BMR Calculator: Resting Burn & TDEE

BMR calculator: resting burn and TDEE

By default (male 30, 165 lb / 75 kg, light activity), Mifflin-St Jeor lands near 1,700 BMR and ~2,336 TDEE; diet from TDEE with a sustainable deficit, not from BMR alone.

What the calculator returns

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 (or sooner if loss stalls after 5-10 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).
Tracking accuracy:

Portion guessing is the usual error in food logs. A kitchen scale or barcode scanner app improves intake accuracy. Calibrate TDEE against a 2-3 week scale trend before assuming the formula is wrong.
Scope and limits:

Mifflin-St Jeor, Revised Harris-Benedict, and Katch-McArdle formulas. TDEE = BMR × activity multiplier (six levels, 1.2–1.9). Estimates within ±10% (Mifflin), similar band for Harris, or ±5% (Katch with measured body fat), plus individual biological variance. 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, little exercise). Light 1.375 (exercise 1-3×/week, ~15-30 min). Active 1.465 (exercise 4-5×/week). Moderate 1.55 (daily movement or intense exercise 3-4×/week, ~45-60 min). Very Active 1.725 (intense exercise 6-7×/week, ~45-120 min). Athlete 1.9 (very intense daily training or physical job). Most desk-bound regulars overestimate by one tier.

The formulas

Three 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 default when you don't have a body-fat number.
Revised Harris-Benedict (1984):

$\text{BMR}_{\text{men}} = 13.397W + 4.799H - 5.677A + 88.362 \quad \text{Women: } \text{BMR} = 9.247W + 3.098H - 4.33A + 447.593$
Same inputs as Mifflin. Long-standing reference in older apps and articles; often runs slightly higher than Mifflin, especially for sedentary adults. Useful for cross-checking legacy calculators.
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, Active 1.465, 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. Some plans use 500-1,000 below TDEE for 1-2 lb/week; that range is more aggressive and raises muscle-loss and metabolic-adaptation risk. This calculator defaults to about 500 below TDEE for loss. Larger deficits (750+) 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 changes, and stress. Use a 7-day rolling average and look at 2-3 week trends before concluding the formula is off.
Wearables vs formulas:

Fitness trackers estimate total burn from movement and heart rate; accuracy varies by device and often overstates exercise calories. Formula TDEE from BMR plus an activity multiplier is imprecise too, but it is consistent day to day. Treat wearables as trend data; use scale trend over 2-3 weeks to judge whether your TDEE target fits. Indirect calorimetry in a clinic remains the most accurate resting measurement.

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.

Is there a good BMR for weight loss?

No single BMR number is "good" or "bad" for fat loss. Your BMR is personal: age, sex, height, weight, muscle mass, and genetics all shift it. What matters for weight loss is TDEE (BMR plus activity), then eating below TDEE with a sustainable deficit. Use your calculated BMR as a floor reference, not a daily calorie target. Compare your result to population averages only to sanity-check inputs, not to pick an ideal number.

What is a normal or average BMR?

Population averages sit around 1,410 calories per day for women and 1,696 for men, but individual values spread widely. Typical ranges are roughly 1,200-1,600 for women and 1,400-1,900 for men. A number above or below the average is not wrong; taller, heavier, or more muscular people run higher. If your result looks off, recheck height and weight inputs before comparing to benchmarks.

Can I increase my BMR to lose fat faster?

You can raise resting burn modestly by adding lean muscle through resistance training and keeping protein adequate during a deficit. Muscle is metabolically active; fat tissue is not. Expect gradual change, not a large overnight jump. Building muscle while dieting is slow; the bigger lever for fat loss remains a moderate calorie deficit below TDEE, not chasing a higher BMR alone.

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?

Body composition is the usual culprit: muscular builds often read low on Mifflin-St Jeor, while higher body fat can push it high. Chronic dieting can pull measured BMR 10-15% below the formula. Thyroid issues, medications, and bad height/weight inputs also shift the number. Switch to Katch-McArdle if you have a reliable body-fat reading, then calibrate against 2-3 weeks of scale trend.

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 for most adults when you only have height and weight (default). Use Revised Harris-Benedict if you want to compare against older apps or articles that still cite it; it often runs slightly higher. Use Katch-McArdle when you have a reliable body fat percentage from DEXA, calipers, or a smart scale, especially for athletes or non-average composition.

Why might my real metabolism differ from this calculator?

Even when age, size, and body composition are accounted for, a 2005 meta-analysis (Johnstone et al.) found roughly 26% unexplained variance in basal metabolic rate between individuals. Formula error bands (±10% for Mifflin, ±5% for Katch with measured body fat) sit on top of that biological spread. Treat output as a starting estimate and calibrate against 2-3 weeks of scale trend.

Does pregnancy change BMR?

Yes. Supporting fetal development raises daily energy needs, so pregnant individuals usually require more calories than a pre-pregnancy BMR or TDEE from this tool would suggest. Use pre-pregnancy stats only as a rough baseline and follow your clinician or dietitian for trimester-specific intake targets.

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: How Many Calories Do You Burn at Rest?

Profile

Equation

Activity & goal

Results

TDEE by activity

BMR vs RMR

Basal metabolic rate

Resting metabolic rate

Reading your BMR and TDEE

Reading your results

Example: 165 lb (75 kg), male 30, light activity → ~1,700 BMR, ~2,336 TDEE

Activity tier is where most people overshoot

Diet from TDEE, not from BMR

When to switch formulas