Get rental car in Bath, then Stonehenge & Highclere Castle in a day? Doable?

You can probably do it, but not sure of the times available to book Stonehenge.

We did Salisbury, Avebury and Stonehenge in one day from our hotel near Heathrow.

Try planning out your two tours, Stonehenge and Highclere, estimate your travel time (then add 50% more, since travel in South England traffic is generally heavy).

We visited Bath and don't know where the rental car agencies are located or what time they open. Check that out, especially if there is a Bank Holiday.

We loved the Brooks Guesthouse in Bath. Also, for fantastic fish and chips, check out The Scallop Shell.

Stone enthusiast? Is there such a thing? I wouldn't describe myself as a 'stone enthusiast' but I am interested in Neolithic history and in that respect Stonehenge interests me a great deal. If you have no interest in it why do you want to see it? If it's simply a tick box exercise I don't see why you would sacrifice so little time in visiting it. I'm assuming you will have already toured Bath prior to picking up your rental car rather than picking up a rental car, touring Bath, Stonehenge etc?

If Stonehenge doesn't really interest you I'd suggest skipping it, it's busy most days but combine a bank holiday and accompanying traffic and it will be very busy indeed. I'd concentrate more on seeing sights that actually interest you.

Ok, point taken about the bank holiday. Is the same plan possible on the day AFTER the bank holiday when everyone is back at work?
JC, Highclere Castle is a must-see for the wife, with the appropriate time devoted to it. I have a strong interest in Stonehenge, but am definitely second in line for the content of this day. I do not bypass World Heritage sites lightly. Sunday & Monday will devoted to Bath.
Highclere is (sort of) on the way to Blenheim, the Cotswolds,and then north. So is Stonehenge and Highclere in a day efficient or crazy?

Yes it is doable. Stonehenge is on the way to Highclere although not the quickest route. Stonehenge opens at 09.30 and at that time it should be reasonably quiet, it starts to get very busy around 11.00 so if you leave Bath around 08.30 you should be at Stonehenge for when it opens. How long you spend there is entirely up to you. Some people spend hours there whilst others spend less than 10 minutes. I recommend taking in the museum beforehand as you'll benefit from a greater understanding of the stones before you see them. I would imagine you would be there for about an hour.

Stonehenge to Highclere will take about an hour so you'll be there with plenty of time to visit the house and grounds.

Definitely leave this trip for after the bank holiday although be aware that many people take advantage of the bank holiday to optimise their annual leave and book a week off for the price of 4 days so the sights won't be devoid of tourists despite most people returning to work.

You may well find that many of the shops in Bath are shut on a Sunday & Monday Bank Holiday.

Thanks for the advice everyone. I appreciate your insights.

Yes! We did something similar. We got up VERY early, drove to Stonehenge for tickets around opening time, and then drove to Highclere and spent the afternoon there. After that we drove to Oxford and spent the night there. It worked out great!