Almost everyone arrested for a DUI in California asks me some version of the same question. How could my number be that high when I only had a couple of drinks? I am Joel Brand, and I defend DUI cases across the state. Blood alcohol concentration is far less precise and far more attackable than the clean little decimal on a police report makes it look. Understanding how it actually works is the first step to understanding why your case is more defensible than it feels.
What blood alcohol concentration is measuring
Blood alcohol concentration, almost always written as BAC, is the percentage of alcohol in your bloodstream by volume. A 0.08 reading means roughly eight hundredths of one percent of your blood is alcohol. It is a tiny quantity, which is exactly why small errors in how it is measured matter so much. A breath machine does not even measure your blood. It measures alcohol in a sample of your breath and then multiplies by an assumed conversion factor to estimate what your blood would show. That assumption is built into the machine, and it is not true for everyone.
How alcohol gets into your system, and how it leaves
When you drink, alcohol is absorbed through your stomach and small intestine into your blood. It does not happen instantly. Depending on what you drank, whether you ate, and your own body, full absorption can take anywhere from fifteen minutes to well over an hour. Your liver then eliminates alcohol at a fairly steady but individual rate, often estimated near 0.015 percent per hour. The key point is that your BAC is a moving target. It rises while you are still absorbing and falls once absorption is done.
Why counting drinks does not give you a number
People try to back-calculate their BAC from how many drinks they had, and it almost never works. A home pour is not a standard drink. Beer and cocktails vary wildly in strength. Body weight, sex, food, hydration, and even the time between your first and last drink all change the result. Two people who drank the identical amount can blow very different numbers. This is also why the roadside guesswork people do, deciding they are fine to drive, is so unreliable. I wrote separately about what your breath test number actually means.
The two legal limits that matter most
California does not just punish you for a number. It has two separate ways to charge a DUI, which I explain in detail in the article on driving with a 0.08 percent blood alcohol level. There is the per se limit of 0.08, where the number alone is the offense, and there is driving under the influence, where impairment is the issue regardless of the exact reading. Lower limits apply to commercial drivers at 0.04 and to drivers under 21 under the state's zero tolerance scheme at 0.01.
Rising BAC, the timing problem the state hopes you ignore
Here is a defense that turns the science back on the prosecution. The law cares about your BAC when you were driving, not when you were tested. The test usually happens forty minutes to an hour or more after the stop. If you were still absorbing alcohol during that gap, your BAC at the wheel could have been below the limit even though the later reading was above it. This is the rising blood alcohol argument, and I use it often. I cover it fully in the rising BAC defense.
Why the machine itself is not above suspicion
A breath result depends on a machine working correctly and being used correctly. Breath instruments must be calibrated and maintained on a schedule, and when that paperwork is missing or wrong, the result is open to challenge. I dig into this in the bad calibration defense. Mouth alcohol is another problem. A burp, acid reflux, a recent drink, or residue from dental work can put alcohol in your mouth that the machine reads as if it came from deep in your lungs, inflating the number. That is the subject of the mouth alcohol defense.
Blood tests are not automatically more reliable
People assume a blood draw is the gold standard, and while a blood sample has the advantage of being preserved for independent retesting, it has its own weak points. Fermentation in an improperly stored vial, a contaminated swab, a gap in the chain of custody, or a phlebotomist who deviated from protocol can all skew a blood result. A preserved sample is a defense asset precisely because your own expert can re-examine it, which is impossible with a breath test that vanishes the moment you blow.
What this means for your case
The single number at the top of your report carries enormous weight in the courtroom, but it rests on assumptions, timing, equipment, and procedure, every one of which can be examined. I have seen readings fall apart once the calibration logs, the observation period, and the absorption timeline are scrutinized. The number is a starting point for the conversation, not the end of it.
Why the same person can blow two different numbers
It is not unusual for a breath test to give two readings a few minutes apart that do not match, and for a later blood draw to disagree with both. That variation is not a glitch you should shrug off. It is a window into how imprecise the measurement is, and the law generally expects breath samples to agree within a narrow margin for a reason. When the readings scatter, it raises a fair question about which number, if any, actually reflects what was in your blood behind the wheel. Those discrepancies are something I look for closely in the records.
The bottom line
Your BAC is an estimate produced by a machine or a lab, shaped by your biology and the timing of your drinks, and measured well after you were actually driving. None of that is as airtight as it looks on paper. If you are looking at a number you do not understand or do not believe, that is exactly the kind of case worth a close look. Get a free written case analysis below, or call me directly at (888) 271-6644. I answer my own phone, 24/7. You can also read more from the DUI blog.