Take everything into account. Accommodation, food, travel (including buses, trams, taxis, ferries) and any other activities or entrance fees etc. And consider how generous you want to be with your spending.
If you haven't purchased your travel gear yet, remember to factor in the cost of your backpack, new clothes, gadgets and any other items for your journey ON TOP of your trip budget.
Just because you're loaded on Day 1, it does not mean you can spend excessively without it coming back to bite you in the arse on Day 40. So DON'T go spending all your money in the first week.
A little goes a long way (even in expensive cities like Paris!) If you can find a nice local bakery, a croissant and coffee for breakfast will cost you about €2 (£1.70)! Massive Tip: Don't be tempted to buy unusual but pointless souvenirs. Enjoy them on the shelf, but leave them there.
European Health Insurance Card - This app goes hand in hand with your actual card. It tells you exactly how you can claim back your treatment costs in any country and also knows what to do if your card is lost in any country! That sort of knowledge in your pocket is very handy!
Translator - Even if it's just the basics, it's fun to show off!.
Maps - Whether it's an integrated GPS, the Google Maps app, or some kind of application letting you know what's nearby. It's all useful!
Flashlight - Handy for rummaging through your backpack - especially when you don't want to wake up your roommates by turning the light on and blinding them all!
Currency Converter - SO valuable!
City Transport Maps - A map of the transport links within each city can help you to get around a lot quicker, especially when it's a particularly big capital.
Tip Calculator - If you aren't traveling alone you're going to need this for when you dine anywhere particularly nice.
Global Help - This app contains any important numbers you need for any country in the world. Emergency services, roadside assistance, embassies, you name it!
Mobile Banking - Whichever bank you use make sure to get yourself an app so you can manage your accounts on the go. You can keep track of your spending as well as any other transactions you may need to make.
Airline Apps - If you are traveling by plane make sure to get yourself an app for the airline you are using. It's handy to check flight times and might even allow you to store your boarding pass on your phone.
HostelBookers - An app that will help you to book your accommodation should you be traveling spontaneously - but also useful if there are any last minute changes of plans.
Music Player - Nothing better when you're rolling past picturesque scenery than some moving music to match.
The Rail Planner App - Useful features to get your trip off to a good start for sure. Including a trip planner, european timetables, stations nearby, city maps.
All of these apps are available on iPhone