How to know if I'm driving sober?

I'm almost 16, and about to get my drivers license. If a situation ever comes up where my ride home has also been drinking, I want to have a picture of a chart that calculates your BAC (blood alcohol content) on my iPhone, like ones that factor in weight, time, and amount of drinks, and any other factors. I want something that is accurate and easy to use. I'm not planning on drinking under age, but it's possible that this situation could come up with one of my friends. I just want a reliable website where I could get a good example of one to have a picture of on my phone.

There are charts at
http://www.brad21.org/bac_charts.html
http://www.safety-devices.com/bac_chart.htm
http://www.dmv.ca.gov/...0Chart.pdf
http://bloodalcoholcalculator.org/bac-charts/
etc.

However, instead of using an image of a chart, I would suggest an online calculator that does the same thing with less work on your part, such as
http://www.ou.edu/oupd/bac.htm

There's a problem with your question. If you suspect you are too drunk to drive, how are you going to operate the iphone to determine if you are sober enough to drive? The best app is one where a designated driver does not drink.

The problem with any app is that your personal metabolism can't be calculated. Two identical people with everything the same except metabolism will have different BAC levels. Food intake, gender, number of drinks, drinking rate, metabolism rate, other drugs (prescription or otherwise) all factor into the BAC level.

Also, even if you are below the BAC level, you can still be arrested for driving under the influence (impaired driving).

Just Google "BAC chart".It's not that difficult. But try this: http://en.wikipedia.org/...ol_content

Buy a breathalyzer, run from about $15 to 150.

You are under age so don't drink at all. And if you drink don't drive under any circumstances because you will be unable to figure out if you are drunk or not. It isn't worth it.

If you are underage and find yourself in a vehicle where the driver has drank anything then call in plan b. Hopefully at your age this would be a family member. Someone that you can call who will not hold judgement on you for putting yourself in the situation. If this is not the case then pay attention at the party to who is not drinking and ask for a ride. If all else fails bring cab money. There's no shame in getting home safely.

If the driver talks and acts sober, life is good. If he is laughing too much, or sullen, and is not himself… Then you know.

My wife and I have used the zero tolerance method for our 39 years of marriage. If we have had anything to drink that day, no driving. That makes the calculation easy: 1 or more drinks = find a different way home.

EDIT - Dan B has the best answer. No age is too young to begin the practice of a designated driver, somebody with the group who is not going to drink. It is the best way to solve the problem at any age.