Jet Lag Calculator

How to use the jet lag calculator

Enter how many time zones the trip crosses and which way you are flying, and the tool estimates how many days your body clock needs to catch up. Count zones as the plain clock difference on the day you land, not the number of borders the aircraft crosses: New York to London is five hours and so five zones, New York to Paris is six, Los Angeles to London is eight.

The field runs from 1 to 12 because 12 is where the arithmetic turns around. A difference larger than that is shorter measured the other way, so New York to Tokyo, nominally fourteen hours, behaves more like a ten-hour shift in the opposite direction, and ten is the number to enter. Half-hour zones exist too. New York to Delhi is ten and a half hours; round it and enter ten.

Direction is not a detail, it is most of the answer. The CDC Yellow Book, the US government travel-medicine reference, gives the average rate of adaptation as about 1 hour a day travelling east and about 1.5 hours a day travelling west, and this calculator uses exactly those two rates.

The reason is a property of human physiology rather than anything about the flight: the intrinsic period of the body clock runs slightly longer than 24 hours, so it delays more willingly than it advances. Stretching your day, which is what flying west asks of you, works with that drift.

Compressing it, which is what flying east demands, works against it. That is why a six-zone hop to Europe costs about six days eastbound and only about four coming home.

Two things genuinely shorten the window, and the tool surfaces both. Pre-shifting is the first: move your bedtime and your meals by about an hour a day toward the destination schedule for a few days before departure, earlier for an eastbound trip and later for a westbound one, which is why the result includes a suggested pre-shift count capped at three days.

What do the first few days actually feel like?

The estimate is the time to feel fully back to normal, not the length of the misery. The first two or three days are usually the worst, after which most travellers are functional but still wake early, or fade badly in the middle of the afternoon, until the count runs out.

The NHS describes jet lag as something that usually improves within a few days as the body adjusts, which is the same shape of answer expressed less precisely.

For a short trip east, four days away against a six-day estimate, you may never fully adjust before you turn round, and that is often the right outcome: some travellers deliberately stay on home time for a quick hop rather than drag their clock across and back inside a week.

The formula, worked line by line

There is no clinical model inside this tool, and it would be dishonest to imply otherwise. It applies the standard travel-medicine rule of thumb, which converts an hourly rate of circadian adaptation into a number of days. The CDC Yellow Book gives that rate as roughly 1 hour a day when the clock has to advance, which is what eastward travel requires, and roughly 1.5 hours a day when it has to delay, which is what westward travel allows.

Turning those rates into days is a division. One hour a day across a zone difference measured in hours gives one day per zone eastbound. One and a half hours a day gives two thirds of a day per zone westbound.

The result is rounded to the nearest half day so the answer reads like advice rather than a false precision, and the pre-shift suggestion is the rounded-up day count capped at three, because shifting further in advance than that is rarely practical for anyone with a job.

eastward: days = zones / 1     (about 1 zone per day)
westward: days = zones / 1.5   (about 1.5 zones per day)
adjustment = rounded to the nearest half day
pre-shift  = min(3, round up of adjustment)

The rounding is worth seeing in action, because half days appear whenever the zone count is not a multiple of three. Five zones westward is 5 / 1.5 = 3.333, which rounds to 3.5 days. Seven zones westward is 4.667, which rounds to 4.5. Eight zones westward is 5.333, which rounds to 5.5.

Eastward the arithmetic never produces a fraction at all, because one day per zone always lands on a whole number, which is another way of saying the eastward estimate is the blunter of the two.

Read the output as a window rather than a date. These are population averages, and real recovery moves with age, how much you slept on the aircraft, caffeine and alcohol, whether you nap on arrival, and plain individual variation that the arithmetic cannot see.

The Yellow Book also discusses melatonin, noting that 0.5 to 1 mg is often enough to produce a circadian shift and cautioning against high doses above 5 mg, but timing and suitability are questions for a clinician or pharmacist rather than a web page. Everything here is general information for planning a trip, not medical advice.

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