What Is the Jet Lag Recovery Time Calculator?
Jet lag happens when your body's internal clock (circadian rhythm) falls out of sync with the local time at your destination. This calculator gives a quick estimate of how long it takes to fully adjust, based on the number of time zones you crossed and the direction you flew. It uses the widely cited rule of thumb that the body recovers roughly one day per time zone when flying east, and slightly faster — about two-thirds of a day per zone — when flying west.
How to Use It
Enter the UTC offset of your origin city and your destination city (for example, New York is UTC−5 and London is UTC+0). Then choose your travel direction, or leave it on "Auto-detect" to let the calculator decide based on which way your clock moves. The result shows the estimated recovery in days, the number of time zones crossed, and an approximate number of hours.
The Formula Explained
First we find the time zones crossed by rounding the difference between destination and origin UTC offsets: \(Z = \text{round}(\text{dest\_offset} - \text{origin\_offset})\). Recovery time is then \(\left| Z \right| \times \text{factor}\), where the factor is 1.0 for eastward travel and 0.67 for westward travel. Eastward trips are harder because you "lose" hours and must advance your clock, which the body resists more than delaying it.
$$\text{Recovery Days} = \left| \text{Dest Offset} - \text{Origin Offset} \right| \times 1.0$$
Worked Example
Flying from New York (UTC−5) to London (UTC+0) is an eastward journey. \(Z = \text{round}(0 - (-5)) = 5\) time zones. Since it is eastward, the factor is 1.0, so recovery =
$$5 \times 1.0 = 5 \text{ days}$$The same route in reverse (westward) would be
$$5 \times 0.67 \approx 3.35 \text{ days}$$
FAQ
Is this medically exact? No. It is an estimate based on a common heuristic. Individual recovery varies with age, sleep habits, and light exposure.
Why is eastward travel worse? Advancing your body clock (going to bed earlier) is harder than delaying it, so eastward trips typically take longer to recover from.
Do I need to count crossings under one hour? Offsets are rounded to the nearest whole hour, so a 30-minute zone difference may round to 0 or 1 zone.
