Why choosing a key for a song based
on the "highest note" you can sing
doesn't always work

The "Highest Note" method of finding the right key
If you are a singer but not a musician, finding the right key for your songs can be a little bit tricky. One method that is popular with singers is to first discover the highest note in your particular vocal range and then kinda "work back" from the highest note in the song you are going to sing. The idea is that you will be able to then choose a key for that song which doesn't exceed your top note...

While generally this certainly does work, unfortunately there are enough exceptions to the rule to make it worth looking at this method in a bit more depth.

In cases where you have worked out the top note in your vocal range and compared that note to the highest note in a song in order to find the best key, ISN'T always the best way to choose the right key for a song.

The exceptions that prove the rule
Some songs have a very small range of notes making up the whole vocal that is sung in the song. So if you try to select a good key for that song by simply working out the top note you can sing and keeping within those confines, you will be singing that particular song all the way through at the very top of your vocal range. All the notes will be within your vocal range, yes, but because you'll be singing the whole song all the way through right up there near the top of your vocal range, this will put great strain on your voice. The result is you will have chosen a key which is within your vocal range, but too high for you. You would have been better off choosing a lower key in cases like this.

In another scenario, the highest note of another song may be a note that you hit (sing) very quickly and slide off again very quickly. So by making this note equal to the top note in your vocal range will result in the key being too low for you. This is because if you are just going to quickly hit and/or slide off the highest note in a song, there's every chance you will be able to reach a note which is a little bit higher than your normal top note. The result is you will have chosen a key which is too low.

Similarly, if the highest note in a particular song is a note which has to be hit, held, and then vibrato introduced to it, you may need to choose a key that makes the highest note in the song 1 or 2 semitones BELOW your normal top note in your vocal range. This is because it's generally easy to hit very high notes, but it's NOT so easy to hit, hold, and control very high notes for any length of time (even a couple of seconds). The result of choosing your key will be that you have chosen a key which is too high for you even though the highest note was within your range.

In Conclusion
Working out keys for songs based on where in that song the highest note is and comparing that to the highest note in your vocal range can work sometimes. But there are enough occasions and exceptions to the rule to make it far from an ideal way for choosing the key of a song.

Always the best way to choose the right key for any given song is to enlist the help of a musician friend to play the song in a variety of keys for you while you sing along until you find the most comfortable key for you.

Also remember that when choosing a key, it's also good practice to sing the song in a large venue rather than in your home. Unless you live in a Hollywood mansion(!) your living room will be much smaller than the venue you'll be eventually singing the song in so you're unlikely to "project" your voice as much at home as you would when you sing out there on stage in a large venue.

It's not uncommon for singers to be able to sing 3 or 4 semitones higher at a gig than they managed to achieve when practicing a song at home.