You can use the Eurail pass on the Eurostar but there is a limited number of pass holder seats on each departure and they may sell out weeks in advance. Depending on what train  you want to travel on from Paris there might also be a limited number of pass holder seats.

When and where, apart from Paris, do you want to travel?

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Indeed, without exact places and dates it’s difficult to give you some advice.

7 days is quite short to “see” Europe. If you add too much destinations, you’ll spend more time on trains than actually seeing stuff. If you want a good visit of London, you’ll spend easily 3 or 4 days there. There’s so many different neighbourhoods.

If you only want to see Paris and you can book quite a bit in advance you shouldn’t buy a Eurail pass. One day is (especially if you return on the same day) in Paris is also not enough to properly see it. There’s a high reservation cost on Eurostar if you have a pass. Often ordinary tickets are financially more interesting, if you only visit London and Paris.

If you visit some other destinations in the UK, a one country pass UK might be an interesting choice! Plenty of other super nice cities, towns and landscapes there as well!

In some senses your question is like the saying “How long is a piece of string?”

For example an Interrail pass is a very useful tool for somebody who wants to go land surfing. That is starting at A and travelling long distances to B,C ……….

It can be useful for somebody wanting to visit a friend or relative across Europe but not wanting to fly

And of course it is traditionally for young people on their gap year back packing travels.

What it isn’t is a low cost alternative to a traditional package holiday or a low cost flight dash.

It is a low cost way of travelling by train over long distances at all times of the day at a value for money price. 

So simply put - If you like adventure and want to see large parts of Europe from the comfort of a train then it is for you.

With a 4 day flexi pass you could easily travel from the UK (Eurostar permitting - but there are options) to Nice in a day. Then across to Venice in a day. Then through the Alp to Munich. Then Munich to UK (just).

Alternatively you could do a one-way or 2-way flight and then use your four days on any routing that takes your fancy. 

There are some things to consider about rules and reservations but these are for later detail. Just remember you can only use any global pass on 2 days in the UK and reservations are needed on most express trains in Spain, Italy and France and a few others (including Eurostar). I just factor them in (like extras on Ryanair).

Finally consider how long you stay in each stop and how long you will be travelling. We are a retired couple and normally try to Travel 1 and stay 2, so we have a full day in each stop and then an early start and a mid afternoon arrival. We budget for an average of £100 per night plus about £50 for food etc.

Our latest trip was flight to Catania (Sicily)for 2 nights in Siracuse, to Salerno (1 night), to Bologna (2 nights), to Nuremberg (via Munich) (3 nights), to Brussels (1 night) and then E* to UK and LNER to Leeds.

Total res costs for 2 about £150 first class , accommodation and spending about £1400 , flights to Catania about £100 and rail pass about £400 (half a 10 day first class pass).

So a budget of about £2000 for a fantastic 10 day adventure. You could save on that by cheaper accommodation, going second class and eating at Macdonalds and of course travelling on slower regional trains. Or of course spend a lot more on gourmet restaurants, scheduled flights and 5 star hotels.

Finally an option for Eurostar if you can’t get a pass reservation - buy a standard ticket well in advance at early bird prices.

We are already booked on a train on the Wed before Easter 2023 at £69 (standard seat) each. Compare that with the £38 reservation fee and the worry that when finally a quota for pass users (apparently 180 days before) they will all be gone - if there are any at all. And by then the price may be much higher - on the day E* tickets can be upwards of £200 each.

In a trip with a budget of £1000 upwards an extra £30 each for security is well worth it. In addition if you can get to London cheaply you will not need a travel day. From Yorkshire I will need a travel day or pay LNER a huge amount for an early peak train to London to connect with my E*.

@Yorkie 

For a Eurail pass there is no limit for travels in any country as the traveller's country of residence is outside Europe

Hello @Billie Howard ! Thanks for your question. This sounds like a very interesting trip!

I would advise using our Trip Planner: Plan your Rail Trip in Europe| Journey Planner and Rail Trip Planner (eurail.com)

You will be shown an estimate of the travel days you need to complete your trips, but it is up to you how many days you would like to spend travelling with the train and how many European countries you would like to visit.

As @AnnaB mentioned, please make sure that you book the Eurostar/Thalys trains in advance to avoid disappointment. 

If you want to book via b-europe.com, please check this link and let me know if it works for you:
Pass fare Thalys and Eurostar | SNCB International (b-europe.com)
On this page, at the bottom right, you need to select the Pass and then click on ''show trains''.

You will also be asked to fill in a Pass cover number. You can acquire this number by logging in to your account:
Click on Trips & Travellers, then the edit option on the right, you should see the Get a Pass Cover Number at the bottom of the options.

You can also use this link directly:

https://www.interrail.eu/en/book-reservations#/generatePassCoverNumber

I hope this helps! Please let me know if you have any follow-up questions.

Best,

Annie