How many days for London and Paris-July

Are you excluding both your jetlagged arrival day and your departure day from your count of 9 full days? If not, I'd give five days to the arrival city since the first day may not be worth much at all. Otherwise, I don't know that it matters, because both London and Paris have way more things to do than you can possibly handle in five days. I spent several nights in Oxford and thoroughly enjoyed it, but I wouldn't recommend a day-trip there during such a short visit to London. The train takes about an hour each way, but to that you must add the travel time between your hotel and the departure station (Paddington or Marylebone) and the travel time between the Oxford station and the historic center. All told, you'll probably lose about 4 hours of sightseeing time if you spend a day going to Oxford, and London has so very many great sights.

“Optimal” will vary from person to person. What are your family’s priorities? Maybe this will guide you as to what would work best for you.

You can easily spend 9 days in both cities with day trips such as Oxford or Versailles. The first day should be discounted as you will most likely be suffering from jet lag and the last day will probably just be getting to the airport. Getting from London to Paris by Eurostar will be around 6 hours door to door, so a good half day is lost there.

Oxford is a full day trip and HP is another 3/4 of a day, which wouldn’t leave much time to see the many sights in London.

It’s not my trip, but I would be tempted to pick one or other country.

I presume you would be flying open jaw to avoid losing a day back tracking.

I agree with Jennifer's one-country suggestion; I just hesitated to say it because I totally understand the desire for a variety of experiences. But if you're serious about the Harry Potter tour and going to Oxford, you've cut the number of full days available for London and Paris down to six.

At least three full days at the end of your trip in Paris, rest of time in London.

Why not just do London and some surrounding places, skip Paris?

May I ask if you have a open-jaw ticket? If not, I would only choose UK so you can enjoy your time and not spend it on traveling between countries. (even though London-Paris has a good connection)
If you are able to book open-jaw then 2 full days (no travel on those days) in Paris is enough to get a little taste of it.

I agree with the PPs, especially if the 9 days includes your flights to and from Europe. There really isn't time for both cities, but if you already have an open jaw ticket booked, then spend the last 2 days in Paris. If you have a full 9 nights in Europe then add an extra day to Paris.

By the way, if you want to do the HP studio tour, you'll want to buy your tickets soon, as they usually all out months in advance.

I’ll add to the pile on to stick with the UK only with that short of time and one day devoted to Harry Potter. Oxford would take a day as well. You’ll lose the better part of a day changing cities and that’s a huge chunk of 9 days.

Apparently the Paris transit strikes are over, but they could always flair up again.

Sunshine, when you write nine full days, I am assuming that you leave Europe the day after your last full day and that your first full day is the day after you arrive in London. I am also assuming that you can fly into London and fly out of Paris.

I would suggest writing down what you and family absolutely would want to do in each city, and then what you and your family would want to do in each city. Once you determine the must-do and would-like-to-do sights and activities in each city, you can more easily determine whether you have time for two cities and, if so, how much time you should spend in each city.

It is your trip so you will have to decide on the time allocation. Personally, with nine (or even eight) full days, I think you have enough time to visit both cities.

As an aside, I will note the sightseeing priorities listed by Rick Steves (which I find much too ambitious):

--5 days--Paris and Swiss Alps

--7 days--add London

--10 days--add Rome

Thank you, all!
To clarify...

We will be gone 11 days total. Day one is a day flight, so we hope to hit the ground running the following morning. We’ve been there before, while the kids have not. I only counted full days of touring in total. Of course one day will be broken up with travel to Paris. Still figuring out best time to take the TGV. Any advice is welcome.

We are flying out of Paris.

Thanks for the clarification. By TGV, I assume you mean the Eurostar - the train that would take you from London to Paris. I would try to get the earliest train that your group could comfortably make, considering the time it would take you to get to St Pancras, and being mindful that you need to be there at least an hour before the train leaves ( for security and Passport Control). Also keep in mind that there is a 1 hour time change between countries.

You wrote: "We will be gone 11 days total. Day one is a day flight, so we hope to hit the ground running the following morning. We’ve been there before, while the kids have not. I only counted full days of touring in total. Of course one day will be broken up with travel to Paris. Still figuring out best time to take the TGV. Any advice is welcome.

We are flying out of Paris."

My advice is not to plan too much on the first day. You never know if you'll be able to get any sleep on the plane and may arrive feeling like zombies. So of your 9 days one will be the Zombie Day (as my family calls it) and one will be the travel day. You're probably going to want four days in London and three days in Paris or the other way around. As other posters have said, you'll be getting a taste of each, don't try to cram in all the "Must Sees" from some guidebook.