If you want something you can carry around, go to this link from O and A: http://www.oanda.com/currency/converter/
On the top, choose "Currency Converter." Put in "US dollars" for "Currency I Have" and "Euros" as "Currency I Want." Then underneath, instead of "Rate Details," choose "Traveler's Cheatsheet." Print this and carry it in your wallet.
Do this the day before you go, so the rates will be as close as possible the ones you are really getting.
Or, you can do the "quick and dirty" approximation - just add a third to the euro prices to estimate US dollars. It will be close enough for most purposes.
One important tip: find out your bank or credit union's maximum withdrawal amount, and convert this into euros. When you arrive, jet lagged, you need to know how much you can withdraw from the ATM, and the machine will not have dollars OR conversions on it; it will just ask how many euros you want. I like to get €480 as my first withdrawal, since this assures me at least one bill smaller than a €50.