Last updated: November 2026
To plan a fasting day in Carbon, open the Calorie Planner, tap the day, and tap the fork-and-knife button beside Calories. The day goes to zero, its calories move to your other unlocked days, and your weekly total stays the same.
That means a fasting day never shortchanges the rest of your week. One thing to know first: leaving a day empty in your Diary doesn't count as a fast. Only a planned fast moves its calories.
How to plan a fasting day
Open the Diary tab and tap Planner.
Lock any day you don't want to receive the extra calories, with Lock day.
Tap the fasting day. It can be today, a past day, or a day later in the period.
Tap the fork-and-knife button beside Calories. The column reads Fast, the button turns purple and reads Undo, and the message under the day says “Undo the fast to adjust this day.”
Tap Save changes.
That day's Diary now shows Fasting day in place of your targets, and its Planner button turns purple.
How to undo a fast
Tap the day, then tap Undo beside Calories. If the fast was your last change, the week goes back to exactly what it was. Otherwise the day returns to its usual targets, and the calories it gave away come back out of your other unlocked days. Reset day to weekly defaults, under the sliders, also ends a fast. Save when you're done.
Frequently asked questions about fasting days
Where do the fasting day's calories go? They're shared across your other unlocked days. Lock days first to choose where they go.
Why can't I plan a fast? Planning a fast needs at least one other unlocked day to take the calories.
Does Carbon track fasting hours? No. The Planner marks a whole date as a fast. It doesn't track fasting windows.
Does a fast affect my check-in? At check-in, Carbon leaves planned fasts out of your protein average, so a fast won’t pull your protein down.
New to the Planner? See Using the Calorie Planner. Fast won’t save? See Why Won’t the Calorie Planner Adjust?
Plan the fast, keep your week whole, and let Carbon handle the math.