Calorie Calculator
Calculate BMR and TDEE using the Mifflin-St Jeor equation with weight loss and gain targets.
Reviewed by Aygul Dovletova · Last reviewed
Using the Calorie Calculator
Enter age, sex, weight, height, and activity level. The tool returns three numbers: BMR (calories burned at rest), TDEE (maintenance calories), and targets 500 above and below TDEE for weight change.
- Age and sex - biological sex materially affects BMR because of body-composition averages. Age adjusts BMR down by about 5 kcal/year after 20.
- Weight - pounds or kilograms.
- Height - feet/inches or centimetres.
- Activity level - sedentary (1.2), lightly active 1-3 days/week (1.375), moderately active 3-5 days (1.55), very active 6-7 days (1.725), extra active (1.9).
The Formula and Why Mifflin-St Jeor
The calculator uses the Mifflin-St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition. For men: BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age + 5. For women: +5 becomes -161. The Academy of Nutrition and Dietetics endorsed it in 2005 as the most accurate BMR prediction for healthy non-obese adults, typically within 10% of measured RMR from indirect calorimetry. It outperforms the older Harris-Benedict equation (1919, revised 1984), which overestimates BMR in overweight individuals. TDEE multipliers come from Roza and Shizgal 1984 and are the standard. Weight-change targets assume 3,500 kcal per pound (Wishnofsky 1958).
Accuracy Caveats
Even the best BMR equation carries a ±10% error band versus measured rate. For a BMR of 1,600, that is ±160 kcal. Athletes with high lean mass, people on certain medications (beta-blockers, antidepressants, thyroid medications), and anyone with untreated thyroid disease can fall outside. For medical precision, get a measured BMR via indirect calorimetry at a sports-medicine clinic.
When to Use These Numbers
- Starting a weight-loss plan and needing a calorie target for the first weeks.
- Reverse dieting - slowly raising calories from a long diet back to maintenance.
- Bulking for muscle gain - figuring the 10-20% surplus over maintenance.
- Meal planning for a new exercise routine.
- Onboarding to a food tracking app (MyFitnessPal, Cronometer) that wants a starting goal.
Pitfalls and What the Numbers Miss
A 500 kcal deficit rarely produces steady 1 lb/week loss over long periods. In weeks 1-2, people often lose 2-4 lbs driven by glycogen-and-water depletion. Weeks 3-8 show the near-linear 1 lb/week the math predicts. Past week 8, metabolic adaptation kicks in: BMR drops 5-15% below predicted, NEAT (non-exercise activity thermogenesis) quietly decreases, leptin falls and hunger rises. Standard counters: diet breaks every 6-8 weeks, refeed days, periodic BMR recalculation at new body weight. Activity level is self-reported and notoriously optimistic - most people overestimate exercise. If real weight change disagrees with the predicted rate after 3-4 weeks, trust the scale.
Energy Balance Terminology
BMR is resting energy expenditure at complete rest, 12+ hours fasted. RMR (resting metabolic rate) is the same in less strict conditions, running 5-10% higher. TDEE breaks into BMR (60-70%), TEF (thermic effect of food, 8-10%), EAT (exercise thermogenesis), and NEAT (non-exercise thermogenesis). TEF varies by macro: protein costs 20-30% of its calories to digest, carbs 5-10%, fat 0-3%. A high-protein diet produces a slightly higher real maintenance than the equation predicts. Mifflin-St Jeor only predicts BMR; the activity multiplier absorbs TEF indirectly.
How This Compares to Alternatives
Fitness trackers (Apple Watch, Garmin, Fitbit, Whoop, Oura) combine a BMR formula with real-time heart rate and movement - more responsive to daily variation, less accurate than clinical calorimetry. Nutrition apps (MyFitnessPal, Cronometer, LoseIt) embed a similar Mifflin-St Jeor internally. A registered dietitian uses Mifflin-St Jeor or Harris-Benedict, often adjusting based on DEXA body composition. The gold standard is indirect calorimetry (metabolic cart) at sports-medicine clinics ($100-300 per session). This calculator is the fastest option for a reasonable starting target and matches what most apps calculate in onboarding.
Frequently Asked Questions
What is the difference between Mifflin-St Jeor and Harris-Benedict?
