How to pronounce birthday in American English

Stress falls on the first syllable — say "BURTH" with a longer, fuller vowel and keep every other syllable short and quick. The respell "BURTH-day" marks the stressed syllable in capitals so the rhythm is easy to read at a glance.

Americans use a relaxed retroflex R: the tongue curls back rather than rolling, and the R is one continuous sound with the vowel before it — not two separate sounds. Don't try to pronounce a separate vowel followed by a separate R. Treat them as a single shape.

American English uses different vowel shapes, a relaxed retroflex R, and connected-speech tricks like flap-T and glottal-stop T that British Received Pronunciation generally avoids. The respell "BURTH-day" reflects the casual American form; British dictionaries typically print a citation form with crisper consonants and different vowel choices.