How long until alcohol leaves your blood, and how long until it drops below a given limit. The liver clears at a constant rate and there is no way to speed it up.
Until it is fully cleared
—
Until it is fully cleared—
Until it drops below the limit—
Blood alcohol now—
Clearance rate—
Standard drinks—
Against the limit—
How this was worked out
Indicative result. The time calculated is a population average. If you have been drinking, the only safe decision is not to drive, whatever this figure says.
The formula
hours = current blood alcohol ÷ 0.15
Where it comes from
Unlike almost everything the body clears, alcohol does not follow exponential decay but linear decay: the enzyme that metabolises it saturates almost immediately, so above a small amount the liver works flat out and clears the same amount every hour, around 0.15 grams per litre. That is why the time needed is simply the blood alcohol divided by that rate.
How to work it out by hand
Work out current blood alcohol with the Widmark equation
Divide by 0.15, which is what clears per hour
The answer is the hours until it reaches zero
To drop below a limit, subtract that limit from the blood alcohol first
What is worth knowing
Nothing speeds clearance up: coffee keeps you awake but does not reduce blood alcohol, and an awake person at 0.8 g/l is still at 0.8 g/l. Sleeping does not speed it up either, which is why people test positive the next morning: three drinks too many at two in the morning can still register at nine. The 0.15 rate is an average; in some people it is 0.10, and then the wait is twice as long again.
Frequently asked questions
Does coffee help clear alcohol?
No. It keeps you awake, which is a different thing. Blood alcohol is unchanged and feeling fine is the dangerous part.
Can I test positive the next morning?
Easily. From a high reading in the small hours, eight hours of sleep is not enough to reach zero.
Why is clearance linear?
Because the enzyme that metabolises alcohol saturates at a very low concentration. Beyond that the liver runs flat out and clears a constant amount.
Does weight affect the time?
Not directly: it affects the starting blood alcohol, and that is what determines how long you wait.
Is it the same for everyone?
No. The 0.15 per hour is an average; some people are at 0.10 and others at 0.20, which doubles or halves the wait.
Cookies
We use our own cookies to remember your preferences: country, number format and the calculators you use most. Analytics and advertising cookies are only switched on if you accept them. More information.