text{Activity Multiplier}$
Activity multipliers: Sedentary 1.2, Light 1.375, Moderate 1.55, Very Active 1.725, Athlete 1.9. TDEE is the number you actually use for diet planning, not BMR.
Goal-based calorie targets:

Weight loss: Target = TDEE − (250 to 500)
Maintenance: Target = TDEE
Muscle gain: Target = TDEE + (250 to 500)
A 500 cal/day deficit yields roughly 1 lb/week of weight loss. Larger deficits (750+) increase muscle-loss risk and need higher protein (about 1 g per lb of body weight). Eating below BMR for extended periods drives metabolic adaptation; medically supervised very-low-calorie diets are an exception, not a template.

What your BMR and TDEE numbers mean

Two outputs do most of the work. The third is more situational.

BMR:

Your survival floor: what you'd burn lying still all day. Typical ranges are 1,200-1,600 for women, 1,400-1,900 for men. If your calculated BMR sits well below those (under 1,200 for a woman, under 1,400 for a man) and you've been dieting, that may be metabolic adaptation rather than your true baseline. Don't eat below BMR for extended periods; medically supervised very-low-calorie diets are an exception, not a template.
TDEE:

Your actual daily burn including activity, and the number you actually diet from. Typical ranges: sedentary adults 1,600-2,200; active adults 2,000-3,000; athletes 2,500-4,500+. Calibrate it: eat at calculated TDEE for two weeks. If 7-day average weight is stable within ±1 lb, the number is right. If you're gaining, reduce TDEE by 10%; if losing, increase by 10%.
Metabolic age:

With Mifflin-St Jeor this just returns your chronological age (the formula is deterministic on age). With Katch-McArdle it compares your lean-mass profile to typical body composition at different ages: lower means more muscle relative to age peers. The only ways to improve it are to add muscle and reduce body fat.
Macro targets:

Calories drive weight change; macro split shapes how that change happens. Hit protein first (0.7-1 g per lb of body weight). Carbs and fat are flexible based on training and preference. Example: 1,800 cal at a 40/30/30 split = 180 g protein, 135 g carbs, 60 g fat.

Where BMR estimates can mislead

Even an accurate formula produces misleading numbers when inputs or assumptions don't match reality. The common failure modes:

Overstated activity level:

The most common error. A 30-minute gym session four times a week is not Very Active; that multiplier assumes hard daily training plus an active job or lifestyle. Most desk-bound regulars are Lightly to Moderately Active. When in doubt, drop a tier; overestimation typically inflates TDEE by 100-200 calories.
Body-fat measurement error:

Katch-McArdle is only as accurate as your body-fat number. A 5-percentage-point error in body fat creates roughly a 100-calorie error in BMR. Bioimpedance scales (±4-8%) are convenient but noisy; if that's your tool, use a 7-day average rather than a single reading.
Metabolic adaptation:

If you've been dieting 8+ weeks or have a yo-yo dieting history, your real BMR may sit 10-15% below the formula prediction. Signs: stalled weight loss despite accurate tracking, persistent cold, fatigue, poor sleep. Reverse-dieting back to maintenance for 4-8 weeks usually reopens the response.
Medical conditions and medications:

Hypothyroidism reduces BMR 10-20%; hyperthyroidism increases it 15-25%. PCOS, Cushing's syndrome, and beta-blockers, SSRIs, or corticosteroids also shift metabolism. Use the formula as a starting point and recalibrate against 2-4 weeks of weight tracking.
Body-composition extremes:

Both formulas are validated for typical body composition (15-30% body fat). Below 10% (lean athletes), Mifflin underestimates and Katch-McArdle is the right tool. Above 40%, both formulas tend to overestimate because excess fat is metabolically near-inert.
Day-to-day weight noise:

Don't adjust TDEE on a single day's weight. Water alone fluctuates 2-5 lb from sodium, carbs, menstrual cycle, and stress. Use a 7-day rolling average and look at 2-3 week trends before concluding the formula is off.

FAQ

What is Basal Metabolic Rate (BMR)?

BMR is the calories your body burns at complete rest to maintain vital functions: breathing, circulation, cell production, and brain activity. It represents 60-75% of your total daily calorie expenditure. Think of it as the "minimum operating cost" of keeping your body alive, the calories you'd burn lying in bed all day without moving.

How accurate is this BMR calculator?

Mifflin-St Jeor is accurate within ±10% (~150-200 calories) for approximately 80% of the general population. Katch-McArdle is accurate within ±5% (~75-100 calories) when body fat percentage is accurately measured. For clinical precision, indirect calorimetry (breathing test) measures actual metabolic rate within ±3%.

Why does my BMR seem too high or too low?

Common reasons: (1) Body composition: if you're muscular, Mifflin-St Jeor underestimates; if you have high body fat, it overestimates. (2) Metabolic adaptation: chronic dieting can reduce BMR 10-15% below predicted. (3) Medical conditions: thyroid disorders, medications, and hormonal imbalances affect true BMR. (4) Measurement error: inaccurate height/weight inputs compound calculation errors.

Should I eat at my BMR or TDEE for weight loss?

Eat at a deficit from TDEE, not from BMR. Example: TDEE 2,200, BMR 1,500. A reasonable deficit lands between 1,700 and 1,950 calories (250-500 below TDEE), well above BMR. Eating below BMR for extended periods drives metabolic adaptation, muscle loss, and hormonal disruption, and almost never holds up over months. Medically supervised very-low-calorie diets are an exception, not a template.

Why do men have higher BMR than women?

Men typically have 10-15% more lean muscle mass and lower body fat percentages due to hormonal differences (testosterone vs. estrogen). Muscle is metabolically active (~6 cal/lb/day); fat is nearly inert (~2 cal/lb/day). A 170-lb man and 170-lb woman of identical height and age will have different BMRs because their body composition differs.

How do I know which formula to use?

Use Mifflin-St Jeor if: you don't know your body fat percentage, you're within typical body composition (15-30% body fat), or you want a quick general estimate. Use Katch-McArdle if: you know your body fat from DEXA/calipers/smart scale, you're an athlete (<15% male, <22% female body fat), or you have above-average muscle mass.

Does BMR change with age?

Yes. BMR decreases approximately 2% per decade after age 20, primarily due to muscle loss (sarcopenia) and hormonal changes. A 50-year-old typically has 5-10% lower BMR than at age 25. However, maintaining muscle mass through resistance training can largely prevent this decline; active 60-year-olds can have BMRs matching sedentary 30-year-olds.

What if I have a medical condition affecting metabolism?

Hypothyroidism can reduce BMR 10-20%; hyperthyroidism can increase it 15-25%. Certain medications (beta-blockers, antidepressants, corticosteroids) also affect metabolism. If you have diagnosed metabolic conditions, use this calculator as a starting estimate only, then adjust based on actual weight changes over 2-4 weeks. Consult your healthcare provider for personalized guidance.

BMR Calculator: Basal Metabolic Rate & TDEE

Profile

Equation

Activity & goal

Results

TDEE by activity

BMR vs RMR

Basal metabolic rate

Resting metabolic rate

What BMR actually measures

How BMR fits into total energy expenditure

BMR is the bulk of the burn

Big deficits trigger metabolic adaptation

NEAT moves more than gym time

Know your formula's error band

Age matters less than the textbook says