Am I too drunk to drive?

Three separate reasons, each sufficient on its own.

1. The estimate is wider than the decision. The Widmark equation needs two constants that vary between people, so an honest result is a band roughly a quarter wide. For most realistic nights that band straddles the legal limit, meaning the calculator genuinely does not know which side of it you are on. The margins are set out in full here.
2. Impairment starts below every limit. Measurable loss of visual tracking, divided attention, and reaction time begins around 0.02 to 0.05, under the lowest mainstream limit in the world.¹ A number under a limit is a statement about prosecution, not about competence.
3. It cannot see you. A formula does not know what was really in the glass, what you ate, what you take, how much you slept, or that alcohol is still absorbing. Tiredness alone degrades driving, and it stacks.

A limit is the point at which the law stops arguing about whether you were impaired and starts arguing about the penalty. It is not a threshold below which driving is fine, and it is not the same number everywhere: it changes by country, by state, by vehicle class, and by how long you have held a licence. Several countries are at zero.

Which is why this site will tell you when an estimate is still above a limit, and will never tell you that being below one clears you. The BAC chart shows what different nights work out to, and what each level does to the faculties driving needs. This is general information about how alcohol behaves, not legal advice about your situation.

Most people asking this question at 1am will make a sensible decision. The dangerous version is at 8am, when the night feels finished, the hangover reads as tiredness rather than alcohol, and nobody thinks of themselves as drink-driving on the way to work.

The arithmetic is unsympathetic. Blood alcohol falls by roughly 0.012 to 0.02 percentage points per hour, about one standard drink an hour, and sleep does not add to that rate.² A genuinely heavy night can still be over a legal limit at breakfast, and morning-after cases are an ordinary category of drink-driving prosecution rather than a curiosity.

The night this happens most is the Wednesday before Thanksgiving: a heavy Blackout Wednesday routinely has people driving to dinner the next afternoon on blood alcohol that has not finished clearing.

If the question is when last night likely clears, the sober-up estimator gives a window rather than a time. Plan around its later edge, and treat it as the earliest point at which the question becomes worth asking, not as a clearance.

A breathalyzer measures what a calculator estimates, and it is the only consumer device that does. That makes it genuinely useful for one thing: finding out what your own drinking actually does to you, tested the next morning, when the answer is information rather than a decision.

It is not a pre-driving check and should never be used as one. If you are sitting in a car about to test yourself before driving: get out of the car and call a taxi or a rideshare. The device is not the thing to fix in that moment, and no reading it gives you changes what to do next.

Treat the impulse as the more important signal. Needing a machine to decide whether to drive home is not a question about equipment, and it does not become one at a lower number. If that decision keeps coming up, the confidential helplines in the footer of every page on this site are there for exactly that, and they are a great deal cheaper than the alternative. How the estimates here compare to a measurement sets out the chain: a blood test measures, a decent fuel-cell breathalyzer estimates closely, a formula estimates roughly, and how you feel barely counts as evidence.