How many hours of fasting until ketosis?
When ketosis typically begins during a fast, why the number ranges from roughly 12 to 18 hours, and what speeds it up or slows it down. A precise, sourced answer.
Ketosis is the phase most fasters are chasing, and the most common question about it is simple: how long does it take? The short answer is around 16–18 hours for most people on a normal diet — but the range matters more than the headline.
What ketosis is
As a fast continues and liver glycogen runs low, your body leans harder on fat for fuel. The liver converts fatty acids into ketone bodies (mainly beta-hydroxybutyrate), which the brain and muscles can burn. Measurable nutritional ketosis is the result (Cleveland Clinic on ketosis).
The timeline
On FastHQ’s fasted-state phase timeline, ketosis is marked from ~18 hours. That sits at the end of the glycogen-depletion and gluconeogenesis stretch:
- ~12h — liver glycogen meaningfully depletes; the metabolic switch toward fat begins (Anton et al., Obesity, 2018).
- 12–18h — gluconeogenesis; fat-burning rises.
- ~18h onward — ketone production climbs into nutritional ketosis.
What moves the number
“Around 18 hours” is a population average. Your number depends on:
- Diet history. If you already eat low-carb or are keto-adapted, your glycogen starts lower and you reach ketosis sooner — sometimes closer to 12 hours.
- Activity. Training fasted burns through glycogen faster.
- Your last meal. A large, carb-heavy meal pushes the whole curve later.
How to actually know
A time-based tracker — including FastHQ — estimates when ketosis is likely based on elapsed hours. It does not measure your blood. If you want to know your real number, a blood ketone meter is the gold standard; breath and urine tests are cheaper but rougher.
Which protocols reach it
A 16:8 fast ends right as ketosis is starting, so it only brushes the edge. 18:6 reaches the start of ketosis; OMAD and 24-hour fasts spend real time there.
This article is informational and not medical advice. If you are diabetic, on blood-sugar medication, pregnant, or have a history of disordered eating, talk to a doctor before fasting.