Whether it is legal to have alcohol in your system before driving and whether it is okay to have alcohol in your system before driving are two different questions. Let’s answer the former before we answer the latter.
First question: Is it legal to have any alcohol in your system before driving?
The answer is yes, otherwise the law would say that we cannot drive with a blood alcohol content of 0.01 percent or more while driving. As long as a person has a blood alcohol content level of 0.08 percent or less and they are not "under the influence," it is legal for them to drive. Well then, this begs the question: How many drinks will put a person at a 0.08 percent blood alcohol content?
There is no hard and fast rule for this. It depends on a number of things including, but not limited to, prior alcohol use, body size and shape, and type of drink consumed. The California DMV has created a chart to help drivers estimate their blood alcohol content which can be found here:
I have also recommended that people get breathalyzers for their own personal use. They may not be as accurate as the police grade breathalyzers, but they’re better than nothing.
Bear in mind, even if a person is below a 0.08 percent blood alcohol content, they can still be arrested, charged, and convicted of a California DUI if they are "under the influence." And a person is under the influence when drinking causes a person’s physical or mental abilities to become impaired to such a degree that they no longer have the ability to drive a vehicle with the caution characteristic of a sober person of ordinary prudence under the same or similar circumstances.
Now whether you should drive a vehicle with any alcohol in their system is another story.
While it may be legal to have some alcohol in your system, it is almost impossible to know if your blood alcohol content will register below a 0.08 percent. And even if you are below a 0.08 percent, an officer might (and likely will) interpret his observations of you and your driving to claim that you could not operate a vehicle as a sober person would.
It is never smart to drive after drinking anything. However, having said that, it is not illegal if, and only if, a person’s blood alcohol content is below a 0.08 percent and they’re driving like a sober person would.