Hi folks! I have a question: if the total length of the flute is 51cm and lowest note is F#, what key is the flute? D?
I’m confused! 
Thanks to all!
what key a flute is… you just answered your own question 
if the lowest note is d, it’s a d flute…your flute’s lowest note is F#
cheers
berti
Not necessarily.
8-key flutes play down to C but since they play a diatonic D scale we (Irish musicians) call them “D” flutes.
A classical musician would call the same thing a C flute no matter what the lowest note is because that convention is different:Any instrument is a “C” instrument if it reads staff notation directly without transposing.
Thanks to all for your reply.
Someone has offered to me a old keyed flute with 6 key, but the seller don’t know if the flute is in D. Sure isn’t in C.
It’s a beautiful flute, but if isn’t in D, hoe can I play irish music?
Rgds
You can play Irish music on any flute tuned to a “normal” (Diatonic) scale.
The question is whether you will be able to play along with other musicians.
Are you able to see the flute in person and perhaps bring along an electronic tuner? Or does the seller have a piano and can they play a D on the piano and does that match D on the flute?
Also, length should tell you something. A 6 key flute if it doesn’t have a vented flute will be somewhere around 24ish inches long.
Just some ideas.
Eric
There’s a problem… the flute is in England and I’m in Italy.
I know that the lowest note is F# and the lenght: 54 cm.
It’s definitely not a D flute, and if you want to play Irish music with others, not the flute you want.
Eric
A brutally sharp F flute?
An Ab flute with an extension to Gb (8-key setup)?
A D flute that had a tragic table-saw accident?
Silvio
This will be an F band flute in High Pitch, made for military band use. Being high pitch will pull the pitch of the F up about 1/2 a semitone, the other half will be an allowance for the fact that players in those days blew flatter than modern flute players.
So fine for playing by yourself, but not suitable for playing against CDs or with other players.
Terry
Now, to less important stuff: is there such a thing as an Ab flute? Ab clarinets are not unheard of. They crop up on British band scores occasionally and the parts are covered by Eb (Sopranino) clarinet players.
I imagine an Ab flute to be a really warm sounding piccolo. Fingerings would be nice (on Simple System) in the usual band keys of Bb, Eb, Ab, Db…
Thanks to all for your very important advices! 
You are very fantastic!
The universal rationale for flute naming is this:-
The lowest tonic for a major scale that the flute can play.
Therefore the Irish flute is a D flute
and the normal Boehm flute with keys is a C flute.
The folk rationale for flute naming is this:-
The lowest tonic for a major scale that can be played on the flute using the three main fingers on each hand (ring, middle and index).
Therefore the Irish flute is still a D flute
and the normal Boehm flute (with keys) is a D flute.
There are many otther rationales but mostly they will be found to be based on my two distillations above.
Finally, if a flute has enough keys to be chromatic you can play everything on it except Byzantine and Indic microtoned music on it to name a few but I don’t think this is relevant in your case.
Also, it looks like you are asking about a small G flute with a low foot key for the F#. This is the G above the C on a Boehm flute and above the D on an Irish flute. If it has a key for the note C# you can play any ITM on it and if it has enough keys to be chromatic you will be able to transpose any ITM to any key with it that you fancy.
So fine for playing by yourself, but not suitable for playing against CDs or with other players.
Terry
I’d even add that you will not be able to play against CD’s with an F flute, but you’ll definitely play against other players with that! 
…and don´t let be impressed by those shiny, glossy, morbid, appealling keys. Go for an unkeyed first for a easier start covering six holes. Most of Irish music is played on a Irish flute with no need of keys.
Salutti
Not to be pedantic about this, but in classical music, instruments are named into keys_based on how they transpose notes from staff notation into actual pitch_ NOT what the bell note is. I know this for a fact because I sit in a music theory class every and had a long debate with the instructor about this. I believe the system works because most orchestral instruments are chromatic, so bell notes become less relevant.
The universal way for classical instrument naming:
Take the note the player names “C” on his instrument. (on an irish flute: oxx ooo, clarinet: xxx ooo, sax: oxo ooo…) The key of the instrument is the actual note you hear.
So a tenor sax is in B flat, an irish and a Boehm flute are in C, an “F band flute” is in E flat.
The first rationale you give does not really work for most instruments IMO. Does a Boehm flute with a B foot become a B instrument? You can play a B major scale on it… And I can play a C major scale on my Rudall. I much prefer your second definition, that’s the one I use.