Both are BMR prediction equations; Mifflin-St Jeor (1990) updated Harris-Benedict (1919, revised 1984) coefficients using a more representative modern population. Mifflin-St Jeor is roughly 5% lower for the same inputs and more accurate for overweight individuals - Harris-Benedict overestimates BMR above BMI 30. The Academy of Nutrition and Dietetics has recommended Mifflin-St Jeor as default since 2005. Katch-McArdle is a third option that uses lean body mass directly - more accurate for athletes and bodybuilders who know their body-fat percentage.
Why does a 500 kcal deficit not always equal 1 lb/week?
The 3,500 kcal per pound rule from Wishnofsky 1958 is a simplification that ignores metabolic adaptation, hormonal changes, and body-composition shifts. Kevin Hall's 2011 NIH work produced a dynamic model showing long-term weight loss slows even at constant deficit. The linear model works for 4-8 weeks; beyond that, expect plateaus. The NIDDK Body Weight Planner uses Hall's model for more realistic long-term projections.
Can I eat below my BMR for faster weight loss?
Not without medical supervision. Very-low-calorie diets (under 800 kcal/day) require physician oversight due to cardiac, gallbladder, muscle, and micronutrient risk. The standard guidance is not to drop sustained intake below 1,200 kcal for women or 1,500 kcal for men. Moderate deficits (15-25% below TDEE) produce better long-term results than aggressive ones.
How does muscle mass change calorie needs?
Skeletal muscle burns 6-10 kcal per pound per day at rest - less than fitness media suggests. Gaining 10 lbs of muscle raises BMR only 60-100 kcal/day, not 500-1000. Mifflin-St Jeor assumes average body-fat for age and sex. If you are a competitive athlete with much higher lean mass, try Katch-McArdle: <code>BMR = 370 + 21.6 × LBM(kg)</code>.
What activity level if I have an office job and work out 3 times a week?
Lightly active (1.375) is the honest answer. Sedentary (1.2) assumes no exercise; moderately active (1.55) assumes 5+ hard workouts. People routinely overestimate activity by one tier - the single biggest source of error in calculator-based targets. If unsure, pick the lower tier and adjust up after 2-3 weeks of scale feedback.
Is the data sent to a server?
No. Age, weight, height, and activity selections live in Preact component state. No fetch call, no analytics event bound to your metrics, no storage between visits. Closing the tab clears everything. The calculation is a polynomial JavaScript resolves in microseconds with no network dependency.
Does this account for pregnancy, breastfeeding, or menstrual cycle?
No. Pregnancy adds about 300 kcal/day in second trimester, 450 in third; breastfeeding adds 450-500. Not in the formula - consult a registered dietitian or OB-GYN. The menstrual cycle can swing daily BMR by 50-150 kcal (luteal higher than follicular); averaging over a full cycle washes this out.
What about GLP-1 medications, steroids, or thyroid conditions?
The calculator is tuned for untreated healthy adults. GLP-1 agonists (semaglutide, tirzepatide) reduce appetite but do not change BMR directly. Anabolic steroids raise BMR and lean mass beyond what equations predict. Hypothyroidism lowers BMR 10-30%; hyperthyroidism raises it similarly. Work with a physician and treat the calculator as a rough starting point.
How often should I recalculate?
Every 10-15 lbs of change. Mifflin-St Jeor's weight coefficient is 10 kcal per kg, so a 22 lb drop lowers predicted BMR by 100 kcal/day. Failing to adjust is why people stall at a given deficit after 10-20 lbs - they are eating the pre-loss TDEE, which is no longer a real deficit.
More Home & Lifestyle
Baby Name Generator
Generate random baby names with filters for gender, cultural origin and starting letter.
Open toolMoving Box Calculator
Estimate moving boxes needed by room type with breakdown by small, medium and large sizes.
Open toolPaint Calculator
Calculate how many gallons of paint you need for a room based on dimensions, doors and windows.
Open toolPet Age Calculator
Convert dog or cat age to human years with breed size adjustment for dogs.
Open toolRecipe Scaler
Scale recipe ingredients up or down proportionally for any number of servings.
Open toolSleep Cycle Calculator
Find when to sleep or wake up based on 90-minute REM cycles. Free bedtime and wake time calculator.
Open tool