When up is the answer

Songs written for a deep voice can leave a higher one scraping the bottom of its range, where the sound turns thin and breathy. Raising the key lifts the lowest notes into the part of your voice that carries. Try +1 first, sing the verse, and add one more step at a time until the low notes speak without effort.

Watch the top as well

Raising the song helps the low end and costs you at the high end, because every note moves up together. Sing the highest part of the chorus after each change. The best setting is the one where both ends fit; if no setting makes both fit, choose the part you care most about and accept the other as a stretch.

A rough way to count

Say the lowest note of the verse is a G and the lowest note you sing comfortably is the B above it. Counting up the piano keys, G, A♭, A, B♭, B, that is four semitones, so +4 would put the verse where you can reach it. You will often find the number faster by ear than by counting, but the counting tells you where to start. The page on counting semitones for your vocal range goes through the arithmetic.

A caution about big jumps

A shift of up to 12 semitones is available, but a big jump moves every instrument in the recording as well, and the more a recording is shifted, the more processed it can sound. A few semitones is where it tends to sound its most natural.