Cheat Day Fat Gain Calculator

Understanding the cheat day fat gain calculator — concept and principle

After a big eating day the scale can jump by a kilogram or more, and many people fear they have undone weeks of dieting. This calculator estimates the most body fat you could have gained from the calorie surplus, so you can separate real fat from temporary water weight.

Only a small block of fat — the bigger jump on the scale is temporary water.

How it is calculated

Surplus = calories eaten − TDEE.
Maximum fat gain (g) = surplus ÷ 7.7 (0 if there is no surplus).

The divisor 7.7 kcal per gram is the same figure as 7,700 kcal per kilogram of body fat tissue.

The principle

Fat tissue is mostly fat (about 9 kcal per gram) plus some water and cell structure, which brings its energy density down to roughly 7.7 kcal per gram. Even if every surplus kilocalorie were stored as fat, gain could not exceed surplus ÷ 7.7. In reality part of the surplus refills glycogen, and overeating raises energy expenditure slightly, so true fat gain is lower. The larger jump on the scale has other causes: each gram of glycogen is stored with roughly 3 grams of water, salty food makes the body hold extra fluid, and the food itself is still in your gut.

Worked example

Inputs: 4,500 kcal eaten, TDEE 2,400 kcal.
Surplus = 4,500 − 2,400 = 2,100 kcal.
Maximum fat gain = 2,100 ÷ 7.7 = 272.7, shown as 273 g.
Any scale jump beyond that is mostly water and glycogen (often 1–2 kg), and it fades in days.

Each point is the calculator re-run with only “Calories eaten” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Calories eaten	Maximum fat gain (g)
1,125	0
2,250	0
3,375	127
4,500	273
5,625	419
6,750	565
9,000	857

How to read the result

Compare the result with your weekly deficit. A 500 kcal daily deficit removes about 450 g of fat a week, so a 2,100 kcal surplus cancels a little over four days of dieting, not the whole effort. Judge your progress by the weekly average weight three to five days later, once glycogen and fluid have settled.

Common mistakes

Treating the morning-after scale reading as fat gain.
Underestimating how much was eaten; restaurant and party meals are often larger than people think.
"Compensating" with a near-fast the next day, which can trigger another binge.

Keep in mind: This is a theoretical maximum based on average tissue energy. If big eating days happen often or feel out of control, talk to a doctor or dietitian. This is not medical advice.

Frequently asked questions

How long does cheat-day water weight last?

Usually two to four days, as glycogen is used and the body sheds extra sodium and water.

Do cheat days boost metabolism?

Overeating raises expenditure slightly for a short time, but nowhere near enough to cancel the surplus.

Is a planned refeed better?

A refeed stays at maintenance and is mostly carbohydrate, so it adds little or no fat; a cheat day usually goes well over.

Sources: Hall KD. What is the required energy deficit per unit weight loss? International Journal of Obesity 2008.

Checked 8 Nov 2026. Published formulas cited on the page. For information only — not medical advice.

Your numbers