A note’s shape says how long it lasts. When the quarter note gets the beat, a whole note is four beats, a half note two, a quarter one, an eighth half a beat. Each step down the ladder halves the value.
Note values are proportions, not durations. "A quarter note is one beat" quietly assumes the lower number of the time signature is 4. What never changes is the ratio: each value is half the one above it.
Whole, half, quarter, eighth — each one half of the last.
Note
Head
Stem
Flags
Beats in 4/4
Whole note
Open
None
None
4
Half note
Open
Yes
None
2
Quarter note
Filled
Yes
None
1
Eighth note
Filled
Yes
One
½
Sixteenth note
Filled
Yes
Two
¼
Read the head first: an open head is never short, a filled one is a beat or less. Then count flags — each flag halves the value again. Eighth notes in a group get their flags joined into a beam; that is typesetting and changes nothing about the length.
Common questions
How many beats does each type of note get?
It depends on the note the time signature counts. In 4/4 the quarter note is the beat, so a whole note lasts 4 beats and a sixteenth note ¼ beat; in 6/8 each eighth note is the unit, so a quarter note lasts two of them.
What note is shorter than a sixteenth note?
The thirty-second note, with three flags, lasts ⅛ of a beat. The sixty-fourth note, with four flags, halves that again, though piano music rarely goes past the thirty-second.
How do you tell a half note from a quarter note?
Look at the note head: a half note has a hollow head with a stem, a quarter note a filled head with a stem. A hollow head with no stem at all is a whole note.