How much cash do I need in Istanbul?

The answer (not very helpful, I know) is "it depends" on where you'll be eating and shopping. Sultanahmet is tourist central, so virtually everywhere accepts cards. Even in the less touristy area of the city, cards are widely accepted. The major exceptions are smaller shops in less toursity areas, market stalls, street food, small bakeries, public toilets (which you will often find in mosques). On a few trips, I have budgeted about ₺500 a day in cash (way too much), and ended up with plenty at the end of the trip (which I used to pay the hotel).

All the tourist sites take card for tickets. The nice thing about Istanbul is that there are so many great things you can do for free.

There are plenty of ATMs (bankomat) in Sultanahment, so no hurry to take cash out at the airport.

I was in Istanbul for five days about two years ago and I did not need any cash. Everywhere I went accepted card. If you're using public transport you can tap and pay or buy an Istanbul Kart. The easiest place to buy one is at the self serve machines located at tram stops, metro stations and the airport.

Thanks for sharing your experiences. I am thinking I will buy an Istanbulkart at the airport and head on to the hotel before worrying about a small amount of cash maybe for tiny purchases or restrooms. (Just today in fact I didn't have the right change for a water closet in Sarajevo.).

Change is one thing you won't need to worry about - attendants at the toilets will give change.

Was there for 4 nights last month. We exchanged $20 USD for Turkish Lira (there are numerous currency exchanges in the city.) Other than one restroom stop for cash, everything else was credit card or Istanbulkart. We left the remainder of TL behind as a housekeeping tip at the hotel.

Thanks for the additional replies. Sounds like the need for cash would be quite minimal