Understanding your Blood Alcohol Concentration (BAC) as it Relates to a California DUI

Your blood alcohol concentration, commonly referred to as your BAC, blood alcohol content, or even blood alcohol level, is the amount of alcohol that is in your bloodstream.  Expressed as a percentage, your BAC is measured as grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.   The more you drink, the higher your BAC.

Even though your BAC is a scientifically validated number, it is still open to attack.  This is because any violation of California's Title 17 can support your DUI defense.  Title 17 of the California Code of Regulations establishes the protocol for collecting, storing, and maintaining California DUI chemical tests.  These violations can lead to inaccurate high BACs in a countless number of situations, causing innocent people to be falsely arrested for DUI.

A skilled California DUI defense attorney knows what type of violations to look for and how to persuasively convince a judge/jury that...based on those violations...your BAC cannot be trusted.

How California DUI blood and breath tests measure your BAC

California chemical tests measure your BAC in two ways...through DUI breath tests and through DUI blood tests.

DUI breath tests

California DUI breath tests are the most common way to measure one's BAC following a DUI arrest.  Although there are a variety of breath testing instruments that California uses, they all have the same purpose...to capture a breath sample that it can convert to a BAC.

DUI breath testing instruments are designed to measure deep lung air (also called alveolar air).  The reason deep lung air is required is because it is the most accurate type of breath sample for determining one's BAC.

When deep lung air enters the breath testing instrument, the instrument multiplies the amount of alcohol found in the breath sample by 2100 to get what's known as your blood-to-breath partition ratio.  By using this ratio, a breath testing machine converts the amount of alcohol detected in your breath to the amount of alcohol in your blood...which is your BAC.

DUI breath tests are susceptible to contamination by a number of factors, including (but not limited to):

  • your breathing pattern (if your breathing pattern is inconsistent, it may affect the accuracy of your BAC result),
  • mouth alcohol fooling the DUI breath test by substituting deep lung air (the type of air which the breath testing instrument is designed to collect) with alcohol lingering in the mouth2, and
  • outside influences (such as radio frequency interference "RFI" and/or temperature).  RFI occurs when an electronic device is located near the breath testing instrument that can interfere with the circuitry of the machine, causing an inaccurate read.

If any of these factors improperly affects your DUI breath test, you can be falsely accused of driving under the influence and wrongfully arrested for Vehicle Code 23152(b) VC driving with a BAC of 0.08% or greater.

An experienced California DUI defense lawyer will be on the lookout for these as well as other errors in an effort to help you defend your DUI breath test.

DUI blood tests

Unlike a DUI breath test that indirectly measures your BAC by converting the alcohol in your breath to obtain a BAC, a DUI blood test directly measures the alcohol in your bloodstream.  As a result, California DUI blood tests are the most reliable way to measure one's BAC.

Even so, they are still subject to scrutiny.  Just because your DUI blood test reports that you have a BAC of a 0.08% or greater doesn't mean that the result is accurate.  Factors such as

  • how, by whom, and when your blood is drawn,
  • whether the vial containing your sample has sufficient amounts of anticoagulant and preservative (both of which are necessary to preserve the integrity of the sample), and
  • whether your DUI blood sample is properly stored,

can all affect the accuracy of your reported BAC.  As San Francisco DUI defense attorney Jim Hammer explains3, "Although blood tests consistently produce the most accurate BAC levels, they are by no means foolproof.  There are a number of issues that can affect the accuracy of DUI blood tests...issues that an experienced DUI lawyer knows how to investigate and effectively challenge."

Calculating your BAC

Although BAC is considered an "objective" way to measure whether you are fit to drive, in reality, your BAC isn't so objective.  While California DUI laws prohibit driving with a BAC of 0.08% or greater, experts say that some people aren't under the influence until their BAC is higher...0.10% for example...while anti-drinking lobbyists claim that everyone is under the influence at 0.05%.

Nonetheless, pursuant to California Vehicle Code 23152b VC, you are presumed to have driven under the influence if you submit to a DUI chemical test that reports a BAC of 0.08% or greater.  While there are general ways to calculate your BAC based on

  • how many drinks you consume,
  • how long you drink,
  • what type of alcohol you drink,
  • your gender, and
  • your weight,

factors such as

  • whether you eat while you drink or whether you drink on an empty stomach,
  • whether you are sick and/or on medication, and
  • (with respect to a DUI breath test), your breathing pattern,

can all affect these calculations.

That said, there are online BAC calculators that you can use to determine "how much is too much".  It is important to understand that these are simply average calculations or rough estimates based on "normal" drinking patterns.

While we can't guarantee or endorse the accuracy of the DUI BAC calculator below, it does provide useful information about responsible drinking.  Simply enter your information below to see examples of estimated blood alcohol calculations.

I have had      over a period of      hour(s)2.

I am      Male      Female (Explanation of gender differences in Blood Alcohol Concentration)

and I weigh      Pounds      Kilograms

and I live in      (so that the result is displayed in the appropriate units).


You may also find helpful information in our related articles on Vehicle Code 23152b Driving with a BAC of 0.08%, California DUI Breath Tests, California DUI Blood Tests, and How California Title 17 Violations Support Your DUI Defense.

Call us for help.
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If you or loved one is charged with a DUI and you are looking to hire an attorney for representation, we invite you to contact us at Shouse Law Group. We can provide a free consultation in office or by phone. We have local offices in Los Angeles, the San Fernando Valley, Pasadena, Long Beach, Orange County, Ventura, San Bernardino, Rancho Cucamonga, Riverside, San Diego, Sacramento, Oakland, San Francisco, San Jose and throughout California.

Legal References:

1California Vehicle Code 23152 -- Driving Under the Influence.  ("(b) It is unlawful for any person who has 0.08 percent or more, by weight, of alcohol in his or her blood to drive a vehicle. For purposes of this article and Vehicle Code Section 34501.16  VC, percent, by weight, of alcohol in a person's blood [BAC] is based upon grams of alcohol per 100 milliliters of blood or grams of alcohol per 210 liters of breath.1

2When a California breath testing instrument measures breath, it is designed to measure deep lung air.  When this type of air is collected by the instrument, the BAC is determined by multiplying the amount of alcohol present in the breath sample by 2,100 (this is known as the blood-to-breath ratio).  Because of this significant calculation, any amount of residual alcohol found in the mouth can result in a major discrepancy in one's true BAC...which is why we say that mouth alcohol can fool a DUI breath test.

3San Francisco DUI defense attorney Jim Hammer uses his former experience as a district attorney to defend California DUI clients throughout the Bay Area, including Oakland, Berkeley, Daly City and San Jose.

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