A calorie budget usually appears as one tidy figure, something like 1,650 kcal a day. That figure is a starting estimate. It comes from an equation, multiplied by a guess about how active you are, and you then compare it with food entries that carry their own error. The budget is still worth having, as long as you know roughly how far off it could be and how to check it.

Where the number comes from

Most apps start with resting energy, the energy your body uses in a day at rest. Hardly anyone has it measured, so it's predicted from weight, height, age and sex. A common choice is the Mifflin-St Jeor equation, built in 1990 from lab measurements of 498 healthy adults, 247 of them women, aged 19 to 78 (Mifflin et al.). For women it reads:

  • 10 × weight in kg + 6.25 × height in cm − 5 × age in years − 161

Take a 35-year-old woman who is 165 cm (5 ft 5 in) tall and weighs 68 kg (150 lb). The equation gives her about 1,375 kcal of resting energy a day.

That resting figure is then multiplied by an activity factor to cover everything else you do, and a goal may shift the result up or down. Imoonk's estimated setup, for example, multiplies Mifflin-St Jeor by 1.2, then subtracts 250 kcal, adds nothing or adds 250 kcal for lose, maintain or gain, and you can type in a manual budget instead. For the woman above, maintain would come to about 1,650 kcal.

Error one: the equation

An equation fitted to a group predicts the average person of your size, age and sex. You may not be that person. A systematic review for the American Dietetic Association compared the four equations most used in clinical practice and found Mifflin-St Jeor the most reliable: it came within 10% of measured resting energy in more people than any other, with the narrowest error range. The same review said noteworthy errors remain when it's applied to individuals, and that older adults and ethnic minority groups in the US were underrepresented in the research behind these equations (Frankenfield et al.).

Within 10% sounds tight until you put it in numbers. For our example, 10% either way spans roughly 1,240 to 1,510 kcal of resting energy, or about 1,485 to 1,815 kcal once multiplied by 1.2, and some people fall outside even that range.

Your cycle can nudge things too, though less than people often expect. A meta-analysis of 26 studies covering 318 women found resting energy slightly higher in the luteal phase, the two weeks or so before a period. In studies published since 2000 the difference was smaller and no longer statistically significant (Benton et al.).

Error two: the activity factor

The multiplier is usually the biggest single assumption. The European Food Safety Authority says physical activity levels depend on your type of work, daily exercise and household tasks. It bases its example adult requirements on a moderately active level of 1.6 and calls these levels only rough indications (EFSA).

For our example, 1.2 gives about 1,650 kcal and 1.6 gives about 2,200. That 550 kcal gap is bigger than the equation's error. A low factor such as 1.2 assumes very little movement and leaves exercise to be added separately. A higher one already includes some of your activity, so if your budget uses one, adding workouts on top can count them twice.

Error three: the food side

A budget only means something next to what you eat, and food numbers have margins of their own.

US labels are allowed some slack. Under FDA rules, a packaged food counts as misbranded if its calories are more than 20% above the value on the label (21 CFR 101.9). When researchers at Tufts tested reduced-calorie meals, 10 supermarket frozen meals averaged 8% more energy than their labels stated, and 29 restaurant dishes averaged 18% more. Some single restaurant items reached 200% of their stated value (Urban et al., 2010).

A larger follow-up measured 269 items from 42 restaurants in three states. On average the stated calories were close, within 10 kcal a portion. Individual foods varied, though: 19% held at least 100 kcal more than stated, and dishes with lower stated calories tended to hold more than listed (Urban et al., 2011).

Then there's the log itself. A systematic review of 59 studies with 6,298 adults compared what people reported eating with energy use measured by doubly labeled water, a lab method that tracks the energy you use over a week or two. Most studies found reported intake came in lower than measured energy use, and the size of the gap varied widely even between people using the same method (Burrows et al.). Small extras like a splash of oil in the pan or a taste while cooking are easy to leave out, which is ordinary and nothing to feel bad about. It does mean a log tends to read a little low.

The budget changes as you do

Even a perfect number would drift. Your energy use depends on your size, and it changes when your weight or routine changes. Researchers at the National Institutes of Health showed that the old rule of 3,500 calories per pound ignores these adaptations and overstates how quickly weight responds to a change in intake (Hall et al.). The NIH's online Body Weight Planner warns that its own results may be too high for people who are very sedentary, and notes that people often underestimate how much they eat (NIDDK).

A budget set from today's weight fits today, and if your weight or routine shifts a lot, it's worth recalculating.

How to check your own number

Since the budget and the log are both estimates, the test is whether they hold up together over time. The scale is the obvious measure, but it's noisy. In a study of adults living normally, about 84% of two-week weight change was fat-free mass, which includes water (Bhutani et al.). In a European study of 1,421 people who used smart scales, weight also followed a weekly rhythm, highest after the weekend and lowest on Friday (Turicchi et al.).

If you weigh yourself, compare like with like:

  • Use a weekly average of weigh-ins taken at the same time of day, not single days.
  • Look across a few weeks before you decide anything.
  • Log as you usually do. A careful week followed by a loose one makes the comparison harder to read.
  • If the trend and your intended direction disagree, change one thing at a time, by a modest amount, and give it a few more weeks.

If your budget says maintain and the average holds steady, your budget and your log agree for you, whatever the equation says. If the average drifts, the size and direction of the drift show which way to adjust.

Weighing regularly isn't right for everyone. If you have a medical condition, are pregnant, or have a history of disordered eating, a doctor or registered dietitian is a better guide to your energy needs than any equation.

Sources

  1. A new predictive equation for resting energy expenditure in healthy individuals — The American Journal of Clinical Nutrition, 1990.
  2. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review — Journal of the American Dietetic Association, 2005.
  3. Effect of menstrual cycle on resting metabolism: A systematic review and meta-analysis — PLOS ONE, 2020.
  4. EFSA sets average requirements for energy intake — European Food Safety Authority, 2013.
  5. 21 CFR 101.9: Nutrition labeling of food — U.S. Code of Federal Regulations, 2024.
  6. The accuracy of stated energy contents of reduced-energy, commercially prepared foods — Journal of the American Dietetic Association, 2010.
  7. Accuracy of stated energy contents of restaurant foods — JAMA, 2011.
  8. Validity of Dietary Assessment Methods When Compared to the Method of Doubly Labeled Water: A Systematic Review in Adults — Frontiers in Endocrinology, 2019.
  9. Quantification of the effect of energy imbalance on bodyweight — The Lancet, 2011.
  10. Body Weight Planner — National Institute of Diabetes and Digestive and Kidney Diseases, 2019.
  11. Composition of two-week change in body weight under unrestricted free-living conditions — Physiological Reports, 2017.
  12. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention — PLOS ONE, 2020.