When do I start shortening prayers on a trip?

You start shortening once you have left the built-up area of your town or city, not while you are still at home. The trip itself has to be long enough to count as travel, and from the moment you pass the last houses of your town, Dhuhr, Asr and Isha can be prayed as two rak'at each.

Leave your town first

Packing your bags or deciding to travel is not enough. The Prophet ﷺ prayed Dhuhr as four rak'at in Madinah, and then prayed Asr as two rak'at at Dhul-Hulayfah, a stop just outside the city, on his way out (Sahih al-Bukhari 1089). So scholars of all four schools agree that shortening begins after you leave the inhabited area of your hometown, its houses and neighbourhoods, not from your front door.

The Quran gives the permission in general words, linked to "going out" into the land:

وَإِذَا ضَرَبْتُمْ فِى ٱلْأَرْضِ فَلَيْسَ عَلَيْكُمْ جُنَاحٌ أَن تَقْصُرُوا۟ مِنَ ٱلصَّلَوٰةِ "And when you travel throughout the land, there is no blame upon you for shortening the prayer..." Quran 4:101

The trip must be long enough

A trip to the next town for shopping doesn't count. The Maliki, Shafi'i and Hanbali schools set the minimum at roughly 80–89 km (about 50 miles) one way, and the Hanafi school at roughly 77–90 km, depending on how the old measures are converted. Some scholars say whatever people normally call "a journey" counts. Shortening applies only to the four-rak'at prayers; Fajr and Maghrib stay the same. See What is qasr prayer?

What if the prayer time started before you left?

Say Dhuhr time came in while you were still at home, and you set off before praying it. Scholars differ:

Hanafi and Shafi'i: you may shorten it, because you are a traveler when you actually pray.
Hanbali (well-known view): pray it in full, since its time started while you were resident.

Either way, the simplest option is to pray it in full at home before leaving if you have time.

Airports and train stations

If the airport is clearly outside your town, many scholars allow you to shorten there. If it sits inside the city limits, it's safer to pray in full or wait until you're on your way. Once you're on the road or in the air, you're a traveler. You may also combine Dhuhr with Asr, or Maghrib with Isha, when it helps. See How do I combine Dhuhr and Asr when traveling?

When it stops

You keep shortening until you get back inside your own town, or until you settle somewhere long enough to count as a resident. The number of days differs by school. See How many days can I shorten my prayers when traveling?

Key points

Shortening starts after you leave your town's built-up area.
The trip must be about 80–90 km or more one way.
Only Dhuhr, Asr and Isha are shortened to two rak'at.
It ends when you return home or settle long enough.

Tip: Before a trip, check the prayer times for your departure day. If a prayer is due before you leave, just pray it in full at home and start shortening on the road with a clear mind.