Is 40 minutes enough time to get connecting flight from Frankfurt to the United States?

No need for concern if it is one ticket bought directly from the airline and not unless you absolutely need to make the connection. The airlines all have minimum connection time rules that they must follow. If you miss it the airline will just arrange to put you on the next flight as long as they sold the ticket that way following the rules of minimum connection times. What airline is it? Are you buying it directly from the airline or are you using a 3rd party site?

Yes, you should be very, very concerned... so concerned that you should not buy the ticket, especially if there are no other flights out to your destination the same day.

I've done it at Frankfurt airport in 50 minutes. A long distance. Running. Out of breath. Arrived at the gate as the last passengers were boarding. Stressful.

On a flight from Nuremberg to Frankfurt to the US there are no "security" checkpoints in Frankfurt, but there are commonly several ticket/ID checks and some extra screening of selected passengers before you board at the gate. If it is a continuing ticket, the airline has the responsibility to get you home so you will be rebooked on the next available flight if you miss your assigned one. I have been delayed more than 40 minutes more than once by fog on the Nuremberg to Frankfurt leg. If you can get a booking that gives you more time that would be best, but the airline will be responsible.

Because your first flight is within the Shengen zone, you will probably have your transatlantic flight in a different-lettered concourse than where you arrive at FRA. One reason for that is the EXIT Immigration line you will have to go through. You can't predict how long the line will be, especially for "All Other Passports." It is true that estimated walking times in Frankfurt typically vary from 10 to 20 minutes, but vary widely.

I was in FRA twice last month, and I don't consider it a horrible airport, or to have longer walks than Denver (for example) can have. But it does require an assumption that you will need substantial transfer time. I personally have no problem with connecting flights, in general. They often save money. Even on Lufthansa, it is increasingly common to get a Bus-Gate in FRA for inbound "domestic" flights, which adds even more time to the transfer.

Yeah I would buy if it is less expensive than the longer connection and as long as it is directly with an airline that has alliances like Lufthansa and United and if you do not mind staying in Frankfurt if you miss it.

If you need to be back at work the next day, can you risk being late or not arriving at all because next AVAILABLE flight is 2 days away? Sure the airline is responsible to get you home, but it is bound to mess up all your plans. I would never choose a flight with that short of time as I am finding flights rarely go to plan. I prefer to choose something attainable even if a few dollars more or requiring a longer wait time.
You also want to ge checking these connection flights on the actual airline website. If bought through a third party site and things go wrong, then much harder to get results promptly. Not all 3rd parties are programmed well for valid minimum connection times.

It is cutting it really close. I have had 45-60 minute transfers (usually to and from Bangalore) through Frankfurt and I have routinely made the 60 minute transfers, but have missed some of the 45 minute transfers. For me, it is worth the risk, but I always look at the next flight options to make sure I can get out the same day on the same airline.

Not specific to Frankfurt, but you have to consider that it can take some time just to deplane. Its often been 15-20 minutes for us (coach people). Then at FRA, sometimes there is a shuttle bus to board the plane.

I'll add that your flight to the US may start boarding an hour before departure time so the flight will likely have started boarding before you have deplaned from your first flight.

I've not transited thru Frankfurt but I don't like to have less than 3 hours for an International flight. My home airport is a small end-of-the-line airport with only 2 flights a day so if I miss either one I'm stuck in SLC for the night. Where you are flying in to makes a difference in my view as well on how much chance you can take with flight times.

You say you are flying back to the USA, but Lufthansa flies to a lot of destinations in the USA. Some, like JFK, EWR, ORD have several flights a day (split with United). So if it is to one of these destinations, there could be a later flight the could put you on.
At the same time, Lufthansa has 5 flights a day from Nuremberg to Frankfurt. Can you book an earlier flight to give you more time?
Why don't you come clean and tell us your destination city in the USA so we don't have to guess?

Wow. Thanks everyone for the responses. I don't think I've every signed up for a forum, asked my first question, and gotten so many helpful responses. We decided not to chance it and will pick different flight because our travel plans are around Christmas and we want to make sure we are home with the family instead of waiting for the next flight.

gmgbfisher, this obligates you to report back on your trip, or to reply when someone else asks a similar question! Have fun.

The official minimum connecting time for (nearly) all connections in Frankfurt is 45 minutes, so no airline should sell a ticket below that time. Are you sure you only have 40 minutes? Is this a purchase directly from the airline?

Kaeleku, sure, bad weather conditions at Frankfurt airport are only ordered by mean German air traffic controllers who want to delay flights all over Europe. This never happens at other airports.

Frankfurt airport runs on Germany efficiently and transiting to a connecting flight there is better than in Madrid, CDG Paris or Amsterdam, but 40 minutes is way too risky.

I never purchase a ticket with a connecting time of less than two hours.

I agree with two hour minimum. You made the right call not to risk it. I have had to go through security twice on occasion at FRA when changing planes. They were screening both at the terminal entrance and the gate, but that was some years back. One time I think I walked about five miles for my transfer. And yes, that sinking feeling when you realize you have to take a bus to the terminal.

"We decided not to chance it and will pick different flight because our travel plans are around Christmas and we want to make sure we are home with the family instead of waiting for the next flight."

That ups the stakes then - with the possibility of winter weather (not particularly in Frankfurt but bad weather can cause a cascade across airports) and increased numbers of travelers. I'd probably want to be in Frankfurt the night before your international flight in this instance.