Do I need to apply for ETA for transit/layover at London Heathrow Airport?

The summary by the U.S. Embassy is not up-to-date. Initially, it appeared as though American travelers just transiting through a UK airport would need an ETA. The law was amended, however, and Americans traveling on a U.S. Passport will NOT need an ETA if you are not passing through Passport Control as you would be if you were leaving the airport and therefore entering the U.K.
You can verify this at www.Gov.uk
Cheers!

As long as you are sure you can stay airside at Heathrow, then you don't need an ETA.

both legs are on the same ticket? your luggage will be checked through?

The ETA is easy, inexpensive and good for two years. Why worry about taking a wrong turn and facing passport control? As I recall I've always had to go back through a passport check before security to get back into the departure area, but that could be me AND I haven't transferred through Heathrow since Covid.

As noted, the information on the website you used is out of date. Providing you are flying on a single ticket, have the boarding pass for the connecting flight, and your checked luggage is tagged through to your final destination, then you can stay airside and do not need the ETA. Of course you can apply for one anyway if you are of the chicken little persuasion, but it's not necessary. And you need to try really hard to find yourself accidentally on the wrong side of passport control.

The airport has a very useful website. In particular, the Connections section. Plug in your Date and flight numbers, and it will guide you safely from A to B.

I got my tickets through my United miles. My final destination is Vienna (first leg via United and second leg via Austrian Airlines) and both legs are on the same ticket. I hope/believe the luggage are check through final destination. Can someone share their experience.
In the US one has to claim and recheck luggage upon entering US border even though it is not your final destination.

I know it not much for the ETA but it seems there is so much fees here and there and it adds up. For the return flight via Swiss Airlines I have a layover in Zurich and I have to pay to choose seats for each leg, $26 for BUD to ZUR and $40 for ZUR to JFK. .

It seems everyone is nickel and dimes for everything,
30.00 euro for a slightly earlier check-in
on one review, I even saw 25 euro rental for pots/pans for a apartment that come with kitchen
extra for request of extra towels and housekeeping
no detergent even thought there is a washing machine in the apartment
only two coffee capsule for the duration of the whole stay

OP you won't go through Customs or Passport Control until you arrive in Vienna.

amui753, from everything I've read, if you are transiting through at the airport, you do not need to apply for an ETA.

If it were me, I wouldn't bother. Worst case, if you are at the airport and find you need to stay in London for a night for some reason, you could always just quickly use the phone app to apply. I got my authorization back in about 30 seconds.

both Austrian (Vienna) and United (Newark) use - and are scheduled to use in May - Terminal 2. No transfer between terminals required.

Go to this website:

http://www.heathrow.com/flight-connections

plug in your flight information and you will get step by step instructions on what to do at Heathrow.

While you won't go through passport control at Heathrow, you will go through security. Just follow the "Flight Connections" signs. It's simple since you don't have to change terminals.

I just received my new US passport to be able to get the ETA which I just did on Tuesday, Feb 17. Yes to go to London which is my destination next month, I needed to get an ETA. Yes, instructions/info stated if you are transiting through a London airport you need an ETA.

In November 2024 was transiting though Heathrow had to to go through security even though the airline rep told me that was not going to occur.

Yes, instructions/info stated if you are transiting through a London airport you need an ETA.

Those regulations were changed. You don't need one if you are just transiting.

In November 2024 was transiting though Heathrow had to to go through security even though the airline rep told me that was not going to occur.

Heathrow makes everyone go through security, even transiting passengers, unless you are arriving from a UK based airport.

This is what I followed: as OP:
Effective January 8, 2025, all U.S. citizens transiting through or traveling to the United Kingdom for tourism, family visits, business meetings, conferences, or short-term study for 6 months or less will require an Electronic Travel Authorization (ETA) prior to travel, according to the U.S. State Department.Jan 6, 2025.

This may have changed since then but I am leaving March 30 and did not want to be caught short needing one when so easy to get. But maybe US passport holders still need one transiting, maybe not!

"But maybe US passport holders still need one transiting, maybe not!"

This is incorrect.

The State Department web site may not have been updated - it's best to stick to the original source, ie the UK Government (gov.uk). It is well documented that you now do not need an ETA for airside transit.

Not sure what other reassurance people need.

The official UK government ETA website says the following;
“Who does not need an ETA
You do not need an ETA if:
……
you are transiting through a UK airport and you will not pass through border control - check with your airline if you are not sure”

Source; https://www.gov.uk/uk-border-control/layovers-and-transiting

So no, US citizens do not need an ETA to transit airside at a UK airport.