Edit: be aware that Istanbul now has a new airport, so what follows can be considered deprecated
How long and complicated is it to go from Istanbul Atatürk Airport to Istanbul Sabiha Gökçen International Airport?
I might need to transfer and am interested in how complicated/risky it is to go from one place to the other.
NOTE: This answer is valid only for the next few months. Sometime in (or after) April 2019 the "IST" airport is moving to a new location, which will change many of the options...
How simple it is depends on how much you're willing to pay.
The two airports are roughly 60-70km apart (depending on the route). The simplest option between them is to catch a taxi. The time this takes (and the cost) will depend on the traffic and thus the time of day, but could be anywhere between 40 minutes and 2+ hours and cost somewhere around 200 TL (possibly more with very bad traffic).
There there no direct public transport between the two, however Havabus has a services from IST airport to Taksim, and from Taksim to SAW airport. This will cost around 32TL, but realistically will likely take well more than 2 hours, and even longer if there is traffic.
Havabus also have a service from Kadıköy to SAW which may be slightly faster than taking the bus the entire way, however this would require catching the Metro and then a Tram to get to Kadıköy, and is not something I would recommend if you're not familiar with the area.
The real catch with this transfer is the traffic. IST is on the European side of Istanbul, whilst SAW is on the Asian side, which means you have to cross the Bosphorus - which means going over one of 2 bridges or through a tunnel - all of which suffer extensive congestion much of the day. Allow more time that you expect it will take, especially if you're going anywhere near peak-hour - which can realistically account for most of the day and well into the night...
When I came across the text, I wanted to make a few updates. Now, transportation via metro is available from all airports, but the metro service operates until midnight.
Let's list all the alternatives:
Metro is actively used at SAW and IST airports, with an average fare of 1 Euro. You need to purchase an Istanbul Kart to use it, which costs around 3 Euros. You'll need to load an additional amount onto the card (only in Turkish Lira, Euros are not accepted). If you load approximately 3 euros onto the card, you can use it for transportation. You can check the schedules here: https://www.metro.istanbul/
HAVAIST is a government-operated service at airports with fares ranging between 5-10 Euros. For instance, the fare from Sabiha Gokcen (SAW) Airport to Taksim is around 6-7 euros, providing transportation to all parts of the city. You can find more details here: https://www.hava.ist/
Taxis in Istanbul can be a bit of a challenge. When taking a taxi from the airport, always inquire about the taxi fares, especially for Sabiha. Information boards are available upon arrival to check approximate fares. The fare is approximately around 25-30 Euros, but it may vary depending on your destination. Additionally, there are "Lux" taxis available, commonly known as minivan-type taxis, which are about 30% more expensive than regular taxis. However, these vehicles are larger and more practical. Istanbul taxi companies have such services; you can explore more at: https://www.bitaksi.com/tr and https://www.istanbultaksi.com.tr/
There are chauffeur-driven private cars operated by tourism agencies. Here, it's essential to ensure that the companies are registered with TURSAB and legally operating. Foreign companies like Booking, Uber, Welcome Pickups, etc., are not legal in Turkey. If caught, it could lead to trouble with tourism police. I'll share a few companies with you here. Prices start from 49 euros, and depending on the quality of the vehicle and distance, they can go up to 150 euros. Usually, during peak times, you can find luxury vehicles at very reasonable prices. Here are two companies that have offices at airports:
a- https://istanbulairporttransportation.com
b- istanbulairporttransfer.com
