How many calories you need
Where the number comes from: resting metabolic rate, activity factor, and which equation to use. Including what is known about their accuracy, which is less than any calculator implies.
Every calorie calculator on the internet does the same thing: it estimates what you burn at rest with an equation, then multiplies it by a number standing in for how much you move. None of them measures anything. Both halves are worth understanding, because each fails differently and only one of them is yours to fix.
If you came for the number
The calculator does both halves, with the four equations and the activity factors from this chapter, and splits the macros too. What follows is where it comes from and how much to trust it.
- EquationBMR
- × activityTDEE
- ± rateYour target
The four pieces of expenditure
Total daily energy expenditure —TDEE— is usually broken into four parts. Knowing how much each one weighs explains why the gym moves the needle less than it seems.
Why an hour in the gym seems not to count
A hard lifting session runs 300-400 kcal. That is less than the gap between two people of the same weight who differ in spontaneous movement. Training earns its place through what it builds, not what it burns; what it burns is close to noise in the daily ledger.
Estimating resting metabolic rate
Actually measuring it takes indirect calorimetry — a mask, a lab and a twelve-hour fast. What everyone does instead is apply a regression equation fitted on measured populations. These are the ones that matter.
Mifflin-St Jeor lands closest most often in healthy adults
Frankenfield's systematic review for the Academy of Nutrition and Dietetics compared the available equations and concluded that Mifflin-St Jeor falls within ±10 % of measured expenditure more often than the rest, in both normal-weight and obese adults. It is the default recommendation, and the one this app uses unless you tell it otherwise.
- Mifflin-St Jeor (1990)
- 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5 (men) or − 161 (women)
- Revised Harris-Benedict (1984)
- 88.4 + 13.4 × weight + 4.8 × height − 5.7 × age (men)
- Katch-McArdle
- 370 + 21.6 × lean mass(kg)
- Cunningham (1980)
- 500 + 22 × lean mass(kg)
The default. Doesn't need to know your body composition.
The Roza and Shizgal revision, not the 1919 original: that one overestimates by 5-15 %.
Lean-mass based. Identical to Cunningham's 1991 revision.
The classic reference in athletes. Reads about 130 kcal above Katch-McArdle.
The last two start from lean mass rather than total weight, and that is the difference that counts: muscle burns energy at rest and fat barely does. In someone very lean or very muscular total weight lies, and an equation built on it will always be wrong in the same direction.
Athletes are better served by a lean-mass equation
The 2023 meta-analysis on the accuracy of resting metabolic rate equations in athletes found that fat-free-mass formulas —Cunningham, and the ten Haaf equation developed specifically in recreational athletes— predict without systematic bias, while weight-based ones tend to miss. The trade-off is real: they inherit the error of measuring body fat, and a household bioimpedance scale is easily off by several points.
The spread is the point
On the same body —male, 30, 180 cm, 80 kg, 20 % fat— the four equations give 1,780, 1,854, 1,752 and 1,908 kcal. That is over 150 kcal of disagreement between published, peer-reviewed formulas. None of them is wrong: they are all estimates carrying error. Picking the "best" one matters less than accepting that the starting point is approximate.
The activity factor
The second half multiplies resting rate by a coefficient standing in for everything you do: the PAL (physical activity level). The values come from the joint FAO/WHO/UNU consultation on human energy requirements.
| Level | Factor | What that week looks like |
|---|---|---|
| Sedentary | × 1.20 | Desk job, no training. Under 5,000 steps. |
| Light | × 1.375 | Desk job plus 1-3 sessions, or 5,000-8,000 steps. |
| Moderate | × 1.55 | Office plus 3-5 sessions, or 8,000-12,000 steps. |
| Active | × 1.725 | 6-7 sessions a week, or a job on your feet. |
| Very active | × 1.90 | Two-a-days, or hard physical work plus training. |
This is where the precision really goes
Between "moderate" and "active" there is an 11 % gap: on a 1,800 kcal resting rate, nearly 300 kcal a day. That is more error than picking the wrong equation introduces, and it is the only one of the two under your control. Most people rate themselves one or two levels too high. When in doubt, take the lower one.
The underlying reason is NEAT: non-exercise activity. Someone who walks everywhere and never sits still can burn hundreds of kilocalories a day more than someone of the same weight who drives, and neither of them notices. Three gym sessions a week do not close that gap.
What to do with the number
- 1
Work out the starting point
Use the default equation and the lower of the two activity levels you're torn between. Right here, in the calorie calculator; in the app, under Settings → Calories.
- 2
Hold it for two or three weeks
Changing nothing else. Under two weeks and what you see is water, glycogen and gut contents, not fat.
- 3
Weigh yourself under repeatable conditions
Same time of day, fasted, after the bathroom. The weekly average is what counts, not any single day's number (what to log and how to read it).
- 4
Correct with what you see
If weight isn't moving the way you expected, the number was wrong: adjust it by 100-200 kcal and repeat. Your scale knows more than any equation.
Takeaways
- Resting rate is 60-70 % of expenditure; exercise is 5-15 %.
- Mifflin-St Jeor by default; a lean-mass formula only if you know your body fat.
- The activity factor introduces more error than the choice of equation. Round down.
- The calculated number is a hypothesis. The scale, read across weeks, is the data.
Sources for this chapter
- [1]Frankenfield D, Roth-Yousey L, Compher C (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. review
- [2]Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO (1990). A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition. trial
- [3]Roza AM, Shizgal HM (1984). The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. The American Journal of Clinical Nutrition. trial
- [4]Cunningham JJ (1980). A reanalysis of the factors influencing basal metabolic rate in normal adults. The American Journal of Clinical Nutrition. trial
- [5]O'Neill JER, Corish CA, Horner K (2023). Accuracy of Resting Metabolic Rate Prediction Equations in Athletes: A Systematic Review with Meta-analysis. Sports Medicine. meta-analysis
- [6]ten Haaf T, Weijs PJM (2014). Resting energy expenditure prediction in recreational athletes of 18-35 years: confirmation of Cunningham equation and an improved weight-based alternative. PLoS ONE. trial
- [7]FAO / OMS / UNU (2004). Human energy requirements: report of a joint FAO/WHO/UNU expert consultation. FAO Food and Nutrition Technical Report Series 1. official guideline
- [8]Westerterp KR (2004). Diet induced thermogenesis. Nutrition & Metabolism. review