Equations We Use
1. Mifflin-St Jeor Equation (1990) — Primary BMR Formula
The Mifflin-St Jeor equation is our default formula for estimating Basal Metabolic Rate (BMR). It is recommended by the Academy of Nutrition and Dietetics as the most accurate predictive equation for healthy adults, estimating BMR within 10% of measured values for the majority of people.
Men:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Source:
Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition. 1990;51(2):241–247.
Why we chose it: A 2005 review by Frankenfield et al. compared four BMR prediction equations across 36 studies and concluded that the Mifflin-St Jeor equation was the most accurate across diverse populations, outperforming the older Harris-Benedict equation.
2. Katch-McArdle Equation (1996) — Body-Fat-Aware BMR
When users provide their body fat percentage, we switch to the Katch-McArdle formula. It uses lean body mass (LBM) rather than total weight, making it more accurate for individuals who are significantly leaner or heavier than average.
Both genders:
BMR = 370 + (21.6 × lean body mass in kg)
Where lean body mass = weight in kg × (1 − body fat % ÷ 100)
Source:
McArdle WD, Katch FI, Katch VL. Exercise Physiology: Energy, Nutrition, and Human Performance. 5th ed. Baltimore: Lippincott Williams & Wilkins; 2001.
Why we include it: For athletes with low body fat or individuals with higher body fat, standard weight-based formulas can over- or underestimate BMR significantly. The Katch-McArdle formula addresses this by focusing on metabolically active tissue.
3. Harris-Benedict Equation (1919, revised 1984) — Comparison
We display Harris-Benedict results as a secondary comparison on our BMR Calculator. While historically significant, this formula tends to overestimate BMR by 5–15% compared to indirect calorimetry measurements.
Men:
BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age)
Women:
BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age)
Original: Harris JA, Benedict FG. "A Biometric Study of Human Basal Metabolism." Proceedings of the National Academy of Sciences. 1918;4(12):370–373.
Revision: Roza AM, Shizgal HM. "The Harris Benedict equation reevaluated." American Journal of Clinical Nutrition. 1984;40(1):168–182.
4. US Navy Method — Body Fat Estimation
Our Body Fat Calculator uses the US Navy circumference-based method. It estimates body fat percentage from height and circumference measurements without requiring specialized equipment.
Men:
BF% = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
Women:
BF% = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
All measurements in centimeters.
Source:
Hodgdon JA, Beckett MB. "Prediction of percent body fat for U.S. Navy men and women from body circumferences and height." Report No. 84-29. Naval Health Research Center, San Diego, CA. 1984.
Accuracy note: The US Navy Method has a standard error of estimate of approximately 3–4% compared to hydrostatic weighing. It is less accurate than DEXA scans but provides a reasonable estimate using only a tape measure.
Activity Multipliers
To convert BMR to TDEE (Total Daily Energy Expenditure), we multiply by an activity factor based on the user's self-reported activity level. These multipliers are derived from the original work by Harris and Benedict and refined in subsequent research.
| Activity Level |
Multiplier |
Description |
| Sedentary |
1.2 |
Little or no exercise, desk job |
| Lightly Active |
1.375 |
Light exercise 1–3 days per week |
| Moderately Active |
1.55 |
Moderate exercise 3–5 days per week |
| Very Active |
1.725 |
Hard exercise 6–7 days per week |
| Athlete |
1.9 |
Twice-daily training or very physical job |
Important: Activity multipliers are population averages. Individual variation in NEAT (Non-Exercise Activity Thermogenesis) — fidgeting, posture maintenance, spontaneous movement — can cause actual TDEE to differ by 200–400 calories from the calculated value.
