A4 = 440 Hz: the international tuning standard and how to tune by ear

How this is calculated

Concert pitch wasn't always 440 Hz. Before the 20th century, pitch varied by region and ensemble, often somewhere between 415 Hz (Baroque) and 456 Hz (some military bands). The 440 Hz standard emerged in the early 1900s and was adopted by the International Organization for Standardization in 1955. Some orchestras today tune slightly higher - the Berlin Philharmonic famously uses A = 443 Hz for a brighter sound - but 440 remains the reference for sheet music, electronic instruments, and most music education. To tune by ear, play the 440 Hz tone and match your A string to it. Once your A is in tune, tune the rest of your strings by interval or harmonic. Use the beat frequency method for the final adjustment: when your string is close to 440, any remaining difference shows up as a slow beat.

Verdict

Tuning to A440 by ear is a skill every string player learns, and a clean sine reference is the cleanest match you can get. The 440 Hz standard isn't magical - it's a 1955 convention - but it's the reference the whole musical world agrees on, so learn it and use it.