TDEE Calculator — Mifflin-St Jeor + Activity Multiplier

TDEE is the sum of four components. BMR (Basal Metabolic Rate) accounts for 60–75% — the calories needed to sustain vital functions at complete rest. EAT (Exercise Activity Thermogenesis) accounts for 15–30% — calories burned during planned workouts. NEAT (Non-Exercise Activity Thermogenesis) accounts for 5–15% — calories from daily movement like walking, typing, standing, and even unconscious fidgeting. TEF (Thermic Effect of Food) accounts for 8–10% — the energy cost of digesting and metabolizing food, with protein having the highest effect (25–30%) and fat the lowest (0–3%). This TDEE calculator uses the Mifflin-St Jeor equation (Am J Clin Nutr 1990, PMID 2305711) to estimate BMR, then multiplies by an activity factor from the FAO/WHO/UNU 2004 Expert Consultation (<cite>https://www.fao.org/4/y5686e/y5686e00.htm</cite>) that estimates the combined EAT and NEAT contribution.

The output is recommended by the Academy of Nutrition and Dietetics and used in clinical nutrition settings worldwide. It provides the foundation for every calorie-based nutrition strategy — whether your goal is fat loss, muscle gain, athletic performance, or simply understanding how your body uses energy. Treat the number as a starting point for a 2–3 week self-calibration, not as a fixed prescription: a ±10% band around the estimate covers the realistic variance between predicted and measured expenditure for most healthy adults (<cite>Frankenfield et al., J Am Diet Assoc 2005, PMID: 15883556</cite>). If the real intake that holds your weight steady lands outside that band, your activity multiplier choice — not the equation itself — is almost always the explanation. The right workflow is simple: get a TDEE number, hold intake flat at it for 14–21 days, track your weight trend as a 7-day rolling average, and then nudge intake up or down by 100–200 kcal/day until the trend matches your goal. The calculator is not the answer; it is the anchor around which real-world calibration happens.

Clinical research consistently shows that severe calorie restriction — eating far below TDEE — triggers adaptive thermogenesis: the body reduces its metabolic rate by 10–25% as a survival response (<cite>Hall KD, Int J Obes 2008, PMID: 17848938</cite>). This metabolic adaptation is the primary biological reason crash diets fail. You lose weight rapidly at first, then stall as your body downregulates metabolism, and regain the weight when normal eating resumes — often with additional fat as the body overshoots in preparing for the next famine. Understanding your TDEE allows you to create a controlled, sustainable deficit that minimizes metabolic adaptation while preserving muscle mass through adequate protein (1.6–2.2 g/kg) and resistance training.

TDEE is also the foundation of athletic performance nutrition. Endurance athletes may have TDEEs of 3,500–5,000 kcal/day during training blocks. Strength athletes may need 3,000–4,000 kcal to support muscle growth. Without knowing TDEE, neither fat loss nor muscle gain can be optimized — you are guessing at the most fundamental variable in nutrition science, and guessing cheaply is why so many motivated people spin for a year without visible progress.

The activity multipliers used here derive from the factorial method described by the FAO/WHO/UNU Expert Consultation on Human Energy Requirements (2004), and the Mifflin-St Jeor BMR equation was validated against indirect calorimetry in 498 adults by Frankenfield et al. (<cite>J Am Diet Assoc, 2005, PMID: 15883556</cite>), where it predicted RMR within 10% in 82% of non-obese and 70% of obese subjects — outperforming the older Harris-Benedict equation. For the ±500 kcal/day loss and gain targets, the TDEE calculator follows the CDC and NIH guidance of 0.5–1.0 kg/week weight change at the 7,700 kcal/kg energy density of adipose tissue (Wishnofsky 1958; refined dynamically by Hall 2008).

The activity multipliers are the largest source of error. 'Moderately active' means different things to different people. Two individuals who both select 1.55 might have real activity levels corresponding to 1.4 and 1.7. NEAT variation between individuals is enormous — up to 2,000 kcal/day — and is nearly impossible to estimate accurately without wearable tracking technology, and even wearables tend to overestimate activity calories by 15–30% (<cite>Shcherbina et al., J Pers Med 2017, Stanford wrist-worn device review</cite>). For the most accurate results, track food intake and body weight for 2–3 weeks after using this TDEE calculator, then back-calculate your actual TDEE from that real-world data. The calculator also assumes consistent habits — a travel week, illness, menstrual phase, or a run of poor sleep (which lowers NEAT by 100–200 kcal/day) can all shift true TDEE outside the predicted band without any change in your inputs.

The calculator does not account for metabolic adaptation from prolonged calorie restriction, which can reduce actual TDEE by 10–30% in individuals who have been dieting for extended periods (<cite>Hall KD, Int J Obes 2008, PMID: 17848938</cite>). If you have been in a calorie deficit for 3+ months and weight loss has stalled, your actual TDEE at that time may be substantially lower than what this calculator predicts for your current body weight. Finally, Mifflin-St Jeor is validated for healthy nonpregnant adults — it is not appropriate for pregnancy, lactation, severe underweight (BMI < 17), uncontrolled thyroid disease, type 1 diabetes with insulin pump micro-boluses, or paediatric populations (<16 years). In those cases, clinician-measured REE via indirect calorimetry and population-specific equations should be used instead, and intake targets should be set with a registered dietitian rather than a web calculator.