Sources & Methodology

Last Updated: September 12, 2026

CalorieDay provides estimates for general informational purposes only. It is not medical advice, and it does not diagnose, treat, or prevent any condition. Calorie and protein targets are generated from published equations using the details you enter, and they can be wrong for you. Talk to a doctor or registered dietitian before changing your diet, especially if you are pregnant or breastfeeding, under 18, have a medical condition, take medication, or have a history of disordered eating. If a target ever feels too low or you feel unwell, stop and seek professional advice.

What this page covers

1. How your daily calorie target is calculated

The target is built in four steps from your sex, age, height, weight, activity level, goal and chosen pace.

Step 1 — Resting energy (basal metabolic rate). We use the Mifflin–St Jeor equation[1], which a systematic review by the Academy of Nutrition and Dietetics found to be the most reliable of the common predictive equations for healthy adults[2].

Men:   BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

If you choose not to specify a sex, we use the midpoint of the two constants.

Step 2 — Daily energy burn (maintenance calories). Resting energy is multiplied by an activity factor that reflects how much you move on a typical day. This is the physical activity level approach described by the FAO/WHO/UNU expert consultation on human energy requirements[3], using the activity factors commonly paired with the Mifflin–St Jeor and Harris–Benedict equations.

Activity level in the appFactor
Mostly sitting1.2
A little active1.375
Fairly active1.55
Very active1.725

The result is your estimated maintenance intake: the amount at which your weight would be expected to stay roughly the same.

Step 3 — Adjusting for your goal. If you want to lose weight, we subtract a daily deficit based on the pace you choose (gentle 0.25 kg, moderate 0.5 kg, or faster 0.75 kg per week). The deficit uses the long-standing approximation that about 7,700 kcal corresponds to one kilogram of body fat[4], so a pace of 0.5 kg per week is roughly a 550 kcal daily deficit. This is a simplification: research shows that weight loss slows over time as the body adapts, so real results will be slower than the straight-line estimate [5], [6]. If you want to gain weight, we add a surplus of half the equivalent amount, because muscle gain is supported by a modest surplus while larger surpluses mostly add fat [7]. Maintaining weight uses maintenance calories directly.

Step 4 — Safety limits. Before showing you a number, we apply the following limits, in this order, and tell you in the app when your chosen pace has been slowed down because of them:

You can override the calculated target at any time in My Plan. If you do, the app uses your number instead of the calculated one.

2. How your protein target is calculated

The protein target is 1.6 grams per kilogram of reference body weight per day. The reference weight is the lower of your current weight and your target weight (and never below 40 kg), so that the target reflects the body you are aiming for rather than excess weight. The result is kept between 60 g and 180 g and rounded to the nearest 5 g.

Why 1.6 g/kg: a meta-analysis of 49 trials found that benefits of protein for muscle mass and strength plateau at around 1.6 g/kg/day[10], and reviews of dieting athletes support intakes in the 1.6 to 2.4 g/kg range to preserve lean mass during a calorie deficit[11]. This is higher than the Recommended Dietary Allowance of 0.8 g/kg/day, which is the minimum to avoid deficiency in healthy adults rather than a target for people managing their weight [12]. People with kidney disease or other medical conditions should follow their clinician's advice on protein instead.

Carbohydrate and fat are recorded for each meal so you can see the full picture, but the app does not set targets for them. Energy from macronutrients is counted using the standard conversion factors of 4 kcal per gram of protein or carbohydrate and 9 kcal per gram of fat [13].

3. Weight goals and the projected goal date

The projected goal date is a straight-line estimate: the difference between your current and target weight divided by the weekly change your calorie target is designed to produce. It assumes the target is followed consistently and does not model metabolic adaptation, water-weight changes, or changes in activity, so treat it as a rough guide rather than a promise[5], [6]. The target weight you enter is your own choice; CalorieDay does not tell you what you should weigh.

4. Where nutrition data comes from

Photo scans (AI estimates). When you photograph a meal, an AI vision model identifies the foods it can see and estimates portion sizes, calories, protein, carbohydrate and fat. These figures are approximate. Image-based dietary assessment is a recognised research method, but portion size, cooking oils, sauces, and ingredients hidden inside a dish are known sources of error[14]. The app shows a confidence level for each item, and you can correct the name, quantity or values before saving. For packaged foods, the label is more accurate than a photo.

Barcode scans. Barcode lookups use Open Food Facts, an open, collaborative database of packaged foods maintained by volunteers and manufacturers[15]. Values are shown per serving or per 100 g as recorded in the database. Entries can be incomplete or out of date, so compare them against the product label when in doubt.

Manual entries. When you type a meal in yourself, the app records the values you enter without changing them. If you want to check a value, USDA FoodData Central is a reliable public reference for the nutrient content of foods[16].

For general healthy-eating guidance, see the Dietary Guidelines for Americans [17] or your national health authority.

References

  1. 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):241–247. https://doi.org/10.1093/ajcn/51.2.241
  2. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775–789. https://doi.org/10.1016/j.jada.2005.02.005
  3. FAO/WHO/UNU. Human energy requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. FAO Food and Nutrition Technical Report Series 1. Rome; 2004. https://www.fao.org/3/y5686e/y5686e00.htm
  4. Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542–546. https://doi.org/10.1093/ajcn/6.5.542
  5. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826–837. https://doi.org/10.1016/S0140-6736(11)60812-X
  6. National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner. https://www.niddk.nih.gov/bwp
  7. Slater GJ, Dieter BP, Marsh DJ, Helms ER, Shaw G, Iraki J. Is an energy surplus required to maximize skeletal muscle hypertrophy associated with resistance training? Front Nutr. 2019;6:131. https://doi.org/10.3389/fnut.2019.00131
  8. National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report. NIH Publication No. 98-4083; 1998. https://www.nhlbi.nih.gov/files/docs/guidelines/ob_gdlns.pdf
  9. Centers for Disease Control and Prevention. Steps for Losing Weight. https://www.cdc.gov/healthy-weight-growth/losing-weight/index.html
  10. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376–384. https://doi.org/10.1136/bjsports-2017-097608
  11. Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. Int J Sport Nutr Exerc Metab. 2014;24(2):127–138. https://doi.org/10.1123/ijsnem.2013-0054
  12. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press; 2005. https://doi.org/10.17226/10490
  13. FAO. Food energy — methods of analysis and conversion factors. FAO Food and Nutrition Paper 77. Rome; 2003. https://www.fao.org/3/y5022e/y5022e00.htm
  14. Boushey CJ, Spoden M, Zhu FM, Delp EJ, Kerr DA. New mobile methods for dietary assessment: review of image-assisted and image-based dietary assessment methods. Proc Nutr Soc. 2017;76(3):283–294. https://doi.org/10.1017/S0029665116002913
  15. Open Food Facts. The free food products database. https://world.openfoodfacts.org
  16. U.S. Department of Agriculture, Agricultural Research Service. FoodData Central. https://fdc.nal.usda.gov
  17. U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025. https://www.dietaryguidelines.gov

Questions or corrections

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