Calculating blood alcohol: why a range is more honest

A single per mille number looks exact, but it is not. Why DrinkBuddy shows a range and counts the most cautious edge.

A blood alcohol calculator usually gives one number, say 0.4. That looks exact, but it is a sum with assumptions. A range is more honest: “somewhere between this and that”. And anyone who still has to drive looks at the most cautious end of it.

What goes into the sum

An estimate uses what you enter: what you drank, how much and when, plus your weight and sex. Each of those has a margin:

• What was in the glass. The pour and the strength differ from what the label or the menu says.
• Your body. How much water your body holds and how fast your liver works differs per person and per day. According to the Trimbos Institute the breakdown pace lies roughly between 0.1 and 0.25 per mille per hour.
• When you drank and whether you ate. That shifts how quickly alcohol reaches your blood.

A formula that rounds all of this to one number pretends the margins do not exist.

What DrinkBuddy does differently

DrinkBuddy shows a band instead of one line, and the battery reads the side of the band closest to the limit. If the lower end is under the limit but the upper end is not, the app does not say “fine”. Uncertainty means: don’t drive.

What you can do yourself

• Log what you drink, including the last round, or the estimate is off.
• Treat the result as a reason to stop earlier, never as permission to keep drinking up to the limit.
• Sort out your way home before you start: a designated driver, taxi, public transport or sleeping over.

Sources

• Trimbos Institute, Opname en afbraak van alcohol in het lichaam (in Dutch)
• AlcoholInfo, Alcoholpromillage berekenen (in Dutch)
• AlcoholInfo, De afbraak van alcohol (in Dutch)