Carry-on size

When you say "standard" you mean the maiximum size published by the airline you are flying?

How strict? They (all of the hundreds of them) never check ... except when they do. Then you pay CHING$ CHING$

We have found them to be very strict 95% of the time. You need to check with your particular airline, but Lufthansa (21.6 x 15.7 x 9 in) and ITA (21.7 x 13.8 x 9.9 inches), for example, both allow 9" bags. They do weigh your carry-on, and it must be 8 kilos or less. Restrictions for handbags apply as well.

Thanks to both of the responses. We don't know which airline we will be flying yet (probably US Airways on the way there), so my thinking is to be conservative and go smaller rather than larger.

Hello jtcrate, and welcome to the forum,

Budget airlines tend to be much stricter and have smaller dimensions because their low fares are offset by charging for everything else - including having to check the bag you brought from the US to Europe as a carry-on.
Easyjet 45x36x20 cm or pay for a 56x45x25 cm cabin bag.
Vueling and RyanAir free underseat 40x30x20 cm (under the seat) larger carry-on (55x40x20 cm, up to 10kg) for added charge.
While larger airlines tend to have higher fares but also more generous allowances and potentially less scrutiny.

And in my limited experience the airport seems to matter as well. I've flown some of the budget airlines and despite all the warnings some airport crews seemed not to care at all people brought whatever on board and just piled everyone into the airplane. If you can pay extra to skip check-in, where they usually examine your bag, seems to the cheat code to get an 'iffy' bag on the plane.

So if you are potentially flying multiple airlines into and out of multiple airports it's hard to answer definitely. I'd count on paying the higher fare and/or additional charges when comparing flights and cost to avoid surprises.

Good luck,
-Tod

Thanks to both of the responses. We don't know which airline we will be flying yet (probably US Airways on the way there),

Good luck with that since US Airways went out of business in 2013. (They merged with American.)

In regards to size, it doesn't matter what countries you are traveling to, it's the actual airline rules that matter. There is no standard.

I agree with all the comments so far on this thread.
Some airlines are very strict, and I've witnessed a few unpleasant scenes at the gate where passengers' bags have not fit into the ubiquitous metal carry-on baggage sizers found at many gates in Europe. (The airline agent always 'wins', and the customer always ends-up paying a supplement—often after much complaining, and making many threats of never using the airline ever again!) I've read a number of times that gate agents are incentivized by some carriers to spot non-compliant carry-on bags, and then issue surcharges ... but I'm not sure if it's true or not.

My advice would be to bring the 'lowest common denominator' carry-on bags to Europe, i.e. those mentioned by @hiredman a few posts above: "... underseat 40x30x20 cm; larger carry-on 55x40x20 cm." Permitted weights of the larger bag differ, and some can be as low as 8 Kg. In my experience, having a non-compliant bag size is more of a risk than having a slightly overweight one.

Don't assume that rules and their enforcement will be the same as you would normally experience in the US. Without wishing to state the obvious, and with respect and apologies, do read the respective airlines' baggage rules carefully—(always prominent in 'manage my booking').

I agree with going for the smallest and lightest bag you can manage to fit your stuff in.

I bought a bag specifically for an Air France leg from Paris to Milan a few years ago as AF has a weight and size limit whereas my departure airline, Delta, does not have a weight limit.

Some brands of small bags are still quite heavy so start a spreadsheet, lol, and check the dimensions and weights of any bag you are considering. I went with one that is 4.5# and fits the AF dimensions - 55 x 35 x 25 cm.

I'll add that when you are comparing sizes, go with the metric measurement instead of the inch measurement. Some companies seem to round up from the metric measure, some round down. The airlines are specific about the metric size.

I fly Wizzair and RyanAir snd Lufthansa the most. The only one that has measured bags on a flight i was on in the last 4 years was Lufthansa.

As people have mentioned, it's a bit of luck and a bit of how obviously oversized it is. I haven't in recent memory had to prove my bag fit in the little metal cube but I haven't flow all of the European discounters like RyanAir. Is your current bag hard-sided or does it compress? Does it have a frame and wheels that stick out?

Alternatively, maybe invest a whopping $32 in something like this (my wife uses this, and I think it's pretty awesome; especially for the $). 40L, suitcase style backpack, 2.2lbs, small enough to safely be carried on every airline, (except EasyJet?)
https://www.amazon.com/dp/B0B14SRVFN?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_3&th=1