FitMess

Where the numbers come from

In effect from 9 August 2026

FitMess shows you calories, macronutrients, BMI and an estimate of what a workout burned. None of those numbers are invented — each one comes from a published equation or guideline. This page says which ones, what exactly we do with them, and where to read the original.

First, the important part

FitMess is not a doctor and does not diagnose anything. The numbers here are estimates for a healthy adult, derived from population-level equations — and you are not a population. If you have any medical condition, if you are pregnant or breastfeeding, if you have or have had an eating disorder, or if you take medication that affects weight or appetite, do not follow the app's plan without talking to a doctor or dietitian first.

The app is intended for people aged 18 and over. It is not built for children or teenagers, whose requirements are calculated differently.

Your daily calories

First we compute basal metabolic rate (BMR) — what you burn at complete rest — with the 1990 Mifflin–St Jeor equation: for men 10 × kg + 6.25 × cm − 5 × age + 5, for women the same − 161. In comparisons it is the equation most likely to land within 10% of a measured value.

BMR is then multiplied by the activity factor you picked: 1.2 (sedentary) through 1.9 (very active). That five-step table is applied convention rather than a figure from a single paper; the idea behind it — total expenditure = BMR × physical activity level (PAL) — is the method the FAO/WHO/UNU consultation prescribes. It is also the largest source of error in the whole chain: the gap between "lightly active" and "moderately active" is over 300 kcal a day, and nobody rates themselves accurately. That is why FitMess later uses your scale to correct the estimate.

  • 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. PMID 2305711. https://pubmed.ncbi.nlm.nih.gov/2305711/
  • FAO / WHO / UNU Expert Consultation. Human energy requirements (FAO Food and Nutrition Technical Report Series 1). Rome: FAO, 2004. https://www.fao.org/4/y5686e/y5686e00.htm

Safety limits

The deficit is never more than 25% below estimated expenditure, and the daily target never drops below 1400 kcal for men or 1200 kcal for women, however aggressive a goal you enter. Those limits follow the NIH/NHLBI clinical guidelines, which define a low-calorie diet as 1000–1200 kcal for women and 1200–1500 for men, and a pace of about 0.5–1 kg a week from a 500–1000 kcal daily deficit.

  • National Heart, Lung, and Blood Institute (NIH). Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication No. 98-4083, 1998. https://www.ncbi.nlm.nih.gov/books/NBK2003/

Protein, fat and carbohydrate

Protein is targeted at 2.0 g per kilogram of body mass, and when the budget cannot fit that, it is walked down no further than 1.6 g/kg. A 2018 meta-analysis puts the plateau for muscle gain at about 1.6 g/kg with a confidence interval reaching ~2.2; the ISSN position stand recommends 1.4–2.0 g/kg for people who train, and more during a deficit — which is exactly where most FitMess users are.

Fat is targeted at 0.8 g/kg and never falls below 0.6 g/kg, because fat carries the fat-soluble vitamins and the essential fatty acids. Saturated fat is tracked as a soft ceiling of 10% of daily calories, per the WHO guideline.

Carbohydrate is what is left: the calorie budget minus protein and fat. It has no recommendation of its own because FitMess does not restrict it — no food in the app is "forbidden".

  • 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. PMID 28698222. https://pubmed.ncbi.nlm.nih.gov/28698222/
  • Jäger R, Kerksick CM, Campbell BI, et al.. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20. PMID 28642676. https://pubmed.ncbi.nlm.nih.gov/28642676/
  • World Health Organization. Saturated fatty acid and trans-fatty acid intake for adults and children: WHO guideline. Geneva: WHO, 2023. https://www.who.int/publications/i/item/9789240073630

How fast weight changes

When you set a target weight and a timeframe, the app derives the required deficit from the rule of roughly 7700 kcal per kilogram of body mass (Wishnofsky, 1958 — better known as "3500 kcal per pound"). That is arithmetic, not physiology: real loss slows as the body changes, which is the published criticism of the rule. So we use it only as a starting estimate of pace — never as a promise — and it always passes through the safety limits above.

BMI

BMI is weight in kilograms divided by height in metres squared, and the zones (under 18.5 / 18.5–24.9 / 25–29.9 / 30 and above) are the standard WHO classification. BMI cannot tell muscle from fat and reads high for a very muscular person — we show it as a bearing, never as a verdict.

What a workout burns

Workouts are scored with MET values from the Compendium of Physical Activities (Ainsworth et al., 2011), kept NET — 1 MET is subtracted from each value, because an hour of training is worth what it burns ABOVE the hour of sitting it replaced. MET values are population averages with wide individual spread (about ±20% for one person), so in FitMess they are a record of effort and are never added back to the daily budget as "earned calories".

