Day trip to Pompeii and Capri in July.

Are you doing this on your own or with a tour?

It's hard to reply without this info, but doing both would be difficult
When you consider transportation times.

You can't see all three in one day coming from Rome. It's not remotely realistic due to transport time, ferry schedule to Capri, etc. Even if you pick just Pompeii, it will be one long day (at least 4 hours just to get there and back). It sounds like you should actually change locations and not stay in Rome the whole time because you'll be better off using Sorrento or Naples as your base for trips to Capri and Pompeii. Most people pick several bases because it makes more sense logistically than long tiring day trips where you'll be cutting yourself way short at each location.

You can certainly do one of the 3 as a day trip.
Maybe you could possibly do two, if one of the two is a quick visit to Sorrento (Pompei and Capri require the longest, so it would be too rushed to do both in a day trip from Rome).
3 out of 3 can be done only if your objective is to get there, check mark your “done it, been there” sheet, and leave right away..

The worst time to do this is on a weekend.
The last Frecciarossa departing Napoli back to Rome is at 7:30pm.
There is a slower Intercity around 8:30pm, if the FR is too early for you. After that it’s night trains only (last one at 9:30pm)

This is going to be a miserable day. Since you have so much time, why don't you plan a couple of nights in Sorrento? It's easy to get from Sorrento to either Capri or Pompeii, but frankly I'd prefer not to do both in one day.

When we went to Capri, we spent almost the entire day there. And the same is true for Pompeii - I think we spent about 5 hours at Pompeii, not counting travel time.

Make it easy on yourself, and find a lovely place to stay in Sorrento.

As others have said, it may be humanly possible to touch base at each of these three destinations in a single day, but why would you want to? Just to say that you've been there?

If you are really interested in these three places, check out of your Rome hotel, stop in Pompeii for the better part of a day en route to Sorrento, spend 2 nights in Sorrento and visit Capri on the intervening day. Then, on the morning of day 3, check out (ask the hotel to hold your luggage after checkout time) and see a little more of Sorrento before returning to Rome.

We are taking a day trip from our hotel at Piazza Navona to Pompeii,
Sorrento and Capri.

WAY too much. Even just Pompeii is a longer day from Rome than I'd want to do...although some people do it. You could squeak in some of Sorrento but Capri needs at entire day, IMHO. It will also be very hot at Pompeii in July, and like Jane, we spent at least 5 hours there and weren't able to cover it all.

With the amount of time you have, I agree that you should take 2 days from Rome to stay in Sorrento if you want to do all three of these locations. Take just an overnight bag and leave Rome early, see Pompeii enroute to Sorrento (check your bag at the scavi) and then explore Sorrento with the rest of your time and energy. Go to Capri the next day, returning to Rome the morning after. You could make a stop in Naples to see the archeological museum on the way, if you wish.

Because there is more than one ferry operator, it's hard to be sure you have all the timetables. But (besides the slight danger of rough water reducing service your single day ... ) there aren't enough boats to make your trip reasonable. Does your partner understand that the sights of Capri are primarily away from the ferry dock? The microscopic public busses of Capri will have long lines in July, and the 99% pedestrianized attractions will have foot traffic slowing down your sightseeing. Taxis can only go a few places on Capri.

I agree that Pompeii from Rome is possible (if inadvisable) in one day. Capri is not possible, except to visit the port and get back on the ferry. Were you planning to take the public bus to the Sorrento ferry port from the Circumvesuviana stop? If so, you need to add average "latency" between busses to the slow progress of any bus.

We visited Rome (in some cases on the way somewhere else) three times before we scheduled an Italy trip that included Sorrento and surroundings.

I doubt the producers of "The Amazing Race" would try to do what you propose.

I totally agree with staying in Sorrento for at least 2 full days (3 nights) if you want to see it, Pompeii, and Capri. It's a beautiful resort town and you will enjoy your trips to Pompeii and Capri so much more if you give yourself the proper time. Pompeii is huge and deserves at least 4 hours. It will also be quite hot in July, so keep that in mind.

I heartily agree with all the other posters. There is no way you can do either of these as a day trip from Rome! We were in Pompeii on July 25 last year. It was packed and the hottest day of our trip, at least 35 degrees. Pompeii was one of the highlights of our month in Italy and you need to spend a good four hours there. At a cooler time of year, you might do it in three but you'll be heading to the cafeteria for lots of cold drinks and ice cream. If you leave Rome at the crack of dawn and get a fast train to Naples, then the Express to Pompeii Scavi, you could see the site and get back to Rome late at night. Be aware that trains in southern Italy rarely run on time. We got in to Naples two hours late and according to a local we talked to on the train, this is typical. It will be a quiet trip back to Rome because you and your partner won't be talking to each other! This does not sound like a day full of fabulous memories at an amazing archeological site. Seeing Capri on the same day would be physically impossible. We stayed in Sorrento for three nights and saw Pompeii one day and Capri on the other. We started out for Capri at about 8 in the morning and returned at around 5. We experienced some of the longest lines in Italy - we waited for nearly an hour just to get a bus on the island to Anacapri. Even if you stayed in Sorrento, you couldn't do both of these trips in one day. Why would you want to do this to yourselves? If you only have time for one trip, I would pick Pompeii over Capri. It would be more doable and a more memorable experience.

Be aware that trains in southern Italy rarely run on time. We got in
to Naples two hours late and according to a local we talked to on the
train, this is typical.

Kerri, I don't know as the Frecciarossa 'fast trains' - which is what one should take from Rome to Naples - are known to frequently run late. There may have been some sort of issue. We have experienced a strike but so far, no serious delay with Italian trains, not that it can't happen.

It will be a quiet trip back to Rome because you and your partner won't be talking to each other!

Kerri, that's hilarious!