The alcohol detection window calculator above is a lookup table with arithmetic attached. It reports the detection windows that named sources publish for each type of alcohol test, as ranges, and separately converts a drink description into grams of pure alcohol. It deliberately does not join those two things together, because the published evidence does not support joining them.
Arb Digital builds free calculators that state what a number cannot do. On this topic that matters more than anywhere else on the site, because the question people actually have when they arrive here is usually about driving, and the honest answer to that question is not a number at all.
What This Page Will Not Do
It will not tell you when you are safe to drive. It will not tell you when you are legally under a limit. It will not count down to a time when a test will come back negative, and it offers no advice about passing one.
The reason is that detection and impairment are different phenomena with different timescales. A metabolite can remain detectable long after any measurable effect on the body has gone, and impairment can be present at concentrations well below any legal threshold. Neither fact tells you anything about the other. Our BAC calculator, which models blood alcohol concentration using the Widmark equation, states the position in its own result: the only blood alcohol concentration that is safe to drive on is 0.00%. That remains true whatever any window on this page says.
There is a second reason. Every figure here is a population range from a review of published studies. Individual results vary with the amount and pattern of drinking, body composition, liver function, kidney function, hydration, medication, the analytical method used and above all the cut-off concentration the laboratory has set. A range that spans a factor of two or more across people cannot be narrowed to a personal answer by a web page.
The Windows and Where They Come From
The defaults on this page come from Biomarkers and Clinical Laboratory Detection of Acute and Chronic Ethanol Use by Tawiah, Riley and Budelier, published in Clinical Chemistry in 2022, supplemented by the SAMHSA Advisory The Role of Biomarkers in the Treatment of Alcohol Use Disorders, Volume 11 Issue 2, 2012, which we name rather than link because it does not reliably serve to automated readers.
Breath and blood ethanol. The Clinical Chemistry review notes that roughly 2 to 5 per cent of ingested ethanol is excreted unchanged through urine, breath and sweat, with a detection window typically under about 12 hours. These tests measure ethanol present at that moment, so what they detect falls continuously as alcohol is metabolised.
Urine EtG and EtS. The same review states that ethyl glucuronide and ethyl sulfate become detectable in urine approximately one hour after consumption and remain detectable for 24 to 48 hours, with detection potentially extending beyond 72 hours following heavy intoxication, depending on the analytical method and the cut-off value used. The SAMHSA advisory describes an intermediate-term window and notes that a standard cut-off of 500 nanograms per millilitre is used to reduce positives caused by incidental exposure to non-beverage alcohol in foods, mouthwashes and other products.
Blood PEth. Phosphatidylethanol has an elimination half-life of four to seven days and can be detected in circulation up to four weeks after drinking stops, per the same review. That makes it a marker of drinking over weeks rather than of a single occasion.
Hair EtG. The review states that hair provides a longer window for heavy alcohol use and is often used in forensic and legal settings, but does not put a single figure on it, because the answer depends on how much hair is analysed. Scalp hair grows at roughly a centimetre a month, so a segment covers approximately that many months of growth, and the calculator does that conversion for whatever segment length you enter. The precise interval depends on individual growth rate and on the laboratory's own segmentation protocol.
The Formula / How It's Calculated
The only genuine arithmetic on this page is the alcohol content conversion, which is standard. Grams of pure alcohol = volume in millilitres × ABV ÷ 100 × 0.789, where 0.789 grams per millilitre is the density of ethanol. Multiply by the number of drinks for a session total.
Work the defaults through. Three drinks of 568 ml at 4.5% ABV is 1,704 ml of liquid. At 4.5% that is 76.68 ml of ethanol, and at 0.789 g/ml that is 60.50 grams of pure alcohol. Dividing by 14 grams gives 4.32 US standard drinks; dividing by 8 grams gives 7.56 UK units. Our alcohol units calculator covers the same conversion in more depth, including how the two national conventions differ.
The hair conversion is equally simple: months of growth ≈ segment length in centimetres ÷ 1 cm per month, which the page presents as an approximation because that is what it is.
Everything else is a lookup. No formula anywhere on this page converts grams of alcohol into a detection time, because no such formula exists that would be honest at the individual level.
Why Cut-Off Concentration Changes Everything
A detection window is not a property of a substance. It is a property of a substance and a threshold together. EtG concentrations in urine fall continuously after drinking; whether a sample is called positive depends entirely on where the laboratory has drawn its line.
The commonly cited 500 nanograms per millilitre cut-off exists to reduce false positives from incidental exposure — hand sanitiser, mouthwash, some foods, certain medicines. Laboratories that use a lower cut-off, such as 100 or 200 nanograms per millilitre, will report positives from smaller quantities and for longer afterwards. Laboratories using a higher cut-off do the opposite. A window quoted without its cut-off is close to meaningless, which is why the values on this page are editable and why the source is named alongside them.