  • Ainsworth BE, Haskell WL, Herrmann SD, et al.. 2011 Compendium of Physical Activities: a second update of codes and MET values. Med Sci Sports Exerc. 2011;43(8):1575–1581. PMID 21681120. https://pubmed.ncbi.nlm.nih.gov/21681120/

Fibre, sugar, sodium, saturated fat

Fibre is targeted at 14 g per 1000 kcal (US Dietary Guidelines / IOM), sugar is tracked as a soft ceiling of 10% of calories (the WHO free-sugars guideline), sodium at 2300 mg a day (WHO is stricter, at 2000 mg), and saturated fat at 10% of calories.

One honest caveat: what we can read from a photo or a label is TOTAL sugars, while the WHO recommendation is about free sugars. Our ceiling is therefore stricter than it needs to be, and deliberately soft — a fruit-heavy day is not a failure and the app never colours it red.

How we read your log and your scale

Self-reported food intake is under-reported by 20–30% on average in studies that compare it against doubly labelled water. So FitMess does not take the daily total on faith: a day whose intake falls below basal metabolic rate is flagged as incomplete (the standard Goldberg cut-off), and your own answer always beats the heuristic.

Weight is shown as a seven-day rolling average rather than a daily figure, because daily swings mostly measure water and salt. Once there are enough trusted days, your actual expenditure is computed from intake versus weight change and used to correct the original estimate — measuring instead of guessing.

  • Livingstone MBE, Black AE. Markers of the validity of reported energy intake. J Nutr. 2003;133(3):895S–920S. PMID 12612176. https://pubmed.ncbi.nlm.nih.gov/12612176/
  • Goldberg GR, Black AE, Jebb SA, et al.. Critical evaluation of energy intake data using fundamental principles of energy physiology: 1. Derivation of cut-off limits to identify under-recording. Eur J Clin Nutr. 1991;45(12):569–581. PMID 1810719. https://pubmed.ncbi.nlm.nih.gov/1810719/

Reference list

Every paper and guideline the app relies on, with links to the originals.

  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. PMID 2305711. https://pubmed.ncbi.nlm.nih.gov/2305711/
  2. FAO / WHO / UNU Expert Consultation. Human energy requirements (FAO Food and Nutrition Technical Report Series 1). Rome: FAO, 2004. https://www.fao.org/4/y5686e/y5686e00.htm
  3. National Heart, Lung, and Blood Institute (NIH). Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication No. 98-4083, 1998. https://www.ncbi.nlm.nih.gov/books/NBK2003/
  4. Jäger R, Kerksick CM, Campbell BI, et al.. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20. PMID 28642676. https://pubmed.ncbi.nlm.nih.gov/28642676/
  5. 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. PMID 28698222. https://pubmed.ncbi.nlm.nih.gov/28698222/
  6. World Health Organization. Saturated fatty acid and trans-fatty acid intake for adults and children: WHO guideline. Geneva: WHO, 2023. https://www.who.int/publications/i/item/9789240073630
  7. Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542–546. PMID 13594881. https://pubmed.ncbi.nlm.nih.gov/13594881/
  8. Hall KD, Chow CC. Why is the 3500 kcal per pound weight loss rule wrong?. Int J Obes (Lond). 2013;37(12):1614. https://www.nature.com/articles/ijo2013112
  9. World Health Organization. A healthy lifestyle — WHO recommendations (body mass index classification). WHO Regional Office for Europe, 2010. https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle---who-recommendations
  10. Ainsworth BE, Haskell WL, Herrmann SD, et al.. 2011 Compendium of Physical Activities: a second update of codes and MET values. Med Sci Sports Exerc. 2011;43(8):1575–1581. PMID 21681120. https://pubmed.ncbi.nlm.nih.gov/21681120/
  11. U.S. Departments of Agriculture and Health and Human Services. Dietary Guidelines for Americans, 2020–2025. 9th edition, December 2020. https://www.dietaryguidelines.gov/
  12. World Health Organization. Guideline: sugars intake for adults and children. Geneva: WHO, 2015. https://www.who.int/publications/i/item/9789241549028
  13. World Health Organization. Guideline: sodium intake for adults and children. Geneva: WHO, 2012. https://www.who.int/publications/i/item/9789241504836
  14. Goldberg GR, Black AE, Jebb SA, et al.. Critical evaluation of energy intake data using fundamental principles of energy physiology: 1. Derivation of cut-off limits to identify under-recording. Eur J Clin Nutr. 1991;45(12):569–581. PMID 1810719. https://pubmed.ncbi.nlm.nih.gov/1810719/
  15. Livingstone MBE, Black AE. Markers of the validity of reported energy intake. J Nutr. 2003;133(3):895S–920S. PMID 12612176. https://pubmed.ncbi.nlm.nih.gov/12612176/

When this changes

This page is part of the code, not the marketing: if an equation in the app changes, this changes with it. If you think something here is wrong or out of date, write to us — that is the one report we always check.