time question at Heathrow--

Well, look at transit requirements.....

Belfast to London is within the UK, so at Belfast you will go through Security to get on a plane, but nothing else that I am aware of, basically a domestic flight.

In London, you likely will need to go through additional security for an International flight plus a passport check on exit, Customs is not really an issue since you are leaving, Customs is concerned with goods (not people) entering a Country.

In the US, you will land, go through immigration (passport check), pick up any checked luggage, then go by Customs (maybe stopped) then either leave the airport or re-check bags to catch a connecting flight.

The time you have (about 2 hours) should be fine, as long as this is all on one ticket/itinerary. If you are travelling a separate airline Belfast to London, then it may be tight if you have a delay.

Are you booking this all on one ticket or separately? I would probably go for a later flight to SFO. If you are booking them separately I would want to be in London the night before.

Thank you for replies--Belfast to Heathrow flight is connector, not part of United flight through flight to SFO

As separate bookings I would give myself more time, especially if you have checked bags.

If you go to the Heathrow Connections Guide and plug in all your details, you will see all the steps and the estimated time: https://www.heathrow.com/flight-connections

However, they emphasize this assumes you are all on one ticket. If you are not, allow extra time - unless you don't mind buying a last minute, one way, LHR to SFO ticket.

The relatively standard recommendation for transatlantic flights is to be at the airport 3 hours before departure time, and just recently I read that some airline was recommending 4 hours at some particular airport (have no idea which one). So for two separate tickets, you have to allow a huge gap to cater to the possibility that your first flight will be late, not to mention the certainty that it will be considerably after the scheduled arrival time before you can make your way through the airport and get to the check-in counter.

Flying in the same morning is really risky. Head to Heathrow the night before. The relatively new Premier Inn near Terminal 4 has had some very, very good rates. I paid less than 50 GBP for a single room last September.

As well as considering a later flight to SFO for a same-day transfer, you might want to look at flying the first Aer Lingus flight from Belfast to Heathrow as this arrives at the same terminal (2) as United uses (this is also sold as a codeshare on BA, and possibly other airlines). This would eliminate the need to change terminal and the need to clear security again.

A flight the night before would reduce, but not eliminate the transfer risk.

Thank you for replies--Belfast to Heathrow flight is connector, not part of United flight through flight to SFO

Not sure if that answers the concerns of most responses. To put it another way...when you purchased, or are looking to purchase, what are you searching for? Is the ticket Belfast to SFO going through LHR? or is your ticket LHR to SFO and you are separately buying a Belfast-LHR Ticket? Is your whole itinerary Roundtrip to Belfast from SFO or Round Trip to London with a later ticket bought for Belfast to London having no relationship to the other ticket?

The concern is, having unrelated tickets to make connections is risky and may leave you stranded or scrambling to get a replacement ticket. If the tickets are on one itinerary, then it is the airlines concern to make connections at less risk to you.

Thank you for replies--Belfast to Heathrow flight is connector, not
part of United flight through flight to SFO

Ok......Heathrow is merely a stopover. Allow at least 4 hours because your LHR inbound flight could be late and if you miss your transatlantic flight, you'll be stuck buying a one-way ticket to SFO. It is well worth having that cushion. I allow a 4 hour cushion all of the time when I am flying separate flights not connected by one itinerary.

FYI, Customs is never an issue unless you have something to declare. It's a mere walk through. In your specific case from Belfast to LHR, you're flying from one UK city to another so there is no customs.

I think it was CDG airport in France that was recommending 4 hours- but I can say that it took at least 2 hours to go Immigration the last time I was at Heathrow- very long lines and approx 4 officers working. To be on the safe side give yourself extra time if you can.

Happy Travels 💫✨