The jet lag calculator estimates how badly a flight will disrupt your body clock. Enter your origin, destination and departure time and it returns a transparent 0-10 severity score, the direction of the shift, how many time zones you cross, an adjustment estimate in days, and a body-clock reading for arrival – all based on the same chronobiology behind timed-light advice.
How does the jet lag calculator work?
It starts from the one thing that drives jet lag: how far, and in which direction, your internal clock has to move. From your route it works out the real time-zone shift (daylight-saving-correct), decides whether that shift is an advance (eastward, harder) or a delay (westward, easier), and estimates your flight duration from the great-circle distance. It then reads your body clock – your habitual sleep window and chronotype – to see where “biological night” falls when you land.
Those pieces feed a single, open scoring model. The result is not a black box: the calculator shows every factor that added to your score, the model version, and a clear label that the number is an orientation estimate, not a medical measurement.
Which factors go into the score?
The score is an additive, weighted, capped model on a 0-10 scale. Each contribution is displayed separately so you can see exactly why your trip scored what it did:
| Factor | What it captures |
|---|---|
| Size of the shift | The main term – the number of hours your clock must move (the bigger, the worse). |
| Eastward penalty | Advancing the clock (flying east) is harder than delaying it, so eastward trips carry an extra load. |
| Arrival in biological night | Landing when your body thinks it is the middle of the night adds to the difficulty. |
| Long-haul and red-eye | Flights over ~10 hours, and overnight flights that eat into your body’s night, each add a little. |
| Sleep and chronotype fit | Short or long habitual sleep, and whether your chronotype suits the travel direction, nudge the score up or down. |
Westward travel deliberately carries no direction penalty, because delaying the clock is the easier adjustment. All weights live in one place in the engine and are versioned, so the model is auditable rather than magical.
Why is eastward travel scored higher than westward?
Your internal clock naturally runs a little longer than 24 hours, so delaying it (staying up later, which is what westward travel asks) is easier than advancing it (going to bed earlier, which is what eastward travel asks). Flying west from Frankfurt to New York earns a much gentler score than flying east from Frankfurt to Tokyo, even though both cross several zones. The east-west calculator isolates this effect, and why does jet lag happen explains the mechanism.
How accurate is the score and the adjustment estimate?
The physical parts are exact: the time-zone shift, the number of zones crossed, the direction, and whether you cross the date line are computed from real time-zone data. The severity score and the recovery time are heuristics – a transparent, well-reasoned orientation figure built on textbook chronobiology (east-harder-than-west, roughly a day per zone, light acting around your temperature minimum), but not fitted to clinical trial data and not a diagnosis. Two people on the same flight can adapt at different rates. Treat the number as a planning aid, not a promise. For a quick recovery-time estimate alone, use the jet lag duration calculator.
What can I do with the result?
The score tells you how much effort a trip deserves. A gentle westward hop may need nothing more than a good night’s sleep; a hard eastward long-haul is worth preparing for. Turn the reading into action with the light exposure planner (when to seek and avoid daylight) and the jet lag planner (a day-by-day schedule from a few nights before departure through your first mornings at the destination). Your entries carry over between tools, so you only type the trip once.
Frequently asked questions
Is the jet lag calculator free?
Yes. It runs in your browser, needs no login, and does not store personal data on a server. A shareable result link carries only your route and is marked noindex.
Is the severity score medically validated?
No. It is a transparent orientation heuristic based on established chronobiology, shown with its model version and every contributing factor. It is not a clinical measurement or a diagnosis.
Do I need to know my exact flight time?
No – the calculator estimates flight duration from the distance between your airports. If you know the scheduled time you can refine it, but a good estimate is enough for planning.
What is a “chronotype” and why does it ask?
Your chronotype is whether you naturally run early (“lark”), late (“owl”) or in between. It shifts biological night slightly and interacts with travel direction, so it nudges the estimate. See circadian rhythm for the background.
Sources and methodology
- American Academy of Sleep Medicine (2014). International Classification of Sleep Disorders, 3rd edition (ICSD-3) – classifies jet lag disorder among the circadian rhythm sleep-wake disorders.
- U.S. Centers for Disease Control and Prevention – CDC Yellow Book (Health Information for International Travel), “Jet Lag” chapter – direction asymmetry, roughly one day per time zone, timed-light guidance.
- NHS – “Jet lag” (nhs.uk/conditions/jet-lag/), last reviewed 31 May 2023.
- Eastman CI & Burgess HJ (2009), Sleep Medicine Clinics – applied light scheduling relative to the body-temperature minimum.
- Sleep Foundation – “Jet Lag” educational overview.
The concepts above (east-west asymmetry, ~1 day per zone, light acting around the temperature minimum) are established chronobiology. The exact scoring function is jetlag.info’s own transparent heuristic, shown with its model version at the calculator – not a figure taken from any single study.