Jet Lag Calculator

Estimate jet lag severity and recovery time based on timezone changes and travel direction, plus flight duration and departure time for your arrival and body-clock readouts.

This calculator estimates jet lag severity and recovery time from the timezone gap between your Departure Timezone (UTC offset) and Arrival Timezone (UTC offset), plus your Travel Direction. Timezone Difference is the absolute gap between the two offsets, wrapped so it never exceeds 12 hours (a 20-hour raw gap is really a 4-hour gap the other way around the clock) -- Arrival Timezone genuinely widens this gap as it moves further from Departure Timezone, and Departure Timezone works the same way in the opposite direction, so together the two zones set how far your body clock has to shift. Estimated Recovery is Timezone Difference multiplied by a recovery-days-per-zone rate that depends entirely on Travel Direction: eastward travel uses 1.5 days per zone crossed (jet lag researchers generally find eastward flights harder to recover from, since they force you to fall asleep earlier than your body wants to), while westward travel uses a gentler 1.0 days per zone. Because that rate swings by 50% between the two directions, Travel Direction alone changes Estimated Recovery more than any single one-hour adjustment to either timezone input. Departure Time (24h) and Flight Duration do not affect Timezone Difference or Estimated Recovery at all -- they only feed the separate Arrival Local Time and Body Clock Time readouts and the Sleep Strategy / Light Exposure tips. This is a general rule-of-thumb model, not personalized to age, prior sleep debt, or individual circadian sensitivity.