A Mile is How Long???

[caronis]

I'm a bit confused by the number of laps in a 25 yard pool that constitute a mile....I thought it was 35 laps, but now I'm reading that it's 33 laps for a "swimmers mile".....I don't get it.

I'm also trying to figure out how fast my lap times need to be in order for me to swim a 30 minute mile.....Are there time calculators out there that I can use?