Security checkpoints are typically the biggest question mark when you're trying to figure out how early to arrive at the airport.

Wait times can vary dramatically depending on the airport, the gate, the day of the week and the time of day, not to mention other unexpected variables.

But tools from OrbitzTLC Traveler Update and the Transportation Security Administration can help give you a better sense of how long you'll have to wait.

At OrbitzTLC Traveler Update, just choose from a selection of top U.S. airports, then go to "security wait times," and you'll find a running average of wait times sorted by airline, day of week and time of day.

A similar tool is available at waittimes.tsa.dhs.gov, where you just indicate where you're departing, what day you're leaving and what time, and the TSA tells you the average wait times over a four-hour period.