Jet Lag Calculator
Estimate how many time zones you will cross and roughly how long recovery takes, with tailored advice for eastward and westward travel. Calculates in your browser.
Estimated recovery
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Show the math
Jet lag tracks how far your body clock has to move. The difference between the two UTC offsets is taken the short way around the 24-hour day, so it is never more than 12 hours: Los Angeles (UTC−8) to Sydney (UTC+10) is an 18-hour gap on paper but only a 6-hour shift back for your body. The body clock shifts roughly one hour per day going east, and about 1.5 per day going west:
shift = (destination − origin), wrapped into −12…+12 hours
recovery days ≈ shift (flying east)
recovery days ≈ shift ÷ 1.5 (flying west)
Eastward travel forces you to sleep earlier, which is harder than staying up later, so it usually takes longer to adjust.
A rough guide, light exposure, meal timing and sleep all play a part. Everything is calculated in your browser.
What this does
A jet lag calculator estimates how many time zones you will cross and roughly how long recovery takes, with tailored advice for eastward and westward travel.
How to use it
- Enter your origin UTC offset.
- Enter your destination offset.
- Read the zones crossed.
- See the estimated recovery.
How it works
The shift is the difference between the two UTC offsets taken the short way around the 24-hour day, so it is never more than 12 hours: Los Angeles (−8) to Sydney (+10) is 18 hours apart on paper but only a 6-hour shift back for your body. The body clock moves about one hour per day flying east and ~1.5 per day west, so eastward trips take longer. Use each city’s standard UTC offset.
recovery ≈ shift ÷ shift rate
Understanding your result
The recovery estimate reflects a real asymmetry: flying east is harder than west because your body adjusts more slowly when it has to advance the clock. Use it to plan rather than endure, shifting your sleep and light exposure a little toward the destination in the days before you fly, which shortens the adjustment more than waiting until you land.
Example
New York (−5) to London (0) crosses 5 zones eastward — about 5 days to adjust.
Last updated .
Frequently asked questions
How is jet lag recovery estimated?
The body clock shifts about one hour per day when flying east and roughly 1.5 per day flying west, so recovery is estimated from the size of the shift (at most 12 hours, taken the short way around the clock) and its direction.
Why is flying east worse?
Eastward travel forces you to fall asleep earlier, which is harder than staying up later, so adjusting usually takes longer.
What offsets should I enter?
Use each city’s standard UTC offset, for example New York −5, London 0, Tokyo +9.
Is my data kept?
No. Everything is computed locally in your browser.
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