Guide
Breathalyser or BAC Calculator?
There are two ways to find out your blood alcohol concentration: measure it or calculate it. They do different things, carry different error margins, and are useful in different situations. This article compares how each works and whether one can substitute for the other.
How a breathalyser works
Breathalysers measure breath, not blood. In the lung alveoli, alcohol reaches equilibrium between blood and air; the device measures the alcohol concentration in that air and converts it to a blood value using a fixed ratio. The commonly used conversion is 1:2100 — the alcohol in 2100 ml of breath is treated as equivalent to the alcohol in 1 ml of blood.
Two sensor technologies exist. Professional devices used in roadside enforcement use fuel cell sensors: specific to alcohol, stable, and requiring regular calibration. Cheap consumer devices typically use semiconductor sensors, which cost less but can be affected by cigarette smoke, perfume and ketones, and drift substantially over time.
Where breathalysers go wrong
Because a breathalyser measures, people assume it is absolutely correct — but it has clear error sources too. The best known is mouth alcohol: if you have just finished a drink, alcohol on the mouth and throat lining produces a reading far above your actual blood level. This is why official testing usually enforces a 15-20 minute waiting period.
For personal devices, calibration is the biggest problem. Semiconductor sensors begin to drift after a few hundred readings and most owners never have them recalibrated. The device still displays a confident number — it has simply lost its connection to reality.
- Mouth alcohol — wait 15-20 minutes after the last drink
- Calibration drift — happens quickly on cheap semiconductor devices
- Blowing technique — a reading comes out low without a deep, sustained breath
- Body temperature — a fever can inflate the reading
- Diabetic ketosis and some diets — can trigger false positives on semiconductor sensors
- Cigarette smoke and mouth sprays — cause temporary deviations
How calculation works
Mathematical estimation rests on the Widmark formula. It divides the total pure alcohol consumed, in grams, by your body's alcohol distribution volume — your weight multiplied by a sex-dependent coefficient — then subtracts the amount metabolised over the elapsed time.
This method needs no device and works both backwards and forwards. A breathalyser cannot answer "where will I be in three hours", but a calculation can. Its weakness is dependence on inputs: get the amount you drank wrong and the result is wrong too.
The Widmark formula has been established in forensic toxicology for decades, but it is built on population averages. Individual differences — body fat percentage, liver capacity, medications — can pull the real value away from the estimate.
When is each one useful?
They are complementary rather than competing. A breathalyser measures your current state but cannot predict the future and requires carrying a device. A calculation lets you plan: what happens if I have this much, how many hours until I am under the limit — only arithmetic answers those.
But there is one thing neither can do: carry legal weight. Neither your personal breathalyser nor any calculator produces an official result. At a roadside stop, only a measurement taken by authorities on an approved, calibrated device, or a hospital blood test, counts.
- To measure your current state: a calibrated breathalyser
- To plan ahead: a calculator
- To track consumption over time: an app that keeps a record
- For official or legal purposes: only an authority measurement or blood test
The right mindset
Both methods share the same dangerous use case: trying to sit just under the limit. Neither the device's nor the calculation's error margin is tight enough to justify "it says 0.48‰, I can drive".
Their real value is making the risk you are carrying concrete. Seeing a number like 1.20‰ at the end of a night is far clearer feedback than saying "I had a few". That is how we recommend using Promil Tracker: not to push the limit, but to make your own consumption visible.
Frequently Asked Questions
Fuel cell models that are regularly calibrated give reasonable accuracy. Cheap semiconductor devices can drift substantially, particularly after a few hundred uses. No personal device produces a result with legal standing.
Because they do different things. A calculation is a statistical estimate based on what you entered and built on population averages, while a breathalyser measures your breath right now. If absorption is still ongoing, or the amount you entered differs from what you actually drank, the two diverge noticeably.
The standard Widmark formula does not use age as a numeric factor; its core variables are weight, sex, alcohol consumed and time elapsed. Age-related changes in body composition and liver function are highly individual and are not built into the general formula.
No. Phones have no alcohol sensor, and any app claiming to measure your breath is not performing a real measurement. Apps can only calculate from the data you enter — which is exactly what Promil Tracker does.