Which letters count as vowels
In English the five core vowels are a, e, i, o and u. This tool counts exactly those five letters, giving you a clean, unambiguous total. The letter y is a special case: it sounds like a vowel in words such as 'happy' and 'rhythm' but like a consonant in words such as 'yellow'. Because of that dual role, the counter reports y separately rather than lumping it in with the vowels.
The analysis is case-insensitive, so capital and lowercase vowels are counted together, and it ignores numbers, spaces and punctuation so you get a pure letter count.