Connecting flight at FRA. Passport control or customs or just go to next gate?

You will go though passport control in Frankfurt.
Customs is done at your last destination, however t consists only on walking through the NOTHING TO DECLARE path. Random checks may be done by customs officers at Zagreb, but they are rare.

Frankfurt Terminal 1 has a unique configuration. Your flight will arrive at the Z gates of the terminal, while your EU flight will depart from the A gates. The A gates (intra EU flights) are on the 2nd floor, while the Z gates (extra Schengen) are on the 3rd floordirectly above the A gates. So going from your extra Schengen arrival to your intra Schengen departure will entail taking the escalator down to the floor below after passport control. If you are lucky your Z gate arrival might be exactly above your A gate departure, requiring little walk, other than going to the level below.

Just so you aren't surprised, there is the possibility that your flight will arrive at a remote gate which means you will be bused to the terminal. From there you would take an escalator up to passport control. Just follow the crowds, you can't miss it.

Possibly your flight FRA - ZAG will be operated at concourse B of terminal 1.

More info on transferring at FRA airport.

All the Lufthansa or United flights from San Francisco that I can find arrive at the new wing of Concourse Z (gates Z50 - Z69). So far so good. If so, you will just go to the root of the concourse, find immigration (passport control), go through, then downstairs to the A Concourse. And most of the flights to Zagreb I see leave from an A gate.

However, I do see some anomalies. Gate A2 might be a waiting room for a bus that goes out to a plane parked on the Tarmac. And, I do see a B gate listed for a LH flight to Zagreb. In that case you will have to go to a different building, B. From the A concourse, there are only two options to get to B, and they both involve going outside security. There is a tunnel between A and B, but I think you have to go through security to get from the tunnel into B. Same with the SkyLine. You can board it from A, but I think when you get off at B, you'll have to go through security to get to a gate. The SkyLine (at least the Schengen side) is considered to be in the 'public', outside security, part of the airport. You can come into the airport off the street and get to the SkyLine without going through security.

If you stay on the Z (out-or-Schengen or transit zone) level, you can get to the SkyLine and go to the transit zone of B without going through immigration. I don't know whether you can get into the B transit zone without going through security, I've not been able to get anyone to tell me, but that's you only chance to avoid security going from Z to B. So once you get to the transit zone of B, you can go through immigration there to get to a Schengen gate in the B concourse.

Thanks so much, everyone! Glad to hear that passport control/customs is the same thing. Arrival at Z gates and then going to A gates sounds simple enough. So here's hoping that will be the case. Arriving at a remote gate still sounds doable with 2 hours 45 minutes to get to the second flight.. Arrival at at a B gate sounds more problematic what with having to go through security again to get to the A concourse so I'm glad to have this information to help me figure it all out in advance. And thanks Mark for the link to transferring at FRA. Very informative. BTW, probably not news to any of you but I just found out that one can Google the flight number with the airline letter designation 24 hours before departure to get updates on arrival or departure terminals, gates, etc. Hope this is useful information to someone

Glad to hear that passport control/customs is the same thing

No its not. Passport control will be in Frankfurt, customs in Zagreb. But it is unlikely customs will be interested in you.

Arrival at at a B gate sounds more problematic what with having to go
through security again to get to the A concourse

Actually, I said you are likely to arrive at a Z gate. All of the UAL/LHA flights from SFO on the day I checked arrived at a Z gate. It was one of the departures for Zagreb that day that I found using a B gate. Same problem, just in reverse.

As of today, I can't find any Lufthansa flights to Zagreb departing from B gates, only A gates. Hopefully, that will be true on the day you fly.

Thanks for the correction, WengenK. I misspoke (mis-wrote?). I meant passport control and immigration are the same in Frankfurt. Customs in Zagreb. And thanks for the clarification, Lee. Let's see if I've got it correctly this time: the possible anomalies might occur on the flight to Zagreb not the arrival in Frankfurt. Sounds like the only difficulty might be if the flight to Zagreb is leaving from a B gate. If I need to go to B, it seems like I should stay in the Z area and get to the skyline and go to the transit zone, possibly avoiding immigration. Do I have this right?

You cannot avoid immigration. FRA is your point of entry into the Schengen zone. Therefore you must go through immigration. Similar to you entering the US in Atlanta even though your final destination is Seattle. From FRA to Zagreb will be like a domestic flight from Atlanta to Seattle.

Thank you, Trotter. Once again I wasn't clear or misspoke. What I meant to say was, regarding the flight to Zagreb, if the flight is leaving from a B gate and I can stay in the Z area and get to the skyline and go to the transit zone I might be able to avoid going through the whole security check again. Sorry about my confusion. And thanks for the correction.

douglas you can look at a website like Flightaware.com to see recent history of flights, includes gate info. But there is no guarantee that past practice will apply going forward.

Douglas, quite often I've seen arrival gate information displayed on the airline's app when I look at my flight info before departure.

By the time we arrive, the gate has changed.

Douglas, I understand you can Google the flight (i.e.: UA 947) 24 hours in advance of your flight and see the details of your flight including the departure and arrival terminals and, I think Gates. I tried this as a dry run and it worked for me. Of course, you should also check the overhead boards upon arrival to make sure nothing has changed. Have a great time in Budapest!

Another fact.....even if a gate is usually used, it can change.

For the last ten trips from LHR-JFK, I've taken the exact same flight. It usually docks at one of three gates near immigration. (No more than a two minute walk.)

On my last trip earlier this month, we used one of the furthest gates and it was a good ten minute walk to immigration (And I walk fast.)

And let's not even mention the number of times at different airports when the flight arrives early but the gate is not clear of the previous aircraft. Sort of kills the "early arrival" benefit.

Frank, yes, they certainly can and do change the gates. I've had the same situation where the gate is not listed yet because the previous aircraft had not cleared the gate. I guess the really good news is that I wasn't trying to fly when all those systems went down on Friday or Saturday!

Dougas, Frankfurt Airport has a website with information about arrivals and departure - whether they arrived on time, what gate (and concourse) they are using, etc. The information covers multiple days.

The site also has airport maps and information about changing concourses.