How to Start Your First Fast in Atomic Fast

The number beside Start Fast is the goal you’re about to use. Tap the main button and the clock starts immediately. If you want to choose a different goal first, use the small sliders button beside it.

That distinction is easy to miss on your first visit. Here’s how to get from the Today screen to a saved fast on iPhone or Android.

The short version

Atomic Fast’s fasting timer is free. On Today, the main Start Fast button starts a fast with the displayed goal; the adjacent sliders open More fasting options. When you finish, tap End Fast, then Save & Close or Save Without Details to keep the record. Reaching the goal doesn’t end the session automatically, and the phase label is based on elapsed time rather than a measurement of your body.

Before you tap Start

Finish the app’s sign-in and initial setup so you can reach Today. These steps use the English labels; wording and spacing may vary with your app language. Both iPhone and Android have the same main start, options and save actions described here.

Choose a fasting goal that fits your circumstances before starting. The number offered by the app is a setting, not a personal medical recommendation. If you’re still deciding on timing, read about choosing a fasting window.

Fasting isn’t suitable for everyone. Check who should avoid fasting or seek advice first before treating a timer as your next step. If you have diabetes, discuss fasting and medication timing with your clinician; NIDDK explains why doing this on your own can be risky.

Start, finish and save your first fast

1. Open Today and read the goal on the start button. You might see Start Fast (16h), for example. That means tapping the main button begins a fast immediately with a 16-hour goal. If that is the goal you intended, tap it and go to step 3. You don’t need to log a meal first.

2. Use the sliders if you want to choose first. Tap the small sliders icon beside the start button, labeled More fasting options for accessibility. Select your goal from the plan picker. On the following Start Fast sheet, leave Start Now selected and tap the start button showing your chosen goal. For example, selecting the 12:12 plan gives you a 12-hour fasting goal. This is an example of the controls, not a recommendation to choose that duration.

3. Check that the timer is running. The Today screen now shows elapsed time, your goal information and an End Fast button. The phase card provides background information based on the hours since the recorded start. It cannot confirm your blood sugar, ketones or autophagy. Our guide to how the timer shows metabolic phases explains that limit.

4. Tap End Fast when you finish. This opens the completion sheet, where you can review the end time and duration. You can end before the selected goal. The app also continues counting after the goal is reached; you decide when the session ends. A timer goal is never a reason to ignore feeling unwell.

5. Save the completed session. Use Save & Close if you want to keep the details you’ve entered, such as a note, or Save Without Details to keep the fast without those extra entries. Both save a completed record. Discard is a different action: it doesn’t keep the session as a completed fast. Opening the completion sheet alone isn’t the final save step.

6. Find the record on Today. Look for the Latest Fast card and check its duration. Tap that card’s header or chevron to open your session history. Tapping the body of the card opens editing, so aim for the header when you only want the history list.

If the screen doesn’t match

You’re seeing an active timer instead of Start Fast. A session is already running. Check that session before trying to create another one. Atomic Fast prevents overlapping active fasts.

You’re seeing a scheduled fast. You chose a future start time. The walkthrough above uses Start Now. Scheduling is a separate option, so check the scheduled time instead of expecting the elapsed counter to begin immediately.

You tapped the main button before choosing a goal. That button starts immediately; it isn’t a preview. Check the active session. If it was only an accidental start, use End Fast and the completion sheet’s Discard action rather than saving it as a completed fast.

You tapped End Fast but can’t find the completed record. Check whether you finished the save step. Save Without Details still saves the fast; dismissing the sheet and discarding are different actions.

Frequently asked questions

Do I need Atomic+ to use the fasting timer? No. Starting, timing and ending a fast are part of the free timer. You don’t need to use an AI meal scan for this workflow.

Does the timer stop when I reach my goal? No. The display can show that you’ve reached the goal, but the session continues until you end and save it. A goal is a target, not an automatic stop command.

Why don’t I see Since Last Meal? That option depends on an eligible recent logged meal. You can use Start Now without a meal log. Logging food by itself doesn’t start the fasting clock.

Can I save a fast that ended before the goal? Yes. Review the end time and use either save action. You don’t need to change the record to make it look as though you reached the goal.

Will the timer work without an internet connection? Yes. The timer and elapsed-time phase display work offline. This doesn’t mean every app function works offline: food search, barcode lookups and AI scans need a connection.

Sources

Sources checked: October 11, 2026.

• NIDDK — Fasting Safely with Diabetes (2020) — Explains why fasting with diabetes needs individual planning, including medication and monitoring. It is not evidence for app controls.

Product verification: The start, options, completion and history flows were checked against the iPhone and Android source code and documentation. This was a source review, not a mobile-device walkthrough. English labels are used; translated wording may differ.

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Get Atomic Fast on the App Store or Google Play. On Today, check the goal beside Start Fast before beginning.

This article is for general education and isn’t medical advice. If you’re pregnant, managing a health condition, taking medication, or have a history of disordered eating, speak with a healthcare professional before starting a fasting routine.