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The science

How PrivyHealth calculates your calories and macros

Every number PrivyHealth shows you comes from a published formula, not a guess. Here's the maths behind your targets — and an honest look at how accurate the photo estimates really are.

Our formulas are clinically established and evidence-based — the same ones a dietitian reaches for first. They're population-level, though, so they give you a strong starting point that gets sharper as you log real data.

Basal metabolic rate

Your BMR — the energy your body burns just to stay alive at rest — is estimated with the Mifflin-St Jeor equation, published in 1990. It replaced the older Harris-Benedict formula because it was re-derived from a larger, more modern sample and tracks measured resting energy expenditure more closely across a wider range of body sizes. The equation takes your weight, height, age, and sex and returns a daily calorie floor. Everyone's metabolism varies a little, so this is a starting estimate, not a measurement.

Formula

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s

s = +5 for men, −161 for women, −78 for other/unknown.

Total daily energy expenditure

Your TDEE multiplies BMR by an activity factor that reflects how much you move. PrivyHealth uses the standard bracket of multipliers from 1.2 (sedentary) up to 1.9 (very active). These are population averages — a useful default, but a heart-rate monitor or fitness device will always be more precise because it measures your actual energy burn rather than estimating it from how active you say you are. That's why we sync Apple Health, Strava and the rest: real movement data makes the number more honest over time.

Formula

TDEE = BMR × activity factor (1.2 – 1.9)

Sedentary 1.2 · Light 1.375 · Moderate 1.55 · Active 1.725 · Very active 1.9

Goal adjustments

From your maintenance calories, PrivyHealth applies a goal-based percentage to set your daily target. A gentle 18% deficit supports fat loss without crashing your energy; a 12% surplus gives muscle gain room to grow; endurance leans +8% for training fuel; maintenance and general wellness hold steady. The percentage approach is gentler and more sustainable than a fixed calorie drop, and it scales naturally to your body size.

Formula

Target = TDEE × (1 + adjustment)

Fat loss −18% · Maintain 0% · Muscle gain +12% · Endurance +8% · Wellness 0%

Macro split

Protein is set per kilogram of body weight, tuned by goal: from 1.5 g/kg for general wellness up to 2.2 g/kg for building muscle. These ranges follow current sports-nutrition guidance from the ISSN and ACSM, which recommend roughly 1.6–2.2 g/kg to support recovery and lean mass. Fat is set at about 28% of calories (using the Atwater factor of 9 kcal per gram), and the remaining calories fill carbs at 4 kcal per gram. A minimum carb floor of 40 g keeps your brain and muscles fuelled even on low-carb approaches.

Formula

Protein = g/kg × weight · Fat = 0.28 × kcal ÷ 9 · Carbs = remaining kcal ÷ 4

Protein 1.5–2.2 g/kg · Fat ≈28% · Carbs from remainder (floor 40 g)

Sugar ceiling

The World Health Organization recommends limiting free sugars to under 10% of daily energy, and ideally under 5%. PrivyHealth uses the tighter 5% cap when your goal is fat loss, 8% for maintenance and endurance, and 10% for muscle gain, then converts that to grams (sugar is 4 kcal/g). The sugar ceiling is a cap to stay under, not a target to reach — it nudges you toward whole-food carb sources rather than added sugar.

Formula

Sugar cap = kcal × (5% – 10%) ÷ 4

Fat loss 5% · Maintain/Endurance 8% · Muscle gain 10% · Keto/Low-carb override to 20–30 g

Diet-aware tweaks

Your dietary preferences adjust the baseline. High-protein diets add 0.2 g/kg to your protein target. Plant-based (vegan/vegetarian) protein is slightly less bioavailable, so we nudge protein up 0.1 g/kg to compensate. Keto raises fat to about 68% of calories and cuts carbs to a 25 g floor; low-carb sets fat to ~42%; Mediterranean leans to ~34% from olive oil, fish and nuts; endurance trims fat so more energy lands in carbs. Gluten-free and dairy-free preferences shift recommended food sources without changing the math.

Formula

Adjusted per preference: protein +0.1–0.2 g/kg, fat 24%–68%

Keto 68% fat · Low-carb 42% · Mediterranean 34% · Endurance 24% fat

From your plate: how accurate is it?

Honesty matters more than confidence here. Different parts of PrivyHealth are accurate at different levels — here's exactly where each one sits.

What we calculateAccuracy levelWhy
Daily calorie & macro targetsEntry-level dietitian / sports-nutrition 101Established formulas (Mifflin-St Jeor, ACSM/ISSN protein ranges, WHO sugar), but population-derived — not tailored to your labs or body composition.
Meal estimates from photosAI best-guess (±20–40%)A vision model guesses the dish and portions, then assigns macros from its training. Useful for tracking trends — not clinical precision. A confidence score and human-edit step narrow the error before anything is logged.
Barcoded & known foodsDatabase-accurateFoods from Open Food Facts carry verified per-serving data, so this path is far more precise than the photo estimate.
  • Every photo estimate shows a confidence score and waits for your accept/edit before logging.
  • You can replace any ingredient with a database lookup or a manual value at any time.
  • Targets adapt over time from 21 days of your real logs, not just a one-time formula.

Sources

  • Mifflin-St Jeor equation — Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.
  • WHO free-sugar guidance — World Health Organization. Guideline: Sugars intake for adults and children. Geneva: WHO; 2015. Recommends reducing free sugars to under 10% of total energy, with a conditional recommendation of under 5% for additional benefits.
  • Protein guidance — International Society of Sports Nutrition (ISSN) and American College of Sports Medicine (ACSM) position stands support 1.2–2.2 g/kg/day for active populations, with higher ends for lean-mass preservation and hypertrophy.
  • Atwater factors — The general Atwater conversion (protein 4 kcal/g, carbohydrate 4 kcal/g, fat 9 kcal/g) underlies all macro-to-calorie conversions, including the USDA FoodData Central tables.

See it work on your meals

Snap a plate and PrivyHealth puts the science to work — targets, trends and swaps tuned to your goals.

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