Macronutrient Calculations
Our Macro Calculator converts a daily calorie target into grams of each macronutrient using standard energy density values:
| Macronutrient |
Calories per gram |
| Protein | 4 kcal/g |
| Carbohydrates | 4 kcal/g |
| Fat | 9 kcal/g |
How we apply percentage splits: Based on the user's goal (fat loss, maintenance, muscle gain) and diet preference (standard, high protein, low carb, keto), we assign a percentage of total calories to each macronutrient. For example, if a user targets 2,000 calories with a 40/30/30 split (protein/carbs/fat):
Protein: 2,000 × 0.40 = 800 kcal ÷ 4 = 200 g
Carbs: 2,000 × 0.30 = 600 kcal ÷ 4 = 150 g
Fat: 2,000 × 0.30 = 600 kcal ÷ 9 = 67 g
These ratios are based on general sports nutrition guidelines. Individual needs may vary based on training type, medical conditions, and personal tolerance.
Calorie Deficit & Weight Change
Our Calorie Deficit Calculator uses the widely accepted energy balance principle:
1 kg of body fat ≈ 7,700 kcal of stored energy
Daily deficit for 0.5 kg/week loss = (0.5 × 7,700) ÷ 7 = 550 kcal/day
Daily deficit for 0.25 kg/week loss = (0.25 × 7,700) ÷ 7 = 275 kcal/day
Safety floors: We flag any target below 1,200 kcal/day for women or 1,500 kcal/day for men. These thresholds are based on general clinical guidelines — eating below these levels without medical supervision increases the risk of nutrient deficiencies, muscle loss, and metabolic adaptation.
Limitations
All calculator results on this site are estimates based on population-level equations. They are not equivalent to clinical measurements. Specific limitations include:
- Individual variance: Predictive equations estimate BMR within ±10% of measured values (via indirect calorimetry) for most individuals. Factors like genetics, thyroid function, and medication use are not captured.
- Self-reported activity: Activity level selection is subjective. Most people overestimate their activity level, which inflates TDEE estimates. When in doubt, select one level lower than you think.
- Body composition not captured: The Mifflin-St Jeor equation uses total body weight, not lean mass. Two people at the same weight but different body fat percentages will have different actual BMRs. Use the Katch-McArdle formula if you know your body fat percentage.
- Metabolic adaptation: Extended calorie deficits cause the body to reduce energy expenditure through decreased NEAT, reduced thyroid output, and improved metabolic efficiency. Our calculators do not model this adaptation over time.
- Medical conditions excluded: Conditions including hypothyroidism, PCOS, Cushing's syndrome, diabetes, and others can significantly alter metabolic rate. Calculator results should not replace medical assessment.
- Circumference-based body fat estimation: The US Navy Method has a standard error of 3–4% compared to DEXA. It is less accurate for individuals at the extremes of body fat (very lean or very high body fat).
- Static model: Our calculators provide a snapshot based on current inputs. As you lose or gain weight, your TDEE changes. Recalculate every 4–6 weeks for ongoing accuracy.
Data Sources & References
1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals." Am J Clin Nutr. 1990;51(2):241–247.
2. Frankenfield D, Roth-Yousey L, Compher C. "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review." J Am Diet Assoc. 2005;105(5):775–789.
3. Harris JA, Benedict FG. "A biometric study of human basal metabolism." Proc Natl Acad Sci. 1918;4(12):370–373.
4. Roza AM, Shizgal HM. "The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass." Am J Clin Nutr. 1984;40(1):168–182.
5. McArdle WD, Katch FI, Katch VL. Exercise Physiology: Energy, Nutrition, and Human Performance. 5th ed. Lippincott Williams & Wilkins; 2001.
6. Hodgdon JA, Beckett MB. "Prediction of percent body fat for U.S. Navy men and women from body circumferences and height." Report No. 84-29. Naval Health Research Center; 1984.
7. American Council on Exercise. "ACE Body Fat Percentage Norms." ACE Fitness. Accessed 2025.
All calculators on this site are for informational and educational purposes only. They are not a substitute for professional medical advice, diagnosis, or treatment. Always consult a registered dietitian, physician, or qualified healthcare professional before making changes to your diet or exercise program.