ONE Intermittent Fasting App includes a free fasting timer for starting, following, and recording your fasting sessions. Quick Start durations, a Custom Timer, and qualifying history are all included at no cost.
The timer measures elapsed fasting time. It does not measure ketones, glucose, insulin, or any other biological marker — any phase labels you see in the app are educational estimates based on how long a fast has been going.
Tap once and the timer begins. Each Quick Start is simply a preset duration — a longer preset is not automatically a better choice, and none of them measure your biological state.
Quick Start · 16 hours
A commonly used daily intermittent fasting length. Fits many overnight-plus-morning routines.
Quick Start · 19 hours
A longer daily option than 16 hours. Not automatically better — just a longer time-tracking preset.
Quick Start · 24 hours
A full-day fasting length. Longer than a typical daily routine and not appropriate for everyone.
Quick Start · 36 hours
An extended timer option. Longer fasting is not universally better and needs individual judgement.
For a broader comparison of schedules such as 12:12, 14:10, 16:8, 18:6 and 20:4, see the intermittent fasting schedule guide.
The Custom Timer lets you set any fasting duration you like rather than picking from a fixed preset. Common schedules that are not their own Quick Start — such as 14:10, 18:6, or 20:4 — can be tracked as elapsed fasting time with a Custom Timer.
Duration choice depends on your routine, hunger tolerance, health context, and what you can sustain. ONE does not prescribe a length. For educational context, see how long should you fast.
Once a Quick Start or Custom Timer is running, the timer shows elapsed time and progress toward the selected duration. You do not need to calculate the end time yourself — the session state is visible on the timer screen.
The timer is a time-tracking view. It reflects elapsed hours, not a live measurement of your metabolism. Any fasting-phase labels displayed alongside the timer are educational estimates tied to elapsed time, not readings of your biological state.
Completed fasting sessions longer than 12 hours are saved to your fasting history. Sessions of 12 hours or less are not saved. This keeps history focused on fasting-scale sessions rather than short intervals between meals.
Saved history is useful for seeing which fasts you have completed, noticing how consistent your fasting week has been, and reviewing your recent pattern. History is a record, not a proof of health improvement.
A streak tracks how consistently you complete qualifying fasting sessions over time. It is a lightweight record-keeping signal that can help you notice patterns week by week.
A streak is a tracking tool, not a health target. Skipping a fast when you are unwell, extending a fast to preserve a number, or fasting to compensate for a missed day is not something the app encourages — the streak simply records what happened.
A normal phone countdown works if you only want to know how many hours have passed. A fasting-specific timer keeps context around each session.
| Aspect | Generic phone timer | ONE fasting timer |
|---|---|---|
| Session workflow | Start and stop a countdown. | Fasting-specific session with clear start, elapsed progress, and completion. |
| Preset durations | You enter a duration each time. | Quick Start presets at 16, 19, 24 and 36 hours plus a Custom Timer. |
| History | No record of previous sessions. | Completed fasting sessions longer than 12 hours are saved to fasting history. |
| Consistency signal | No streak or consistency view. | Streak record based on completed fasting sessions. |
| Related tracking | Isolated from the rest of your health tracking. | Sits alongside water, weight, and Apple Health or Health Connect integration. |
The fasting timer is one part of the broader ONE Intermittent Fasting App. Around each session, the app also offers water tracking, weight tracking, and integration with Apple Health on iOS or Health Connect on Android.
ONE Premium adds an AI Coach powered by OpenAI that provides weekly and monthly analysis of your tracked fasting patterns, along with protein tracking with natural-language food entry. These are supporting features around the timer, not replacements for it.
The fasting timer records elapsed hours. It does not detect ketosis, glycogen depletion, autophagy, insulin levels, or any other physiological event. Those transitions are gradual, they overlap, and their exact timing varies substantially between individuals.
For an educational overview of the changes that may occur across a fasting window, see fasting stages. Any phase labels shown alongside the timer inside the app are simplified educational ranges, not personal measurements.
A longer Quick Start preset is not a signal that a longer fast is more effective. ONE does not prescribe fasting duration, and longer fasting is not appropriate for everyone. Individual tolerance, nutrition, medications, pregnancy, medical conditions, and history of disordered eating all matter.
For educational context on choosing a duration, see how long should you fast. Talk to a qualified healthcare professional before starting or changing a fasting routine if you have any relevant health context.
A fasting timer measures the elapsed time of a fasting window — the interval between your last meal and your next one — so you can follow the fast without calculating times manually. It is a time-tracking tool, not a biological measurement.
Yes. ONE Intermittent Fasting App includes a free fasting timer with Quick Start durations of 16, 19, 24 and 36 hours and a Custom Timer that lets you set any duration you choose.
Four Quick Start presets — 16, 19, 24 and 36 hours — plus a Custom Timer that lets you set any duration you like. A longer preset is not automatically a better choice; it is simply a longer time-tracking option.
Yes. The Custom Timer lets you set any duration, so schedules like 14:10, 18:6, 20:4, or any other length can be tracked as elapsed fasting time.
Completed fasting sessions longer than 12 hours are saved to your fasting history. Sessions of 12 hours or less are not saved. This keeps the history focused on fasting-scale sessions rather than short gaps between meals.
No. The timer tracks elapsed fasting time only. It does not measure ketones, glucose, insulin, glycogen, or any other biological marker. Any fasting phase labels are educational estimates based on elapsed hours, not measurements of your metabolic state.
Not automatically. Longer fasting is not universally more effective and can be harder to sustain, harder to nourish adequately, and unsuitable for some people. ONE does not prescribe a fasting duration. Choose a duration that fits your routine and health context.
A normal phone timer counts elapsed seconds but has no fasting-specific workflow, no saved history for qualifying sessions, no streak record, and no connection to your other fasting-related tracking. A dedicated fasting timer keeps context around each session.