How Wellix calculates your calorie and macro targets

The full methodology, including the coefficients we cannot cite and the limitations we would rather you knew about up front.

Step 1 — Resting metabolic rate

Wellix uses two different equations depending on whether you are trying to gain weight or to lose or maintain it. This is a historical inconsistency in our code rather than a considered scientific position, and we explain it in What we do not claim.

If your goal is to lose or maintain weight

Wellix uses the original Harris–Benedict equations, first published in 1919,¹ in their commonly circulated rounded form. Weight is in kilograms, height in centimetres, age in years.

Wellix then applies a reduction based on your body mass index. Predictive equations of this kind are known to overestimate resting energy expenditure as body fat rises, because fat mass is less metabolically active per kilogram than lean mass, and the equations were fitted on a largely lean population.

If your goal is to gain weight

Wellix uses the revised Harris–Benedict equation published by Pavlidou and colleagues in Metabolites in 2023.² Note that height is in metres here, not centimetres, and that no BMI reduction is applied on this path.

Step 2 — Activity coefficient and maintenance energy

Your resting metabolic rate is multiplied by a coefficient reflecting how much you move. The result is your maintenance energy requirement — the intake at which your weight would be expected to hold steady. Wellix shows this figure to you directly.

The two columns differ, and the narrow 1.25–1.45 band on the lose/maintain path is deliberately conservative: it produces a lower maintenance estimate, and therefore a lower calorie target, than the widely used 1.2–1.9 band. Both bands are conventions rather than measurements. Neither is derived from a source we can cite.

Step 3 — Turning a weekly pace into a daily calorie delta

You choose how fast you want to change weight, in kilograms per week. Wellix clamps that choice to between 0.1 and 1.0 kg per week and converts it to a daily energy delta using a fixed conversion of 7,700 kcal per kilogram of body tissue:

Daily delta (kcal) = pace (kg/week) × 7,700 ÷ 7

So a 0.5 kg/week loss target produces a deficit of 550 kcal/day, subtracted from your maintenance figure. Wellix also reports an estimated number of weeks to reach your goal weight, computed from the delta that survives the safety floor in the next step.

Step 4 — Safety floors

Wellix will never set a daily calorie target below 1,500 kcal for men or 1,200 kcal for women. If the pace you selected would push the target beneath that floor, Wellix reduces the deficit until the target sits exactly on the floor, recalculates the pace you will actually achieve, and tells you that it did so. Your selected pace is honoured everywhere it is safe to honour it, and silently overridden nowhere.

These floors are widely used clinical conventions for self-directed calorie restriction. They are not a substitute for supervision: very low calorie intakes belong under the care of a clinician, and Wellix is not one.

Step 5 — Protein, fat and carbohydrate

The macro split depends on which path you are on. They genuinely differ, and the difference is not cosmetic.

Lose and maintain: a fixed percentage split

Wellix allocates your calorie target across the three macronutrients in a fixed 25 : 30 : 35 ratio of protein : fat : carbohydrate. Because those shares sum to 90% rather than 100%, Wellix renormalises them so the macro targets reconcile exactly against the calorie target:

• Protein — 27.8% of calories ÷ 4 kcal/g
• Fat — 33.3% of calories ÷ 9 kcal/g
• Carbohydrate — 38.9% of calories ÷ 4 kcal/g

For a 1,800 kcal target this yields roughly 125 g protein, 67 g fat and 175 g carbohydrate. For an 80 kg person that protein figure is about 1.6 g/kg, which sits inside the 1.4–2.0 g/kg/day range the International Society of Sports Nutrition considers sufficient for most exercising individuals.⁴ Note that this is a coincidence of arithmetic, not a design constraint: because the split is a percentage of calories, a heavier person on a low calorie target will land at a lower g/kg figure.

Gain: protein and fat dosed on a reference weight

On the gain path Wellix doses protein and fat per kilogram of a reference weight rather than your raw bodyweight, and gives carbohydrate the remainder. Dosing on total bodyweight over-doses heavier users, because excess fat mass is not metabolically demanding, and it collapses the carbohydrate allowance toward zero.

• Reference weight — your current weight, capped at whatever you would weigh at a BMI of 24.9.
• Protein — 1.6 g per kg of reference weight, plus 0.2 g/kg if you are 50 or older, as a guard against age-related muscle loss. Capped so protein can never exceed 40% of your calories.
• Fat — 30% of calories, with an essential-fat floor of 0.5 g per kg of reference weight.
• Carbohydrate — whatever calories remain, with a 50 g floor.

Step 6 — Water

Wellix sets a target for what you drink, not for your total water intake, because the app logs beverages and a meaningful share of your water arrives inside food.

• 28 ml per kg of bodyweight for men, 25 ml per kg for women.
• Floored at 2,000 ml for men and 1,600 ml for women.
• Plus 500 ml at medium activity or 1,000 ml at high activity.
• Capped at 4,000 ml, as a guard against over-drinking.
• Rounded to the nearest 50 ml.

What happens when we do not know something

If you have not told Wellix your age, height, weight or sex, it substitutes a default rather than refusing to compute: age 30, height 170 cm, and 70 kg for men or 60 kg for women. Users recorded as neither male nor female are computed with the male equation, which is a limitation of using sex-split equations rather than a statement about anyone. Every default degrades the accuracy of your targets, so filling in your real values is worth the thirty seconds.

