Turn physiology into measurable power and heat. Choose equations, units, and activity for precision better. See your metabolic output in joules and watts now.
| Scenario | Inputs (Age, Sex, Height, Weight, Activity) | Method | Outputs (BMR, Total kcal/day, Power) |
|---|---|---|---|
| Office day | 30, Male, 175 cm, 70 kg, 1.55 | Mifflin-St Jeor | ~1649 kcal/day, ~2818 kcal/day, ~136 W |
| Light training | 28, Female, 165 cm, 60 kg, 1.725 | Revised Harris-Benedict | ~1431 kcal/day, ~2716 kcal/day, ~131 W |
| Lean-mass model | 35, Male, 180 cm, 82 kg, 1.55, BF 15% | Katch-McArdle | ~1877 kcal/day, ~3207 kcal/day, ~155 W |
Metabolic rate is an energy flux: nutrient chemical energy becomes heat and useful work. Treating expenditure as average power is practical. For example, 2400 kcal/day ≈ 10.0 MJ/day, which averages about 116 W over 24 hours. In calorimetry, this power appears as a heat flow to the environment.
BMR represents the minimum resting requirement for circulation, breathing, and cellular maintenance. Total daily energy expands BMR using an activity factor and optional add-ons. Many adults fall near 1200–2000 kcal/day for BMR and 1800–3200 kcal/day for TDEE.
Predictive models trade simplicity for measurement needs. Mifflin–St Jeor and Revised Harris–Benedict use age, height, and mass, making them broadly usable. Katch–McArdle instead uses lean body mass, which can track composition changes when body-fat estimates are available.
Activity factors approximate daily movement costs, typically 1.20 (sedentary) to 1.90 (extra active). The calculator also reports an average MET estimate derived from specific power. Since 1 MET ≈ 1 kcal/kg/hour ≈ 1.162 W/kg, the output is easy to interpret.
Conversion is direct: 1 kcal = 4184 J and one day is 86,400 seconds. Average metabolic power is therefore P = (kcal/day × 4184)/86400. This supports thermal calculations, where a 150 W human heat source resembles a small lamp. For quick checks, 100 kcal/day is about 4.84 W of average power.
The thermic effect of food (TEF) is modeled as a percent of the activity-scaled baseline. A 10% TEF on 2500 kcal/day adds ~250 kcal/day, about 12 W on average. The extra kcal/day field can represent measured workouts, cold exposure, or task loads.
For indirect-calorimetry intuition, oxygen use is estimated with the Weir energy equivalent: kcal per liter O2 ≈ 3.815 + 1.232·RQ. With RQ = 0.85, this is ~4.86 kcal/L O2. Dividing total kcal/day by that factor yields VO2 estimates. Changing RQ shifts kcal per liter O2 by a few percent, which is useful for sensitivity scans.
Exported results support repeatable analysis: compare scenarios, quantify heat release, and document assumptions. CSV is useful for plotting power versus conditions or fitting empirical corrections. PDF is convenient for lab notebooks, reports, and sharing calculation settings with collaborators. Consistent units also let you compare human output with device power budgets.
BMR estimates resting energy needs. Total daily energy adds activity scaling, thermic effect, and any extra loads, producing the full-day budget used for experiments, thermal modeling, and workload planning.
Mifflin–St Jeor is a reliable default. Revised Harris–Benedict is a classic alternative. Choose Katch–McArdle when you know body fat %, because it uses lean mass and can better track composition changes.
Watts is average metabolic power over the day. For example, 120 W means 120 joules of energy are dissipated each second on average, mostly as heat, with some portion linked to mechanical work.
It is an average equivalent intensity based on specific power (W/kg). Because 1 MET ≈ 1.162 W/kg, dividing your W/kg by 1.162 gives a useful comparison to activity intensities.
TEF accounts for digestion and nutrient processing. Many practical models use 5–15% depending on diet and measurement context. In this tool, TEF scales from the activity-adjusted baseline for consistency.
RQ mainly affects the oxygen proxy, not the kcal/day estimate. Lower RQ increases kcal per liter O2 slightly. If you do not have measured RQ, 0.85 is a common mixed-substrate choice.
This is an estimation tool intended for physics-style energy accounting and planning. It does not replace clinical assessment. For high-stakes health decisions, use validated measurement methods and qualified guidance.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.