lle St Louis to CDG on Thurday morning March 29

Metro and RER start running then. Least expensive way to go. From Ile St Louis, cross the Seine over the Pont Marie. Take Metro line 7 to Châtelet. Take RER B from Châtelet to the rail station at CDG terminal 2. If you get the 7 Metro at 5:13, you’ll arrive at CDG at 6AM.

We’ve left Ile St Louis that early many times... we walk to the RER at St Michel which takes you directly to CDG. Very easy. No need to take the Mêtro and change trains.

For a flight at that time, you want to be there 3 hours ahead.

Ditto for the RER at St Michel directly to CDG. Did that last September. Got up real early for a flight at 10:30. Arrived with hours to spare, but YMMV.

If you take a taxi, you will be 3 or 3.5 hours at CDG, with nothing to do.
If you take a taxi, there is a flat rate - 55 for Right Bank, and 50 for the Left Bank.
No luggage fees.
And if you take a taxi, you will be there in about 25 minutes.

Thank you all very helpful..

I think arriving at 6 works fine if you only have a carry-on bag. The big, long, boring line is for those who need to check bags - reminds me again why I don't check bags. If you checked in online and have your boarding passes pre-printed, you can go through security and to the gate. If not, I think every airline at CDG has automated kiosks where you can check in and print your boarding passes (sometimes they aren't in an obvious place).

I'll go against the grain here and say I NEVER arrive 3 hours ahead of time for a flight (which drives my husband--a nervous traveler--absolutely nuts). I think 2-2.5, even at CDG, is plenty of time. A lot of times, the excessively long lines are actually due to people arriving too far in advance and trying to check in for a flight that doesn't leave for several hours. Airline workers will often go through the lines and pull out the people whose flights are leaving in the next 1-2 to the front, leaving the early birds waiting even longer.

Also, check the counter for your airline--you may find that that they are not open to check anyone in that early or with that large a window before an early am flight. The last time I flew out of CDG, I ended up there about 3.5 hours early because I was training from the south and it was the only option. Our airline counter didn't open until 2 hours before the flight--the only people who had to wait in line were the early arrivers, since no one was there to check in our bags until 2 hours before the flight time.

Obviously if it makes you more comfortable to get there far in advance, do it--there's no reason to add stress to your trip. But, if you are only doing it because the prevailing advice says to, thought I'd add my 2 cents in...There's actually some interesting research out there about the psychology of waiting in lines, particularly in airports, and how our brains perceive time differently in those situations (i.e., we estimate it took us 90 minutes to check in and get through security when in reality it took closer to 45 minutes).

Oh, and I would use the metro/RER option. It's easy and cheap.