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If you drive with a CDL, the rules are not the same as they are for everyone else. A regular driver can go up to 0.08% before they are considered legally impaired. You lose that privilege at 0.04%. That is half the normal limit, and it catches a lot of experienced drivers off guard.This calculator lets you pick the commercial driver limit directly. Enter your weight, log what you drank and when, and it will show you your estimated BAC against the 0.04% line, plus how long until you are back under it.
● 3 calculation models ● State and country legal limits ● Live BAC curve
Used to estimate body water distribution — the biggest factor in BAC accuracy.
Average is 0.015%/hr. Genetics, liver health, and tolerance shift this between 0.010–0.020%/hr.
Log each drink as you go for the most accurate live curve.
Modeled absorption & elimination curve for your full session.
0 / 8 glasses
Water helps prevent dehydration and eases next-day symptoms, but it does not lower your BAC or speed up alcohol metabolism. Only time does that.
General effects by approximate BAC level. Individual response varies significantly.
| BAC | Typical Effects |
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The 0.04% limit comes from federal law, not a state law. It is set by the Federal Motor Carrier Safety Administration under 49 CFR 382.201, and it applies to anyone operating a commercial motor vehicle that requires a CDL, in every state, no matter what that state’s regular limit is.
The reasoning is simple. A commercial vehicle is heavier, harder to stop, and often carries passengers, hazardous materials, or just a lot of weight that turns a small mistake into a serious one. Regulators decided the margin for error needed to be smaller.
There is a second rule that catches people out even more than the BAC number does: you cannot use alcohol within 4 hours of going on duty, operating a commercial vehicle, or being required to be ready to drive (49 CFR 392.5). So even if you would test under 0.04% by the time you get behind the wheel, drinking too close to your shift can still put you in violation.
A lot of drivers assume 0.04% means “half a beer.” It is a bit more forgiving than that, but not by much. For a 200 lb man, two standard beers over the space of an hour can put him close to or over 0.04%, depending on how fast he drinks and whether he has eaten. For a smaller driver, or a woman (who generally has less body water to dilute the alcohol), it can take less than that.
This is exactly why a flat rule of thumb does not work and a real calculator does. Body weight, sex, how much time has passed, and whether you ate all change the number. Our tool runs the same math your body actually uses (the Widmark and Watson models), not a rough guess.
The penalties are set federally too, so they apply the same way regardless of which state pulls you over:
States are allowed to add stricter penalties on top of the federal minimum, but they cannot go easier than this. If you hold a CDL, a DUI is not just a fine and a bad weekend. It is your livelihood.
Most people search for a BAC calculator after they have already been drinking, to check if they are okay to drive right now. If you drive for a living, it is worth using it the other way around too: figure out how many hours before your shift you need to stop drinking entirely, based on your own weight and how much you plan to have.
Log your drinks as you go, set the legal limit to 0.04%, and the calculator will show you the exact time your estimated BAC crosses back under the line. Give yourself a real buffer past that point. Federal inspectors and your dispatcher will not care that the calculator said 0.039%.
QUESTIONS
No. The 0.04% limit applies specifically to operating a commercial motor vehicle. When you are driving your own personal vehicle off duty, your state’s regular limit applies, usually 0.08%. That said, a DUI conviction in your personal vehicle can still disqualify you from your CDL, so the practical risk is not that different.
Yes. It is a federal standard under FMCSA rules, so it applies the same way in all 50 states for anyone operating a commercial motor vehicle that requires a CDL.
Yes. A BAC of 0.04% or higher while operating a commercial vehicle is a violation on its own, separate from a state DUI charge, and it carries its own federal disqualification.
It uses the same Widmark and Watson body-water formulas used in clinical and forensic estimation, based on your weight, sex, age, and drink timing. No calculator can be 100% exact because things like metabolism, medication, and food intake vary by person, but it gives a much more realistic estimate than guessing or a flat “one drink per hour” rule.
The same 0.04% limit applies, but the penalty for a first offense is higher, a 3 year disqualification instead of 1 year. Treat hazmat routes with an extra margin of safety.