What is the best period to use for calculating ATRs.
14?
21?
14 is by far the most common, and when it comes to indicators common is usually best. 
Do you know why 14 has become the most common?
Basically, I’m looking for something to use on a 15min chart and a bit worried that using a 14 period average will give me too tight a range as that’s only a 3hr range roughly.
After poking around on the 'net it appears 14 is the most common because Welles Wilder, the inventor of the ATR, always used 14…
He was using it on daily charts, but I don’t see the difference in using it on daily, 15m, or anything else. It’s only 3 hours range, but it’s a 15m chart. I would think it would work just as well on any time frame. After all, fourteen candles is fourteen candles…
I use an ATR on my Sunday breakout system. When I was backtesting various ATR settings to see which one worked the best 14 came out on top, and that was on a [I]weekly [/I]chart.