How this works

Methodology reviewed July 2026

The timer tracks your fasting window and marks the metabolic milestones that research associates with elapsed fasting time — glycogen drawdown, the shift toward fat as fuel, and rising ketones — with honest labeling of which claims are well-established in humans and which are extrapolated from animal work.

Formula

elapsed = current time − fast start
Approximate milestones (individual timing varies):
  ~12 h  liver glycogen largely drawn down, fat oxidation rising
  ~16 h  ketone production increasing
  ~24 h+ deeper ketosis; autophagy claims here are largely animal-derived

Inputs

Fast start time · Target window (e.g. 16:8)

Outputs

Live elapsed time · Progress to target · Metabolic milestone reached

Assumptions

Milestone timings are population approximations and depend on your last meal, activity, and metabolism.

Limitations

Human evidence for some later-stage benefits (notably autophagy) is limited and extrapolated from animal studies — the tool flags this rather than overselling it.
Fasting isn't appropriate for everyone.

Safety

Not for people who are pregnant, have a history of disordered eating, diabetes on glucose-lowering medication, or other conditions — check with a clinician first.

This calculator is for educational purposes only and is not medical advice, diagnosis, or treatment. Results are estimates based on published formulas and population averages — your individual values may differ. Nothing here is calculated on a server: everything runs in your browser and no data is stored or sent anywhere. Always consult a qualified clinician before making health, medication, or training decisions.