BMR Calculator: Basal Metabolic Rate & TDEE Guide
How to calculate BMR and TDEE. Mifflin-St Jeor and Katch-McArdle formulas. Trusted by trainers. No sign-up—all calculations run locally.
What This Calculator Does & Who It's For
Calculator Purpose & Ideal Users
- Primary Use Cases:Weight loss planning: Calculate deficit targets that preserve muscle and avoid metabolic damage. Muscle gain (bulking): Determine surplus calories needed for lean mass gains. Maintenance tracking: Establish baseline for intuitive eating or reverse dieting. Athletic performance: Ensure adequate fueling for training demands.
- Ideal User Profiles:Adults 18-65: Formulas validated for this age range. Generally healthy individuals: No uncontrolled thyroid, metabolic, or hormonal conditions. Those tracking body composition: The Katch-McArdle option rewards users who know their body fat percentage. Anyone starting a diet or training program: Baseline metabolic data prevents guesswork.
- Who Should Use Caution:Adolescents/elderly: Formulas less validated outside 18-65 range; use as rough estimates only. Pregnant/nursing women: Metabolic needs increase 300-500 cal/day beyond standard calculations. Those with eating disorders: Calculator should not be used to justify extreme restriction. Post-bariatric surgery patients: Metabolic rate changes significantly; clinical measurement recommended.
- When NOT to Use This Calculator:Do not use as a sole guide if you have: diagnosed thyroid disorders (hypo/hyperthyroidism), PCOS or other hormonal conditions affecting metabolism, history of eating disorders, or are taking medications known to affect metabolism. In these cases, use results as a starting point only and adjust based on real-world body response over 2-4 weeks.
- Scope & Limits:Mifflin-St Jeor and Katch-McArdle formulas. TDEE = BMR × activity multiplier. Estimates ±10% (Mifflin) or ±5% (Katch with known body fat). All calculations run locally; no data stored. Consult a healthcare provider for personalized metabolism guidance.
This BMR calculator estimates the minimum calories your body needs at rest, then calculates Total Daily Energy Expenditure (TDEE) based on your activity level. Understanding who should use each feature helps you get accurate results.
Understanding Each Input & Why It Matters
Input Field Reference Guide
- Age (18-100 years):BMR decreases ~2% per decade due to muscle loss and hormonal changes. The formula subtracts 5 calories per year of age. Impact: A 20-year age difference changes BMR by ~100 calories. Accuracy note: If you've maintained significant muscle mass, actual BMR may be higher than calculated.
- Sex (Biological):Men receive +5 calories; women receive -161 calories in the Mifflin-St Jeor formula. This accounts for typical differences in lean mass and hormonal profiles. Impact: Same height/weight/age male vs. female differs by ~166 calories. Note: If you're transgender on hormone therapy, consider using the sex aligning with your current hormonal profile, or use Katch-McArdle which is sex-agnostic.
- Height (cm or ft/in):Taller bodies have more tissue requiring energy. The formula adds 6.25 calories per centimeter of height. Impact: Each inch of height adds ~16 calories to BMR. Accuracy tip: Measure in the morning; you're slightly shorter by evening due to spinal compression.
- Weight (kg or lbs):The largest BMR driver—heavier bodies require more energy. The formula adds 10 calories per kilogram of weight. Impact: A 10 lb difference changes BMR by ~45 calories. Accuracy tip: Weigh in the morning, after bathroom, before eating, in minimal clothing. Use 3-7 day average for best accuracy.
- Body Fat % (for Katch-McArdle):Converts total weight to Lean Body Mass (LBM = Weight × (1 - Body Fat%)). Only metabolically active tissue counts. Impact: Two people weighing 180 lbs—one at 15% body fat (153 lbs lean) vs. 30% (126 lbs lean)—have BMRs differing by ~580 calories. Measurement methods: DEXA scan (±1-2%), hydrostatic weighing (±2-3%), calipers (±3-4%), bioelectrical impedance (±4-8%).
- Activity Level (for TDEE):Multiplies BMR to estimate total daily burn. Sedentary (×1.2): Desk job, <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 + daily training or 2×/day training. Common mistake: Most people overestimate by one level; when in doubt, choose lower.
Each input directly affects calculation accuracy. Understanding why each field matters helps you provide better data and interpret results correctly.
The BMR Formulas: Complete Mathematical Reference
Calculation Methods Explained
- Mifflin-St Jeor Equation (1990):
Where W = weight in kg, H = height in cm, A = age in years. Validated against indirect calorimetry. Accuracy: ±10% for 80% of general population. Best for those without body fat data.
- Katch-McArdle Equation (1996):
Where LBM = Weight (kg) × (1 − Body Fat %). Best for athletes and those with known body fat. Accuracy: ±5% when body fat is accurately measured.
- TDEE Calculation:
Activity multipliers: Sedentary 1.2, Light 1.375, Moderate 1.55, Very Active 1.725, Athlete 1.9. TDEE is the number you use for diet planning.
- Goal-Based Calorie Targets:Weight Loss: Target = TDEE - (250 to 500)
Maintenance: Target = TDEE
Muscle Gain: Target = TDEE + (250 to 500)Never eat below BMR—this triggers severe metabolic adaptation. A 500 cal/day deficit yields ~1 lb/week loss. Larger deficits (750+) increase muscle loss risk and require higher protein intake (1g+ per lb body weight).
Two peer-reviewed formulas power this calculator. Each has validated accuracy ranges and specific use cases.
