Paris metro passes/discounts?

FYI https://easycdg.com/rer-charles-de-gaulle-airport-cdg-to-paris-by-train/how-to-use-paris-metro-rer/

You can buy everything from a kiosk at the airport or live person. Get a ticket for the RER B and a Navigo Easy pass.

To get to Chartres you need to go to Montparnasse train station (Gare de Montparnasse) and catch a regional train. You could either buy that in person from a kiosk or live person at Montparnasse train station or in advance online at this site:
https://www.sncf-connect.com/en-en/train/timetables/paris/chartres

Which four days of the week will you be in Paris? I ask because, if you intend to use the RER into the city, and more especially if you'll return to CDG via the RER, a Navigo Découverte will almost certainly be a better value. A return ticket to the airport is €26 by itself. A Navigo Découverte is €31.60.

The Navigo Découverte always runs Mon-Sun though, so if you bridge these days, the ND may not be the way to go.

Thanks for your replies.
I’ll be arriving on a Thursday. I won’t be going back through CDG but instead going to Chartres from Paris that Sunday.

Chartres is not in the Ile de France and thus Paris region transport passes like the ND don't cover it. You buy a regional train ticket Paris/Chartres (2 one to go and one to return)

There is no good value pass for a 4 days Paris stay. The Visite which can be purchased for a 3 or 5 days stay is the most expensive transport product sold. The Navigo Decouverte weekly pass can be good value for a short stay if it involves an airport trip or two.

Do the math.

The Navigo Découverte weekly pass is €31.60 unlimited travel and will include your travel from CDG to Paris.

RER ticket CDG to Paris €13
Single metro ticket €2.50
Single bus ticket €2

If it's just a few euros difference between single tickets and the pass, I would get the pass for it's convenience.

You won't have to worry about single tickets. If you get on the wrong bus or head in the wrong direction on the metro, no loss.

Your Paris to Chartes ticket will be separate.

Great info! Just what I needed.
Many thanks—

And example of how convenient the pass is. No need to get both bus and metro tickets and you can hop a bus for 6 blocks in the rain without thought -- or we like to buy our butter at a shop in Mouffetard and our metro line has a stop Censier Daubenton near the market street. So come home, I might just hop off and go pick up the butter and then hope back on the train. With the pass it is all 'free' -- with tickets on the Easy it would be an extra 2.50 for that short hop.