When you're out drinking, how do you know when it's time to give up your car keys?

Don't try to guess then, experts say.

"The decision to drive, once you have been drinking, is affected by the alcohol," said Dary Fiorentino, of the Southern California Research Institute.

To be legal, drivers must have a blood alcohol concentration (BAC) below 0.08 percent. How a person gets there is affected by gender, weight, food eaten, size and mix of a drink and other variables.

People with high tolerance may feel more sober, but alcohol still impairs their judgment, reaction time and driving skills like other drinkers, experts say.

"When the cop stops you ... it's about the scientific test. It's not about 'I feel OK,'" said treatment center educator Mary Mitzuk, speaking to first-time DWI offenders at a recent seminar.

Charts estimating BAC by gender and weight are a guide, but people shouldn't rely on them, experts say. There is some impairment at every level. If you've had enough to be near the legal limit, plan ahead: Arrange a ride before going out. Bring a phone number for a taxi.

A judge is looking at giving some first offenders the option of buying personal breathalyzers to reduce DWI fines.

"Budweiser used to say 'Know when to say when,'" Isanti County Judge James Dehn said. "We have never taught America when 'when' is."