Interpreting Your Results: What the Numbers Mean
Output Interpretation Guide
- BMR (Basal Metabolic Rate):Your "survival minimum"—calories burned if you stayed in bed all day. Typical ranges: Women: 1,200-1,600 cal. Men: 1,400-1,900 cal. Red flag: If your calculated BMR seems very low (<1,200 women, <1,400 men), you may have significant metabolic adaptation or need clinical evaluation. Action: This is your floor—never eat below this number for extended periods.
- TDEE (Total Daily Energy Expenditure):Your actual daily calorie burn including all activity. This is the number you diet from. Typical ranges: Sedentary adults: 1,600-2,200. Active adults: 2,000-3,000. Athletes: 2,500-4,500+. Validation: Eat at calculated TDEE for 2 weeks; if weight is stable (±1 lb), the number is accurate. If gaining, reduce by 10%; if losing, increase by 10%.
- Metabolic Age:Compares your metabolism to age-group averages. With Mifflin-St Jeor: Returns your chronological age (formula is deterministic). With Katch-McArdle: Compares your lean mass to typical body composition at different ages. Interpretation: Lower = more metabolically efficient (more muscle, less fat). Higher = less efficient. Improving it: Build muscle through resistance training; reduce body fat percentage.
- Macro Targets (Protein/Carbs/Fat):Distribute calories across macronutrients based on your goal. Protein priority: Always hit protein target first (0.7-1g per lb body weight). Carbs/fat flexibility: Adjust based on preference and training demands—total calories matter more than carb/fat ratio for weight change. Example: 1,800 cal at 40/30/30 split = 180g protein, 135g carbs, 60g fat.
Raw numbers need context to be actionable. Here's how to interpret each output and what to do with the information.
Common Calculation Pitfalls & Edge Cases
Avoiding Calculation Errors
- Activity Level Overestimation:The #1 error. A 30-minute gym session 4×/week does NOT make you "Very Active"—that multiplier assumes hard training 6-7 days PLUS an active job or lifestyle. Most desk workers with regular gym routines are "Lightly Active" to "Moderately Active" at most. When in doubt, choose one level lower than you think.
- Body Fat Estimation Errors:Katch-McArdle is only as accurate as your body fat measurement. A 5% error in body fat percentage creates a ~100+ calorie error in BMR. If using bioelectrical impedance (bathroom scale), expect ±4-8% error. For best accuracy, use DEXA scan results or 7-day average from smart scale.
- Metabolic Adaptation Blindspot:If you've been dieting for 8+ weeks or have a history of yo-yo dieting, your actual BMR may be 10-15% lower than calculated. The formulas assume "normal" metabolic function. Signs of adaptation: weight loss stalled despite accurate tracking, extreme fatigue, always cold, poor sleep. Solution: "Reverse diet" back to maintenance for 4-8 weeks.
- Medical Condition Impact:Hypothyroidism reduces BMR 10-20%. Hyperthyroidism increases it 15-25%. PCOS, Cushing's syndrome, and certain medications (beta-blockers, SSRIs, corticosteroids) also affect metabolism. If you have these conditions, use calculated BMR as a starting point and calibrate based on 2-4 weeks of tracking weight vs. intake.
- Extreme Body Compositions:Formulas are validated for "typical" body compositions (15-30% body fat). At extremes (<10% or >40% body fat), accuracy decreases. Very lean individuals: Katch-McArdle is essential—Mifflin-St Jeor significantly underestimates. Very overweight individuals: Both formulas may overestimate because excess fat has very low metabolic cost.
- Water Weight Fluctuations:Don't adjust your TDEE based on daily weight changes. Water weight can fluctuate 2-5 lbs daily from sodium intake, carb consumption, menstrual cycle, and stress. Use 7-day average weight and 2-week trends before concluding your TDEE calculation is wrong.
Even accurate formulas produce misleading results with incorrect inputs or misapplied assumptions. Here are the most common issues and how to avoid them.
Putting It All Together: Practical Application
Step-by-Step Implementation Guide
- Week 1-2: Establish Baseline:Eat at calculated TDEE (maintenance). Track food accurately using a food scale for portions. Weigh yourself daily at the same time, calculate 7-day average. Outcome: If weight is stable (±1 lb), your TDEE is accurate. If not, adjust by 10% and repeat.
- Week 3-4: Implement Goal:Fat loss: Reduce to TDEE minus 300-500 calories. Prioritize protein (minimum 0.7g/lb body weight). Muscle gain: Increase to TDEE plus 250-350 calories. Ensure resistance training program is in place. Continue tracking: Aim for 0.5-1% body weight loss per week (fat loss) or 0.25-0.5% gain per week (muscle gain).
- Ongoing: Monitor & Adjust:Recalculate TDEE every 10-15 lbs of weight change. Watch for metabolic adaptation signs (stalled progress, fatigue, poor recovery). Plan periodic "diet breaks" at maintenance every 8-12 weeks of deficit. Adjust activity multiplier if lifestyle changes (new job, injury, changed training).
- Validation Checkpoints:Working correctly: Losing 0.5-1 lb/week on deficit, maintaining on TDEE, energy stable, sleep good, training performance maintained. Needs adjustment: Stalled for 2+ weeks despite accurate tracking, energy crashing, sleep disrupted, strength declining. Action if stalled: Either reduce intake by 100-150 cal OR increase activity before assuming calculator is wrong.
Turning calculator output into a sustainable nutrition plan requires calibration and patience. Here's the evidence-based approach.
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, never below BMR. Example: If your TDEE is 2,200 and BMR is 1,500, a safe deficit is 1,700-1,950 calories (250-500 below TDEE). Eating below BMR triggers severe metabolic adaptation, muscle loss, hormonal disruption, and is unsustainable. The calculator shows your TDEE automatically based on activity level.
? 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.