How many units can you drive on?
There is no number of units that is safe to drive on.
The UK limit is not written in units. It is measured as alcohol in your blood, breath or urine, and the amount that reaches your blood from the same pint differs from person to person. Anyone giving you a number of pints is guessing with your licence and somebody else's safety.
If you are driving, the only reliable amount is none.
What the law actually measures
The limit is a concentration, not a count of drinks. Scotland sets a lower one than the rest of the UK.
| Measured in | England, Wales & N. Ireland | Scotland |
|---|---|---|
| Blood | 80mg per 100ml | 50mg per 100ml |
| Breath | 35µg per 100ml | 22µg per 100ml |
| Urine | 107mg per 100ml | 67mg per 100ml |
Why the same two pints reads differently on two people
These are the reasons a unit count cannot be turned into a breath reading.
Body size and composition
Alcohol spreads through body water. Less body water means a higher concentration from the same drink.
Sex
On average women reach a higher blood alcohol concentration than men from an identical drink, for the same reason.
Food
Alcohol on an empty stomach reaches the blood far faster and peaks higher.
Speed of drinking
Two pints over two hours and two pints in twenty minutes produce very different peaks.
Metabolism
Clearance rates vary between people, and with liver health, medication and illness.
The drink itself
Strengths vary widely, and a home pour is rarely a measured one.
The morning after catches more people than the night before
Alcohol leaves the body at roughly one unit an hour, and that clock only starts once the alcohol has been absorbed.
Someone who finishes ten units at midnight can still be over the limit at eight the next morning. Nothing speeds it up. Coffee, a cold shower, a big breakfast and a night's sleep make you feel more awake without making you any more legal to drive.
Roughly one unit an hour is an average, not a promise. It is fine for deciding that you definitely should not drive, and no use at all for deciding that you can.
If you are asking this question often
Regularly working out whether you are still over the limit is worth paying attention to in itself. Keeping an honest record of what you drink and when is a straightforward place to start, and it is what Alcohol-3 is for.
Related
This page explains the law as it stands in the UK. It is not legal advice. If you have been charged with a drink driving offence, speak to a solicitor.
Keep an honest record
Log what you drink and when. It answers the morning-after question better than any estimate, and it is free.
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