How this instrument works
This calculator works in three steps. First, it estimates resting metabolic rate (RMR) — roughly what the body burns over 24 hours doing nothing but staying alive — from weight, height, age, and sex, using the Mifflin-St Jeor equation. Second, it multiplies that resting figure by an activity multiplier reflecting how physically active a typical day is, from sedentary (1.2×) up to very active (1.9×), to estimate total daily energy expenditure (TDEE): the calories needed to maintain current weight. Third, it adds or subtracts roughly 500 kcal/day depending on the goal selected — losing, maintaining, or gaining about a pound a week — to produce a final daily calorie target.
The activity multipliers (1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 active, 1.9 very active) are a widely used, commonly republished convention seen across sources including the NIH and Mayo Clinic, rather than a figure derived from a single controlled trial — they're a reasonable standard approximation, not a precise measurement of any individual's actual activity-driven energy cost. The ±500 kcal/day adjustment is likewise a simplification, built on the longstanding rule of thumb that roughly 3500 kcal corresponds to about a pound of body weight, spread across a week. Real-world weight change is messier than that constant relationship suggests — it doesn't fully hold over longer stretches of dieting, where metabolic adaptation, changes in lean mass, and individual variation all affect the pace of actual change.
This site's separate RMR, Harris-Benedict, and Katch-McArdle calculators stop at resting energy expenditure alone. This calculator goes two steps further, layering an activity multiplier and a weight-goal adjustment on top of that resting figure to produce an actionable daily calorie target rather than just a resting baseline.
- Set "Sex is female" to Yes or No — this changes the constant in the resting-metabolic-rate formula.
- Enter weight in kilograms and height in centimeters.
- Enter age in years.
- Choose an activity level, from Sedentary (little/no exercise) up to Very active (hard exercise + physical job).
- Choose a goal — Lose ≈1 lb/week, Maintain, or Gain ≈1 lb/week.
- Read the resting metabolic rate, maintenance calories (TDEE), and target calories per day.
Worked example — 70 kg, 170 cm, 30-year-old man, moderately active, maintaining
A 70 kg, 170 cm, 30-year-old man: RMR = 10 × 70 + 6.25 × 170 − 5 × 30 + 5 = 1617.5 kcal/day. At a moderate activity level (×1.55), maintenance calories are 1617.5 × 1.55 = 2507.125 kcal/day. With a Maintain goal (+0), his target calories equal maintenance: about 2507 kcal/day.
A 60 kg, 160 cm, 25-year-old woman, lightly active, aiming to lose about a pound a week: RMR = 10 × 60 + 6.25 × 160 − 5 × 25 − 161 = 1314 kcal/day. At light activity (×1.375), maintenance is 1314 × 1.375 = 1806.75 kcal/day, and subtracting the standard 500 kcal/day deficit gives a target of about 1307 kcal/day. An 85 kg, 180 cm, 22-year-old active man aiming to gain about a pound a week has an RMR of 1870 kcal/day, maintenance of 1870 × 1.725 = 3225.75 kcal/day, and a target of about 3726 kcal/day after adding the 500 kcal/day surplus.
Questions
How is this different from the site's RMR, Harris-Benedict, or Katch-McArdle calculators?
Those calculators stop at estimated resting metabolic rate — the calories burned doing nothing but staying alive over 24 hours. This calculator starts from the same kind of resting estimate (using Mifflin-St Jeor) but goes two steps further: it multiplies by an activity level to estimate total daily calories at maintenance, then adjusts that by a weight-goal figure to land on an actionable daily calorie target for losing, maintaining, or gaining weight.
Where do the activity multipliers (1.2 to 1.9) come from?
They're a widely used, commonly republished convention for translating resting metabolic rate into total daily energy expenditure, appearing in similar form across sources like the NIH and Mayo Clinic — not a figure pinned to one single controlled study. Treat the multiplier you choose as an approximation of your typical week rather than a precise measurement; genuine day-to-day activity varies enough that any fixed category is a simplification.
Why does the goal adjustment use roughly 500 kcal/day?
It follows the long-standing rule of thumb that about 3500 kcal corresponds to roughly a pound of body weight, so a 500 kcal/day deficit or surplus, sustained for a week, works out to about a pound of change (3500 ÷ 7 ≈ 500). It's a useful planning simplification rather than an exact prediction — real weight change also depends on metabolic adaptation over time, changes in body composition, water weight, and individual variation the constant doesn't capture.
Is a 500 kcal/day deficit safe for everyone?
This calculator doesn't screen for that — it applies the same fixed 500 kcal/day adjustment regardless of the resulting target's absolute level. A 500 kcal/day deficit could push some people, particularly smaller or already lower-calorie individuals, to an unreasonably low target. Anyone considering a sustained calorie deficit, especially alongside a medical condition, medication, or pregnancy, should check the resulting target with a doctor or registered dietitian before following it.
Why does the resting-metabolic-rate formula differ between men and women?
The Mifflin-St Jeor equation's regression found men and women differ systematically in resting metabolic rate even after accounting for weight, height, and age, largely reflecting that men, on average, carry more metabolically active lean mass at a given weight. That's captured with a single constant at the end of the equation — +5 for men, −161 for women — rather than a separate measurement of body composition.
Does maintenance-calorie accuracy depend on picking the right activity level?
Yes, substantially — the activity multiplier is the single biggest lever in this calculator after resting metabolic rate itself, since it can scale the resting number by anywhere from 1.2× to 1.9×. Picking a level that doesn't match how physically demanding your typical week actually is will shift the maintenance estimate, and therefore the final target, by a meaningful margin. When in doubt, a slightly lower activity level is usually a safer starting assumption than an inflated one.
References
- 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):241-7
- NIH National Heart, Lung, and Blood Institute — Body Weight Planner (activity-level and energy-balance methodology)
- Mayo Clinic — Calorie Calculator (activity-level convention and weight-goal guidance)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.