A breathalyser can usually detect alcohol for around 12 to 24 hours after your last drink, depending mainly on how much you drank. Because a breathalyser measures current Blood Alcohol Concentration (BAC) — not lifetime use — it stops reading positive once your body has metabolised the alcohol back down to zero.
Key facts
- Breathalysers measure current BAC, so they read positive only while alcohol is still in your system.
- The body clears alcohol at roughly 0.015% BAC per hour on average.
- Typical detection window is about 12–24 hours, driven mainly by how much was consumed.
How your body clears alcohol
On average, the body eliminates alcohol at roughly 0.015% BAC per hour, though this varies between individuals. So someone who reaches 0.08% BAC will take roughly five to six hours to return to zero; someone at 0.15% may take around ten hours or more.
This is why a single heavy night can still register the next morning: if you peaked high, you may not be fully clear by the time you wake.
What affects the detection window
- Amount consumed — more alcohol means a higher peak and a longer time to clear.
- Time — BAC falls steadily; the breathalyser reads zero once alcohol is fully metabolised.
- Body weight & composition — affects how alcohol distributes and dilutes.
- Sex & metabolism — enzyme activity and body water differ between people.
- Food — eating slows absorption, lowering the peak (but not the total to clear).
Breath vs blood vs urine vs hair
Different tests have very different windows. Breath and blood reflect current BAC (hours). Urine tests can detect alcohol or its markers for up to 12–24 hours, or longer with EtG testing. Hair tests can indicate use over months. A breathalyser is a real-time tool — it tells you whether someone is impaired now, which is exactly what roadside and workplace screening needs.
Why 'now' is what matters for safety
For driving, machinery and pre-shift checks, the relevant question is whether the person is over the limit at this moment. A breathalyser answers that directly and immediately, which is why it's the front-line tool for enforcement and workplace safety rather than longer-window lab tests.
Frequently asked questions
- Can a breathalyser detect alcohol from the night before?
- Yes, if you drank enough that your BAC hasn't returned to zero. Heavy drinking can still register the next morning.
- Does water, coffee or food clear a breathalyser faster?
- No. Only time lets your body metabolise alcohol. Food eaten before drinking lowers the peak, but nothing speeds elimination once alcohol is in your blood.
- How long after one drink can a breathalyser detect it?
- A standard drink raises BAC for roughly 1–2 hours before it clears, though mouth alcohol can be detected immediately after drinking.