For about fifteen years my method for working out whether I could drive on a Saturday morning was to stand in the kitchen, assess how I felt, and decide I felt fine. I always felt fine. That's the problem with the test. The thing doing the testing is the thing that's been drinking.

So here's the version with numbers in it.

* * *

The one-hour-per-unit rule

The NHS line is that one unit is roughly what an average adult clears in an hour. Your liver works at its own pace. It does not care that you have a 9am at the garden centre.

The catch is the starting gun. The clock starts when you stop drinking, not when you start, and the last drink takes a while to even reach your blood. So "I had my first pint at 7" is irrelevant. What matters is the last one, and how many came before it.

Worked examples (for those of you who are busy)

What you drank	Units (approx)	Minimum wait from last drink	Stopped at 11pm, so...
2 pints of 4.5% lager	5.1	6+ hours	5am at the earliest
4 pints of 4.5% lager	10.2	11+ hours	10am at the earliest
6 pints of 4.5% lager	15.3	16+ hours	3pm at the earliest
Bottle of 13% wine	9.8	11+ hours	10am at the earliest
4 double G&Ts (40%)	8	9+ hours	8am at the earliest

Units are the NHS formula: volume in ml times ABV, divided by 1,000. A pint is 568ml. These are the at the earliest numbers. Heavier, lighter, female, empty stomach, tired, on medication: all of it moves the line, and mostly in the wrong direction.¹

The six-pint row is the one that got me. Big Saturday, sensible Sunday lunch drive to your mum's, and you're plausibly still over at 1pm.

* * *

Things that don't help (I've tried them)

Sleep. Your liver does the same job asleep or awake. Eight hours of sleep is eight hours, not sixteen.

Coffee. Makes you a more alert drunk person.

A fry-up. Food before drinking slows absorption. Food the morning after is just breakfast. Good breakfast. Not a remedy.

A shower, a run, a pint of water. Nice. Irrelevant.²

What's the actual limit?

In England, Wales and Northern Ireland it's 35 micrograms of alcohol per 100ml of breath. In Scotland it's 22. There's no safe number of pints that converts to that, because everyone processes alcohol differently, which is why GOV.UK refuses to give one. I've written up the limit properly here, including why Scotland is basically a zero-pint country.

And if you get it wrong

A minimum 12 month ban, a fine set as a chunk of your weekly pay, and a conviction on your licence for 11 years. I built a drink drive ban calculator that shows the guideline numbers for any reading. It's sobering, which is sort of the point.

* * *

The thing that actually fixed this for me wasn't learning the maths. It was not drinking on the nights before I needed the car. Weeknights now, it's an IMPOSSIBREW and the Saturday morning question just doesn't come up. That's my referral link (tenner off for you, tenner of credit for me). Thought I should say rather than pretend otherwise.

When in doubt, don't drive. Get a lift. Paul will do it. Paul owes everyone a lift.