Where the numbers in an eating plan come from

When you set up an eating plan, the coach recommends a daily figure. This is what that figure is made of, so you can change it knowing what you are changing.

What you burn

Everything starts from an estimate of how much energy you use in a day. The coach takes the best basis it has, in this order:

What you have logged, against what the scale did. Two weeks or more of your own food alongside your own weight is arithmetic about you rather than an estimate of you: if your weight held, what you ate is what you burn, and if it moved, the difference says by how much. Nothing beats it, so it goes first.
What your device recorded. If you have a few weeks of Apple Health, your resting and active energy are already there. That is measured too, and it beats any formula — it is second only because a watch is measuring movement and inferring the rest.
A formula. With neither, it uses your weight, height, age and sex — and your body fat, if you know it — to estimate resting energy, then scales it by how active your days are. The formulas are the standard ones (Mifflin–St Jeor; Katch–McArdle when body fat is known, because knowing your lean mass makes the estimate a better one). They are typically within about ten percent for most people, which is why the coach treats the result as a starting point and revises it from your own weeks.

The setup shows which of these it used. If the device or your record is there, the formula is not.

How fast to move

A cut or a gain is a rate, and the rate is what the coach is careful about.

For a cut, losing around half a percent to one percent of your body weight a week keeps most of what you lose fat rather than muscle, and keeps the deficit small enough to live with. Faster than that, the coach says so, and it will not recommend more than one percent a week.
For a gain, a quarter to half a percent a week puts on mostly muscle if you are training. Faster mostly puts on fat.
The daily figure has a floor. Below a certain intake it is very hard to get the protein, fat and micronutrients a body needs, whatever the goal. The coach will not recommend under it, and if your target and dates would need it, it says which one to move.

A kilogram of body fat holds roughly 7,700 kcal, so a deficit of about 500 kcal a day is about half a kilogram a week. That is the arithmetic behind the projected line on the calories step.

Why protein is set first

Protein is the macro that decides whether a cut costs you muscle and whether a gain builds it, so it is set before anything else and the rest of the day is divided around it.

If you know your body fat, protein comes from your lean mass — about a gram per pound of it. This is the better version of the rule, because the muscle you are protecting is the part of you that needs the protein, and your fat mass does not change the answer.
If you do not, it comes from your body weight — about eight tenths of a gram per pound. It lands in the same place for most people; it drifts high for someone carrying a lot of fat mass, which is why the setup says a body-fat figure would sharpen it.

Either way the figure is rounded to the nearest five grams, because a plan that asks for a hundred and sixty-three grams is a plan pretending to a precision that food does not have.

Fat has a floor. It is set at about thirty percent of the day by default — enough for hormones, and enough for meals to be worth eating — and the coach will not take it below about a third of a gram per pound of your body weight, or below fifty grams a day for anyone, whichever is higher. Fat is not an optional macro: below that the day stops carrying enough for hormonal function, and the coach says so rather than quietly accepting the number. If you type a lower one, it tells you the floor and why.

What is left over is carbohydrate, which is where your training energy comes from. You can set all three yourself — there is a Set my own mode on the calories step, and the daily figure then follows your macros instead of the other way around.

The week, not the day

A week’s figure is what actually decides the outcome; the days are how you spend it. If you train on some days and not others, the setup can put a little more of the week on training days and a little less on rest days. The total stays the same. Nothing about this is required — the same figure every day works fine.

What the weekly review changes

Once a week the coach reads the week and, when there is enough of it to read, proposes a figure rather than asking you to pick one:

It needs five logged days and three weigh-ins in the week. Below either, it shows you the week and proposes nothing, because there is nothing to compare.
It never proposes anything off week one. Most of what the scale does in the first seven days is water and glycogen, so a figure moved off it is moved off nothing.
Moving faster than the rate you chose — by more than about a third — takes the daily figure up, by one small step. Faster is not better: past your chosen rate the extra tends to come off muscle.
Moving slower, two weeks running, takes it down by that same step, or leaves your food alone and moves the end date out instead. One slow week is a week; two is the plan. You pick which.
And if you logged that week as tired or sore, or slept short through it, the timeline moves before the food does, and nothing ever proposes that you train more. A cut into a week that already feels like that is how plans end.

The step is always one step, never a recalculation: the estimate was not wrong, the week was.

What it will not do

It does not recommend rapid loss, very low intakes, or fasting protocols.
It does not recommend a figure at all if you have told it that eating has become something difficult. It stays in the conversation and points you to people whose job that is.
It does not use your age, a medication or a condition to guess anything about you. It uses what you have said.
It is not a clinician. If you are pregnant, breastfeeding, managing a condition that changes how you should eat, or under eighteen, talk to yours before following a figure.

The plan is yours to change at any time from the plan’s own page, and the coach revises the recommendation from your own weeks as they come in.