Subtitle line length
Keep subtitle length to about 42 characters; avoid more than 2 lines per subtitle.

Minimum duration
Subtitles should not be less than 1 second.

Maximum duration
Split subtitles lasting more than 7 seconds.

Reading speed
8-25 characters/second is recommended; 8-16 characters/second for children.

Sounds
Describe meaningful sounds that are relevant to the plot.
ex: (water dripping)

Music
Describe relevant music which does not have relevant lyrics.
ex: ♪ (jazz music) ♪

Speaker Identification
Use parenthesis to indicate when someone is speaking off-screen.
ex: (Michael) Wait for me!

Advanced Guidelines
Translate the whole idea
Translations don't need to be word for word. Look at the context. Make sure you are using the correct punctuation for the whole unit of text.

Maximum timing offset
Subtitles shouldn't start more than 0.5 seconds before or after the audio begins.

On-screen texts
Translate the texts on the video that are relevant to the plot.

Use meaningful speech
Exclude things like “um” “ah” and other disfluencies.

Indicate inaudible parts
If the audio is inaudible, it should be marked as [inaudible].

Lyrics
Wrap relevant lyrics in musical notes.
ex ♪ Tell me dear Billy, ♪

Speaker identification
Identify off-screen and unclear speakers in parentheses.
ex: (Michael) Wait for me!

Foreign language
Identify relevant speech in foreign language. 
ex: (Russian): Thank you.

Simultaneous dialogue
Used exclusively when two speakers talk in the same subtitle. Each speaker should have one line, and both lines should have an hyphen. 
- Sir, you are no gentleman.
- Rhett Butler: And you, Miss, are no lady.