Standard bass tuning: E A D G notes and Hz

A four-string bass is tuned E A D G, one octave below the four lowest strings of a guitar.

Notes and frequencies

Frequencies are equal temperament with A4 = 440 Hz, rounded to two decimals.

Tune in a quiet place and play one open string at a time; let it ring rather than plucking repeatedly.
A chromatic tuner shows the nearest note and how far off it is in cents (hundredths of a semitone). Find the note in the table above for that string.
If the note is too low (flat), tighten the string slowly. If it is too high (sharp), loosen it a little below the target and then tighten up to it: approaching from below keeps the string from slipping.
Go through every string, then check them all once more: tuning one string changes the tension on the bass guitar neck a little.
The Sowlea App’s tuner is free; it marks a string as in tune when it stays within ±2 cents. This website itself has no microphone tuner.

Tune the E string with a tuner or a reference note first. In each pair below, leave the fretted string alone and turn the peg of the open string until the two match.
Press the E string at fret 5 and compare it with the open A string.
Repeat with the A string fret 5 against the open D string, and the D string fret 5 against the open G string.
For a longer-ringing comparison, the harmonic at fret 5 of one string matches the harmonic at fret 7 of the next higher string.

A five-string bass adds a low B0 (30.87 Hz) below the E string; tune it so that its fret 5 matches the open E string.