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Jet Lag Calculator

TravelFlights and Air Travel

Offset from UTC at your departure city on the travel date, e.g. -5 for New York in winter.

Offset from UTC at the destination, e.g. 9 for Tokyo or 5.5 for Delhi.

Estimated days to adjust —
Time zones shifted
—
Direction
—
Your home bedtime in local time on arrival
—
Assumed adjustment (hours per day)
—

Jet lag lasts roughly one day per time zone crossed when flying east and about two-thirds of a day per zone flying west. New York to Paris in winter is a 6-hour eastward shift, so expect around 6 days before sleep, alertness and appetite settle fully on local time.

About this tool

Long-haul travellers, flight crew planning rest days and anyone booking a meeting soon after landing want to know how long the body clock will lag. Enter the UTC offset of your home city and your destination, plus the time you normally go to bed. The calculator takes the shorter way around the clock (so Los Angeles to Sydney counts as 6 hours west, not 18 east), applies the common re-entrainment rates of about 1 hour per day eastward and 1.5 hours per day westward, and lists the lag left on each day after arrival with the local time your body will still treat as bedtime. It is an average: light exposure, sleep on the plane, age and caffeine can move your own recovery by a few days either way.

How to use it

  1. Enter the home offset

    Type the UTC offset of your departure city on the travel date, allowing for daylight saving, such as -4 for New York in summer.

  2. Enter the destination offset

    Use quarter-hour steps where needed, such as 5.5 for India or 5.75 for Nepal.

  3. Set your usual bedtime

    The tool shows when that bedtime lands in destination time on arrival and on each following day.

  4. Read the daily table

    Each row shows how many hours of lag remain and the local time your body clock still expects sleep.

Examples

New York to Paris (winter)

Home time zone
-5
Destination time zone
1
Usual bedtime at home
23:00

Result Estimated days to adjust: 6
Time zones shifted: 6
Direction: Eastward
Your home bedtime in local time on arrival: 05:00
Assumed adjustment (hours per day): 1

  1. Time difference = 1 − (-5) = 6 h
  2. Direction: eastward, 6 time zones
  3. Recovery ≈ 6 ÷ 1 h per day = 6 days (rounded up)
  4. Your 23:00 bedtime at home falls at 05:00 local time on arrival

Six zones east at about one hour a day means close to a week before sleep and appetite line up with Paris time. A 23:00 bedtime at home falls at 05:00 in Paris on arrival.

London to New York (winter)

Home time zone
0
Destination time zone
-5
Usual bedtime at home
22:30

Result Estimated days to adjust: 4
Time zones shifted: 5
Direction: Westward
Your home bedtime in local time on arrival: 17:30
Assumed adjustment (hours per day): 1.5

  1. Time difference = -5 − (0) = -5 h
  2. Direction: westward, 5 time zones
  3. Recovery ≈ 5 ÷ 1.5 h per day = 4 days (rounded up)
  4. Your 22:30 bedtime at home falls at 17:30 local time on arrival

Flying west lengthens the day, which the body clock handles more easily: 5 ÷ 1.5 is 3.3, rounded up to 4 days.

Los Angeles to Sydney

Home time zone
-8
Destination time zone
10
Usual bedtime at home
23:00

Result Estimated days to adjust: 4
Time zones shifted: 6
Direction: Westward
Your home bedtime in local time on arrival: 17:00
Assumed adjustment (hours per day): 1.5

  1. Time difference = 10 − (-8) = 18 h, taken the short way round the clock: -6 h
  2. Direction: westward, 6 time zones
  3. Recovery ≈ 6 ÷ 1.5 h per day = 4 days (rounded up)
  4. Your 23:00 bedtime at home falls at 17:00 local time on arrival

The raw difference is 18 hours east, but the body clock takes the short way: a 6-hour westward shift, about 4 days.

How it is calculated

shift = destination offset − home offset, wrapped into −12…+12 h; days = ⌈|shift| ÷ rate⌉, rate = 1 h/day east, 1.5 h/day west

shift
Clock change in hours; positive means eastward
rate
Hours of body-clock adjustment per day
days
Estimated days until the lag is gone, rounded up

The circadian clock runs slightly longer than 24 hours, so delaying it (flying west) is easier than advancing it (flying east). Field studies summarised by Waterhouse and colleagues put typical adjustment at about 1 hour per day eastward and around 1.5 hours per day westward. A shift over 12 hours is folded the other way, because the body clock re-aligns through the shorter route.

When not to use it

  • Do not use it for trips of one or two days, where staying on home time is often easier.
  • It is not a medical tool for shift-work sleep disorder or diagnosed circadian rhythm conditions.
  • It does not model melatonin timing or light-exposure schedules.

Common mistakes

  • Forgetting daylight saving time, which changes an offset by one hour.
  • Counting flight hours instead of the difference between the two clocks.
  • Treating an 18-hour eastward difference as 18 zones instead of 6 zones west.

Frequently asked questions

Why is flying east harder than flying west?

The human body clock naturally runs a little longer than 24 hours, so stretching the day (westward travel) is easier than shortening it (eastward). That is why the estimate uses 1 hour of adjustment per day going east and 1.5 hours per day going west.

Does a north–south flight cause jet lag?

Not in the circadian sense. London to Johannesburg crosses only one or two hours of clock time, so the tool shows little or no recovery. Tiredness after such flights comes from travel fatigue, dry cabin air and broken sleep, which usually clear in a day.

What happens after crossing nine or more time zones eastward?

With very large eastward shifts the body clock sometimes delays instead of advancing, effectively going the long way round. Adjustment can then take longer than the simple one-day-per-zone figure, so the tool adds a note for these trips.

Where do I find a city's UTC offset?

Check a world clock for the exact travel date. Offsets change with daylight saving: New York is UTC−5 in winter and UTC−4 in summer, while most of Europe moves from +1 to +2. A few places use half or quarter hours, such as India at +5.5.

What does the body-clock bedtime column mean?

It is the local destination time at which your body still expects to fall asleep. After New York to Paris with a 23:00 home bedtime, that is 05:00 on arrival, moving about an hour earlier each day until it matches 23:00 again.

Can I shorten jet lag?

Shifting bedtime by an hour a day before departure, timing daylight exposure at the destination and avoiding long naps on arrival are the methods most often cited. Individual results vary widely, so treat the day count here as a typical figure, not a fixed rule.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 19). Jet Lag Calculator. Tools Directory. Retrieved October 26, 2026, from https://tools.milunir.com/travel/flights-and-air-travel/jet-lag-calculator/

MLA

Tools Directory Editorial Team. “Jet Lag Calculator.” Tools Directory, 19 Oct. 2026, https://tools.milunir.com/travel/flights-and-air-travel/jet-lag-calculator/. Accessed 26 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Jet Lag Calculator.” Tools Directory. Last modified October 19, 2026. Accessed October 26, 2026. https://tools.milunir.com/travel/flights-and-air-travel/jet-lag-calculator/.

Link<a href="https://tools.milunir.com/travel/flights-and-air-travel/jet-lag-calculator/">Jet Lag Calculator – Tools Directory</a>