Taxi from Gare du Nord to Notre Dame in Paris

We have taken a taxi from Rue Cler to Gare du Nord and that was about Euro 25. I expect to Notre Dame to be about the same, maybe a little less. You can use the Taxi Fare Finder for (possibly) more accuracy.

You can get back to Gare du Nord from the Eiffel Tower by just walking over to the Champs de Mars, cross over to the other side of the road and look for the sign marking the stop for the No. 42 bus. It will take you directly to the Gare du Nord.

If your talking about your son's day trip, tell him to just buy a day pass for the Metro/bus system, called a Mobilis for 7.50 EUR. Gare du Nord to Notre Dame in a matter of minutes on the No 4 Metro. St Michel-Notre Dame to Champs de Mars-Tour Eiffel on the RER C. If we wants to travel above ground to see the city, he can use buses.

Notre Dame is two stops on the train from Gare du Nord -- the RER B. And using the RER to the Eiffel Tower and back will also be quicker than using a taxi. He should get a Mobilis day pass unless he also needs to get back to the airport in which case he would need a ridiculously expensive Paris Visite day pass.

About 25 EU for the trip from Gare du Nord to Notre Dame - about 30 mins.
About 23 EU for the trip from Eiffel Tower to Gare du Nord - about 25 mins.

Depending on what time of day it is...

The RER B to St. Michel as stated above, is the easiest and fastest. The 4 metro to Cité or St. Michel, as suggested, stops more but gets you there. The 38 bus originates at Gare du Nord and goes by N.D.
But 30 minutes by taxi— if that’s what works for him.
He could walk the route almost that fast, if he knew Paris. It’s a pretty straight shot.

Sounds like he doesn't have any idea about where he's going in Paris...
He could use a good map, however.

So, David, is it not okay to offer other ideas to a person asking a specific question? Please advise soonest so I can stop thinking of ways to be helpful.

I don't think anyone was rude, either. Brevity doesn't equal rudeness.

Also try reading other posts by the OP. She's asking for her grown-up son. We aren't "Alexa" here. We have brains, and assume the OP has them too. Just trying to impart knowledge and experience. And all of this is supposed to happen on tomorrow, Nov 13, which in Europe, it already is.

Rude posts should be reported to the web master. I did see one...

When information is posted, it goes not only to the OP, but also to the many people searching for information on line.

I did not realize that posts are now being screened not just for helpfulness, but for accuracy within the parameters of the original post. Of course, since most posts here are by people who are unclear about the city, they do not realize what kind of options there are. In most European cities, I simply would never think of taking a taxi, and would work with the mass-transit options (trains, buses, trams, subways). In Berlin, for instance, there are about 4 different networks, and you can often get within 3-4 blocks of your destination for a very modest sum. Paris is the same. Thus, the many suggestions of non-taxi approaches are simply acquainting the poster of options. Plus others read these threads for related reasons.

I know the original request was for information to be used last Monday. Since we arrived by train at Gare du Nord a couple of weeks ago, I thought I'd add our experiences for anyone who might do the same in the future.

We took the Eurostar from London to Paris and had a wonderful trip. We rode economy and the seats were big and comfortable. The trip is short enough that paying for business or first class isn't needed.

If you are considering taking a taxi once you arrive, be aware that you will be in the queue with the rest of the train. There's not a lot of space for the taxis to pick up passengers and only two or three would arrive every five minutes or so. Because of this, we stood in line 50 minutes waiting for a taxi and we weren't near the end of the queue.

Our ride from Gare du Nord to Montparnasse took about 25 minutes and cost 20 Euro.

It turns out taking the Metro would have required only one line change, from Blue line (St. Remy) to Green line (Nation). Two stops later on the Green line at Glaciere would have put us less than 5 minutes from our B&B.

My suggestion is to download the Paris Metro App and take the Metro instead of a cab. With the app, you just put in your destination and it tells you all the lines you need to take along with the stops where you need to change lines if any.

You can buy a pass for the number of days of your visit. We bought a two day pass which cost us less than 20 euros each. Using the Metro really is easy and we traveled from the Eiffel Tower and Arch de Triumph to Notre Dame to Montmartre and other sites during our stay. We were even serenaded by an accordion player on our way to Cimitiere Père Lachaise!