What we do not claim

Five things about the above that we would rather you heard from us.

1. We use two different RMR equations depending on your goal. The lose/maintain path uses the 1919 original; the gain path uses the 2023 revision. There is no scientific reason for a person's metabolism to be modelled differently according to their intention. This is an artefact of how the two paths were written, we regard it as a defect, and we are documenting it rather than concealing it.
2. Harris–Benedict is not the most accurate available equation. Mifflin–St Jeor is generally considered the better predictor of resting energy expenditure in healthy adults, and the authors of the 2023 revision report their equation outperforming both.² Our lose/maintain path currently uses neither.
3. The BMI reduction and the activity coefficients are conventions, not measurements. They are labelled as such above.
4. Individual metabolic rates vary by roughly ±10% around any predictive equation. Your true maintenance intake may sit meaningfully above or below the number Wellix shows you. The only way to find it is to track intake and weight for several weeks and adjust. This is why Wellix tracks your weight trend alongside your intake.
5. A calorie target is not a diagnosis. These formulas assume an adult with no condition that alters energy expenditure or nutrient requirements. Pregnancy, breastfeeding, thyroid disease, kidney disease, diabetes, eating disorders and many medications all change the answer, and none of them are inputs to our model.

How the food numbers get in

The targets above describe what you are aiming at. The other half of the system is what you actually ate, which Wellix estimates from a photograph using a vision model. That estimate has its own error characteristics, which we cover honestly — including the published figures and the cases where photo estimation fails — in how accurate are AI calorie counting apps.

Frequently asked questions

What formula does Wellix use to calculate calories?

Wellix estimates resting metabolic rate with a Harris–Benedict equation — the 1919 original for weight loss and maintenance, and the 2023 revision by Pavlidou et al. for weight gain — multiplies it by an activity coefficient between 1.2 and 1.9 to obtain maintenance energy, then applies a daily calorie delta derived from your chosen weekly rate of weight change at 7,700 kcal per kilogram.

Does Wellix use the Mifflin–St Jeor equation?

No. Wellix currently uses Harris–Benedict equations. Mifflin–St Jeor is generally regarded as a more accurate predictor of resting energy expenditure in healthy adults, and we state this openly on our methodology page rather than implying our choice is the best available one.

What is the lowest calorie target Wellix will give me?

Wellix will never set a target below 1,500 kcal per day for men or 1,200 kcal per day for women. If the weight-loss pace you selected would require going below that floor, Wellix reduces the deficit to sit exactly on the floor, recomputes the pace you will actually achieve, and tells you it has done so.

How does Wellix decide my protein target?

It depends on your goal. If you are losing or maintaining weight, protein is a fixed 27.8% of your calorie target. If you are gaining weight, protein is dosed at 1.6 g per kilogram of a reference weight capped at BMI 24.9, plus 0.2 g/kg if you are 50 or older, and capped at 40% of your calories.

Why does Wellix reduce my metabolic rate estimate if my BMI is high?

Because predictive equations like Harris–Benedict were fitted on largely lean populations and overestimate resting energy expenditure as body fat increases, since fat mass burns fewer calories per kilogram than lean mass. Wellix applies a reduction of 5% to 20% depending on BMI. The direction of that correction is well supported; the specific step sizes are a Wellix product default and not a published coefficient.

Is the estimated time to reach my goal weight reliable?

Treat it as an optimistic approximation. It is derived from a fixed 7,700 kcal per kilogram conversion, which cannot capture the fact that the body’s response to an energy deficit is slow — with a half-time of roughly a year — and varies with body composition. Expect real progress to lag the estimate, especially over long goals.

References

1. Harris JA, Benedict FG. A Biometric Study of Basal Metabolism in Man. Carnegie Institution of Washington, Publication No. 279. Washington, DC; 1919.
2. Pavlidou E, Papadopoulou SK, Seroglou K, Giaginis C. Revised Harris–Benedict Equation: New Human Resting Metabolic Rate Equation. Metabolites. 2023;13(2):189. doi:10.3390/metabo13020189. pmc.ncbi.nlm.nih.gov/articles/PMC9967803
3. Hall KD, Sacks G, Chandramohan D, Chow CC, Wang YC, Gortmaker SL, Swinburn BA. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826–837. PMID: 21872751. pubmed.ncbi.nlm.nih.gov/21872751
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. doi:10.1186/s12970-017-0177-8. pmc.ncbi.nlm.nih.gov/articles/PMC5477153

Wellix provides general nutrition information and is not medical advice. It does not diagnose, treat, or prevent any condition. Calorie and macronutrient targets are estimates produced by predictive equations, and individual requirements vary. Consult a qualified healthcare professional before making significant dietary changes, particularly if you are pregnant or breastfeeding, or have a history of disordered eating, diabetes, kidney disease, thyroid disease, or any other medical condition.

Related

• How accurate are AI calorie counting apps?
• How to track calories without weighing your food
• How to track calories at a restaurant

Targets you can inspect, from a log that takes a photo

Wellix computes the numbers above from your body and your goal, then shows its work.