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Jet Lag Calculator
Use this jet lag calculator to estimate how long your body clock will take to adjust after a flight. Pick your departure and arrival time zones and your arrival time, and it shows the time difference, the direction of travel, and roughly how many days you will need to feel normal.
Time zones use standard UTC offsets — if either place is on daylight saving time, pick the offset in force on your travel date. The adjustment estimate is a rough guide, not a prediction.
How it works
Jet lag happens because your body runs on an internal clock, the circadian rhythm, which expects roughly 24 hours of light and dark in a fixed pattern. When you fly across time zones, the sun rises and sets at different times from the ones your body expects, so your clock is suddenly wrong by several hours. You feel sleepy in the afternoon, wide awake at 3:00 AM, hungry at odd hours, and foggy during the day. The bigger the time difference, the worse it feels, because your body can only shift its clock by a limited amount each day.
This calculator starts with the time difference: arrival UTC offset minus departure UTC offset. Flying from New York (UTC-5) to London (UTC+0) is a difference of 0 − (−5) = 5 hours, and because the arrival offset is higher, the direction is eastward. Flying the other way is a difference of −5 hours, which is westward. The direction matters, because the body clock finds it easier to stay up later than to fall asleep earlier: going west lengthens your day, which the clock handles well, while going east shortens it, which the clock resists.
The adjustment estimate uses the rough rule that most sleep researchers quote: about one day per time zone crossed flying east, and about half a day per time zone flying west. A 5-hour flight east, New York to London, works out to about 5 days; the same 5 hours west works out to about 2.5 days. These are rough rules, not guarantees — age, sleep before the flight, and daylight on arrival all move the number, and the calculator adds half a day for travellers over 50, because adjustment slows with age.
Light is the strongest tool for resetting your clock, stronger than meals, exercise, or melatonin. Bright morning light pushes your clock earlier, which is what you want after flying east: get outside into daylight as soon as you can, and avoid bright light in the evening. After flying west the advice flips: seek light in the late afternoon and evening, and avoid it early in the morning.
Sleep timing helps too. Flying east, go to bed an hour earlier each night for a few days before you travel; flying west, go to bed an hour later. Set your watch to destination time as soon as you board. Short naps of 20 to 30 minutes after arrival can take the edge off, but long daytime naps push your clock the wrong way, so set an alarm.
For trips shorter than three days, full adjustment is usually not worth chasing — by the time your clock catches up, you are flying home and starting the process again. On a short trip keep roughly to your home schedule, get morning light to stay alert, and accept a couple of groggy days. On a trip of a week or more, adjust properly: the better sleep is worth managing light and bedtime for the first few days.
Worked example
A traveller flies from New York (UTC-5) to London (UTC+0) and lands at 8:00 AM local time.
Step 1: Find the time difference — arrival offset minus departure offset: 0 − (−5) = 5 hours.
Step 2: Find the direction — the arrival offset is higher, so the flight is eastward.
Step 3: Apply the rough rule — eastward travel takes about 1 day per hour, so 5 × 1 = 5 days to adjust.
Step 4: Work out what the body thinks — 8:00 AM in London is 3:00 AM in New York, so the body feels like it is the middle of the night.
Step 5: Light advice — seek bright morning light and avoid bright evening light.
So the answer is about 5 days to adjust. The same 5 hours in the other direction, London to New York, would take about 2.5 days, because westward travel takes about half a day per hour.
FAQ
How long does jet lag last?
About one day per time zone crossed flying east, and about half a day per time zone flying west. A 5-hour flight east takes about 5 days; the same 5 hours west takes about 2.5 days. Age, sleep, and daylight on arrival can move the number by a day or so.
Why is jet lag worse flying east than west?
Your body clock finds it easier to stay up later than to fall asleep earlier. Flying west lengthens your day, which the clock handles well; flying east shortens it, forcing you to sleep earlier than your body wants. That is why New York to London feels harder than the flight home.
Does bright light really help with jet lag?
Yes — light is the strongest signal for resetting your body clock. After flying east, get bright morning light and avoid bright light in the evening. After flying west, seek light in the late afternoon and evening and avoid it early in the morning.
Should I take melatonin for jet lag?
Melatonin can help some people fall asleep at the new bedtime, usually in a small dose taken 30 to 60 minutes before bedtime for the first few nights. It is not a cure, and the timing matters — taken at the wrong time it can push your clock the wrong way. If you take medication or have a health condition, ask a doctor or pharmacist first.
Is it worth adjusting on a short trip?
Usually not. If your trip is under three days, your clock will barely catch up before you fly home. Keep roughly to your home schedule, use morning light and short naps to stay alert, and save full adjustment for trips of a week or more.