Calculate how many hours and minutes a fast has lasted from a start time to now or a chosen end time, with session-type classification and practical context notes.
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| Duration | Session type |
|---|---|
| < 12 h | Not a formal fast |
| 12–14 h | 12:12 / Early TRF |
| 14–18 h | 16:8 TRF |
| 18–24 h | Extended TRF / OMAD |
| 24–36 h | 24-hour fast |
| 36–72 h | Extended fast |
| > 72 h | Prolonged fast |
Also in Intermittent Fasting
Health & Nutrition
Knowing the exact duration of a fast — and how it compares to common fasting frameworks — helps with planning, tracking progress, and communicating about fasting practices accurately. This calculator converts start and end times into hours and minutes and classifies the session type.
Fasting durations under 12 hours represent a normal overnight break rather than an intentional fasting period. The 12–14 hour range is early time-restricted eating (TRE), commonly associated with metabolic benefits in observational research. The 14–18 hour window covers the popular 16:8 pattern. Above 18 hours (18:6, 20:4, OMAD) represents extended TRE, while 24-hour and multi-day fasts are less common and carry greater risk.
For fasts beyond 24 hours, electrolyte management becomes important. Sodium, potassium, and magnesium losses through urine accelerate during prolonged fasting. Symptoms of electrolyte imbalance include headache, dizziness, muscle cramps, and heart palpitations. Extended fasts should only be undertaken by healthy adults with no underlying conditions and, for durations beyond 48–72 hours, ideally with medical oversight.
Frequently asked questions
Yes. The vast majority of time-restricted eating windows begin with the last meal of the day and end with the first meal of the next day, with sleep naturally falling within the fasting window. A midnight-to-8am fast with a 10pm dinner and 8am breakfast, for example, is a 10-hour fast — not a 16-hour one.
Water, black coffee, and plain tea contain negligible calories and do not meaningfully break a fast for most metabolic purposes. Adding milk, cream, or sugar does. For autophagy-focused fasting protocols, even small amounts of amino acids from coffee creamers may be relevant, but the evidence base here is limited.
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