CDG to Chartres

Take Le Bus Direct (www.lebusdirect.com) to Gare Montparnasse. You'll need Line 4. The bus trip takes about 75 minutes, depending on traffic. Take the train from Montparnasse to Chartres. Train trip is about an hour.

If there are 2 or more of you take a taxi to the train station. The cost will be just a bit more but no dragging luggage on and off the bus and through this cavernous train station. Particularly important if you're jet lagged and lock like pickpocket bait.

Great suggestions. Any idea how much it'd cost for uber, or is there no longer uber in Paris? Oh, I think there's blah blah car, right?

Look at https://www.rome2rio.com/s/Paris-CDG-Airport-CDG/Gare-Montparnasse

Oh, that's excellent--thank you so much!!! And, I'm assuming that other traffic won't be as bad on a Saturday early afternoon, but maybe that's not necessarily the case?

Uber is not allowed to operate at the airport; be sure to use the official taxi line. We were approached at least 4 times by scam operators as we made our way to the line including one guy very close to the line obviously trying to vamp a dispatcher. It is 55 to the station.

www.parisbytrain.com combines advice with photos of the Montparnasse station, a fairly big interchange so it may help to see it in advance. At Parnasse there is a five-minute walk on city streets between the Metro/RER station and the main-line train station.
Here is the link to Paris rapid transit, where the Getting Around section provides interactive search for routes. http://www.ratp.fr/en/ratp/r_61597/visit-paris/
By commuter train it estimates a little less than one hour, changing trains at St-Michel (which is south of Gare du Nord.) But you can see how the suggestion for a taxi, spreading the cost over a group, may be more comfortable. There is no way of estimating how long you will stand in line waiting for your taxi but be sure to use the official queue and fend off hustlers offering their own transport.

Saturday afternoon is prime errand time. Traffic will be bad. The rome2rio estimates are off. Most taxi drivers I've used when we lived near there hop on the outter ring road and don't even try to cross the city to Montparnasse. I'd just get on the bus JHK talked about, your luggage stored underneath in the luggage hold and fall asleep until you get to the Montparnasse station. It's cushy seats and air conditioned. First stop is Gare de Lyon; don't get off there.

Ugh. City travel is the worst. Wish I could hire a charter plane to Chartres! Thank you all, so very much, for all your advice. It is definitely wise to map out in advance. I'm leaning towards taxi or bus to Montparnasse.

Taxi will be 55 euros for up to 4 passengers.

You have to navigate your way through the train station whether you arrive by taxi or bus. So there's no preference there. (No vehicle is going to let you off train-side!!)

Yes, absolutely. I was thinking more of eliminating idea of transfers with luggage in tow.

If you want to reduce transfers with luggage in tow, then as you mentioned, it's a taxi or the bus.