Energy budget

Every day's budget comes from one formula. Only the movement figure that goes into it varies, and the diary says in words where that figure came from. There is no second formula for days Apple Health knows nothing about.

The formula

Resting energy from the Mifflin–St Jeor equation, using the profile's weight, height, age and sex: 10 × kg + 6.25 × cm − 5 × age, plus 5 for men or minus 161 for women.
Movement-free budget: resting energy × 1.1. The 0.1 is the energy cost of digestion. Nothing else is assumed, because the movement figure already counts everything above resting.
Movement credit: half of the day's active energy. Wrist estimates run high and the body compensates for part of an exercise burn; crediting half is the conventional correction, and full credit measured generous against the scale in live use.
Goal adjustment: the pace you set, converted at 3,500 kcal per pound (7,700 per kilogram) and spread over seven days, subtracted to lose or added to gain.

Protein, fat, carbs and fiber then follow:

Protein: by default 0.8 g per kg of body weight to maintain, 1.2 in a deficit and 1.0 in a surplus, or a strength-training figure between 1.4 and 2.0 g/kg, or a number of grams you type and the app takes as given. The first two are kept between 10% and 35% of the full day's calories.
Fat: 30% of the movement-free budget. A workout never rewrites the fat line that was true at breakfast.
Carbs: whatever calories remain. Earned movement calories land here alone.
Fiber: 14 g per 1,000 kcal of the movement-free budget.
Caps: saturated fat under 10% of the movement-free calories, sodium under 2,300 mg.

Worked example

A 40-year-old man, 180 cm and 80 kg, aiming to lose a pound a week, on a day Apple Health measured 600 kcal of movement. Grams are rounded.

Step	Figure
Resting energy	800 + 1,125 − 200 + 5 = 1,730 kcal
Movement-free budget	1,730 × 1.1 = 1,903 kcal
Movement credit	600 × 0.5 = 300 kcal
Goal adjustment	−500 kcal
Day's budget	1,903 + 300 − 500 = 1,703 kcal
Protein	1.2 × 80 = 96 g
Fat	30% of (1,903 − 500) ÷ 9 = 47 g
Carbs	(1,703 − 96 × 4 − 47 × 9) ÷ 4 = 225 g
Fiber	14 × 1.403 = 20 g

Where the movement figure comes from

Connect movement from Progress › Movement. The app reads active energy from Apple Health and nothing else for this purpose. Then, for any given day:

A finished day Health measured uses that measurement. A day under 50 kcal is treated as a day the watch was not worn, not as a still day.
Today starts at a baseline and rises only as the measured total overtakes it. A quiet morning never reads as a sedentary day, and a watch that has not synced never lowers the number.
A day with nothing usable, or stale data, gets the baseline.

The baseline is the median of up to 14 valid days from the last 60. A median rather than a mean, so one missing day or one long hike does not move it. Nothing is ever budgeted above your own 90th-percentile day.

Cold start

Until five valid days exist, the baseline is a conservative figure implied by the activity level you picked in your profile: the movement that would make the one formula equal resting energy × 1.15 (sedentary) up to × 1.45 (very active). It blends into the measured median over those first five days so there is no cliff. After that, the activity level feeds no arithmetic anywhere.

What counts as stale

Health's newest sample is too old to describe today when there is none since the day before yesterday, or, after 10:00, none in the last six hours. An early morning is never stale: the last sample is from last night, and that is normal.

The budget cannot jump

Within one day the budget shown may not rise more than 250 kcal from what was shown before, except as the direct result of measured movement accumulating. A median that shifts when a late sync lands never moves the number far. Missing, stale, failed or unauthorised data can hold a budget or lower it, never raise it.

What the diary says

Basis	Meaning
Measured	Apple Health had the complete day.
Projected	Today: the trailing median until the measured total overtakes it.
Estimated	A day Health has nothing usable for, or a watch that has not synced: the median stands in.
Cold start	Fewer than five days of history: the activity-level figure, blending out as days accrue.

The same basis is recorded per day in the export, so a reader comparing a day's food against its target sees the budget the app actually displayed.

Safety limits

The budget never goes below 1,500 kcal for men or 1,200 for women. When the floor applies, the app says so.
The fastest loss pace computed is the lesser of 2 lb a week and 1% of body weight a week. The fastest gain is 0.5 kg a week. A faster request is capped, and the profile shows the requested pace beside the applied one.

Past days are a review, not a ledger

The Progress tab's energy history judges past days by the current profile and goal, the same way the weight projection redraws when the plan changes. Change your pace and every day is re-read against it.

The sources behind the numbers are credited in the app under About & Licenses: the Mifflin–St Jeor equation, the Dietary Guidelines for Americans 2020–2025, the Dietary Reference Intakes and CDC guidance on weight-change rates. General guidance, not medical advice.