One bad day of eating does not ruin a diet — it subtracts from one week's deficit and nothing more. The arithmetic is short: whatever you ate above your expenditure, divided by 7,700 kcal per kilogram, is the fat-equivalent cost, and for any realistic day it is a fraction of a kilogram. The scale will disagree the next morning, because water and food mass weigh more than the surplus ever could. This page does the sums for three sizes of day, explains the morning after, and says what to log. It is arithmetic, not a trial result.
The arithmetic of one day
Use the 75 kg example from the calorie deficit calculator: TDEE 2,269 kcal, moderate-rate target 1,857 kcal, so a planned weekly deficit of about 2,891 kcal and a planned loss of 0.375 kg. Six days go to plan. The seventh does not.
| The seventh day | Surplus over expenditure that day | Fat-equivalent that day | Week's intake | Week's balance | Week's fat-equivalent |
|---|---|---|---|---|---|
On plan (1,857 kcal) | −412 kcal (a deficit) | −0.05 kg | 12,999 kcal | −2,884 kcal | −0.37 kg |
3,000 kcal | +731 kcal | +0.09 kg | 14,142 kcal | −1,741 kcal | −0.23 kg |
4,000 kcal | +1,731 kcal | +0.22 kg | 15,142 kcal | −741 kcal | −0.10 kg |
5,000 kcal | +2,731 kcal | +0.35 kg | 16,142 kcal | +259 kcal | +0.03 kg |
The week's expenditure is 7 × 2,269 = 15,883 kcal; the balance column is intake minus that. Read the last column. A 3,000 kcal day costs less than half a week's planned loss. A 4,000 kcal day — more than double the target — still leaves the week in deficit. It takes a 5,000 kcal day, nearly three times the target, to bring the week to roughly maintenance, and even then the damage is a week of no progress, not a week of regress.
The reason is the shape of the numbers. A deficit of 412 kcal a day is small per day but it is applied every day; a surplus is large but applied once. Six small deficits add up to 2,472 kcal before the seventh day happens, and the seventh day has to overcome all of that just to reach zero.
There is no mechanism by which one day erases six. The weekly budget page explains why the week is the unit the arithmetic cares about; refeeds and diet breaks show that a higher day can even be planned into it.
In plain terms: the diet is the average, and one day moves an average by a seventh of itself.
The next morning's water weight
The scale will tell a different story, and it is worth knowing why before you step on it. A large day arrives with more carbohydrate than usual (stored with water), more salt than usual (which holds water), and simply more food and drink by weight — a big dinner with drinks is easily 1.5–2 kg of mass before any of it has been digested. None of that is the 0.22 kg of fat-equivalent in the table; it is mass that leaves over the next few days.
Two rules for the morning after:
- Weigh anyway. MacroDiet's trend weight uses a 7-day half-life, so a
+2 kgreading moves it by only about0.19 kg, and that excess halves every week if the scale settles. Skipping the weigh-in removes a data point; taking it costs the trend almost nothing. Why the scale jumps overnight has the day-by-day arithmetic. - Judge the week on the trend a week later, not on the scale the next day. If the trend is back on its line by the following weekend, the table above was right; if it is still up two weeks later, something other than one day is going on and why weight loss stalls is the page.
What to log — and what not to do
Logging the day matters more than the day itself, because the expenditure estimate is built from what you log. MacroDiet reads your rate of change "over the past three weeks", and a day that is logged wrong stays in that window for three weeks.
| What you do with the day | Effect on the estimate |
|---|---|
| Log it fully, estimating what you cannot weigh | Correct. One high day inside 21 shifts the window average by a fraction of the surplus and the trend confirms it |
| Log it as best you can, even if the estimate is rough | Nearly correct. A restaurant main entered at 1,000 kcal when it was 1,300 is a 300 kcal error once; the arithmetic barely notices |
| Leave the whole day blank | Acceptable. A blank day is skipped; the window is one day shorter |
| Log breakfast and lunch, leave dinner out | Wrong. The day is trusted as complete at half its intake and pulls the estimate down for three weeks — see how accurately you need to track |
| Cut the next day hard to "make up" for it | Not asked for. The week already absorbed the day; a punitive day risks the calorie floor (1,200 kcal for this profile), tends to produce another unusual day, and is itself harder to log well |
The last row deserves one more line. Next week's target is computed from your current weight and the rate you chose — weight × 0.005 × 1,100 — not from what happened last Saturday. The calculator does not carry a debt forward, and there is no reason for you to. The daily versus weekly page explains why catch-up days are where floors get broken.
The short version
One day is one seventh of a week. On the 75 kg example a 4,000 kcal day leaves the week at a 741 kcal deficit and costs about 0.22 kg of fat-equivalent; a 5,000 kcal day brings the week to roughly maintenance and no worse. The 1–2 kg on the scale next morning is water and food mass that the 7-day trend barely registers. Log the day honestly, weigh as usual, do not compensate, and let next week's target be next week's target.
Educational overview only — not medical advice. Zyra is a training and nutrition tracking tool; the rates, caps and floors on this page describe how the calculator behaves, not what any individual should do.