What Blood Alcohol Concentration (BAC) Actually Measures
Last reviewed: 2026 · Driviology Road Safety Education Team
Quick Answer
Blood alcohol concentration (BAC) measures the percentage of alcohol in a person's bloodstream, tested by breath, blood, or urine sample, and Ontario sets different legal limits for fully licensed drivers versus novice and young drivers.
BAC is the technical measurement behind Ontario’s impaired driving laws. Understanding what it actually measures helps explain why the legal limits are set where they are.
What BAC Measures
BAC expresses the concentration of alcohol in the blood as a percentage. It rises as alcohol is absorbed and falls only as the body metabolizes it over time, at a fixed and unavoidable rate.
How BAC Is Tested
Police most commonly use an approved roadside breath testing device, with more precise breath, blood, or urine testing available if a driver is taken in for further testing. See our guide to alcohol consumption and legal limits for the specific thresholds involved.
Ontario's Different Legal Limits
Fully licensed adult drivers face criminal penalties above the federal legal limit and administrative penalties in the lower "warn range." Novice G1/G2 drivers and drivers under 21 must maintain a BAC of zero.