The same logic applies across every test type. Breath testing devices differ in sensitivity and calibration. PEth assays report against different thresholds. Hair testing has consensus cut-offs that have been revised over time. If you are facing a specific test, the number that matters is the one that test uses, and only the body administering it can tell you what that is.
What a Positive or Negative Result Actually Means
A negative result does not establish abstinence. It establishes that whatever was present was below the cut-off at the moment of sampling, which can be true after drinking for reasons ranging from dilution to timing to the choice of threshold.
A positive result does not establish intoxication, and for EtG in particular does not establish beverage drinking. The SAMHSA advisory is explicit that incidental exposure to non-beverage alcohol is the reason a cut-off is applied at all, and that low-level positives require careful interpretation rather than being read as proof of consumption.
None of these tests measures impairment. Impairment is a functional state that depends on tolerance, sleep, medication, food and individual response, and it is assessed by observation and by concentration at the time, not by whether a metabolite is present a day later. If you are being tested in a legal or employment context, the interpretation of your result is a matter for a qualified professional with the full picture, not for a calculator.
Where to Get Help With Drinking
A very common reason for looking up detection windows is worry about one's own drinking. If that describes you, the useful next step is not arithmetic. The NHS alcohol support page sets out what to expect from a GP conversation and signposts community services, Drinkline on 0300 123 1110, and self-help groups. Alcohol Change UK is a registered charity that publishes free tools, information and routes to support for people worried about their own or someone else's drinking.
If you are outside the UK, the equivalent service where you live will be listed by your national health authority. Withdrawal from heavy, dependent drinking can be medically dangerous, which is a reason to involve a clinician rather than a reason to avoid one.
Nothing on this page is a substitute for that conversation, and nothing here should be used to decide whether to have it.
Why the Same Person Gets Different Answers
Even holding the test and the cut-off fixed, the biology varies. Ethanol elimination rates differ substantially between people and are influenced by liver health, enzyme variants, sex, food intake and habitual drinking. Metabolite formation and renal clearance vary on top of that. Urine concentration itself varies with hydration, and a very dilute sample can fall below a cut-off that a concentrated one would exceed.
The practical consequence is that a range published across a study population is the most specific honest statement available, and narrowing it for an individual is not something this page, or any page, can do. If you want the general arithmetic of a substance clearing from the body, our half-life calculator covers it, and the caffeine calculator applies the same idea elsewhere.
Arb Digital designs and builds free interactive calculators that cite their sources, publish ranges rather than false precision, and stay firmly on the right side of the line between information and advice. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Reading a detection window as a safe-to-drive time. They are unrelated quantities. Detection can outlast any effect, and impairment can exist below any legal threshold.
- Quoting a window without its cut-off. The threshold the laboratory uses is part of the answer. The same sample is positive at one cut-off and negative at another.
- Treating a range as a schedule. A published range describes variation across a study population. It is not a timetable that applies to one person.
- Assuming a negative result proves abstinence. It shows a concentration below a threshold at one moment, which is a much weaker statement.
- Assuming an EtG positive proves drinking. Incidental exposure to non-beverage alcohol is the documented reason cut-offs exist, and low-level results need professional interpretation.
Related Free Tools From Arb Digital
Use the BAC calculator for blood alcohol concentration modelling and the plain statement about driving, the alcohol units calculator for converting drinks into grams, US standard drinks and UK units, the half-life calculator for the general arithmetic of substances clearing over time, the caffeine calculator for the same idea applied elsewhere, the water intake calculator for hydration, and the weight converter if your figures are in different units. Everything else is on the free online tools hub.
Frequently Asked Questions
No, and it never will. Detection windows describe when a test can find a substance, not when anyone is unimpaired or under a legal limit. Our BAC calculator states the position directly: the only blood alcohol concentration that is safe to drive on is zero.
Published sources give ranges rather than single numbers. The 2022 Clinical Chemistry review reports that urine EtG and EtS become detectable about an hour after drinking and remain detectable for 24 to 48 hours, potentially beyond 72 hours after heavy intoxication, depending on method and cut-off.
Because detection depends on the amount and pattern of drinking, individual physiology, hydration, the analytical method and above all the cut-off concentration the laboratory uses. A range across a study population is the most specific honest statement available.
The SAMHSA advisory describes it as a threshold used to reduce positives arising from incidental exposure to non-beverage alcohol in foods, mouthwashes and similar products. A laboratory using a lower cut-off will report positives for longer and from smaller amounts.
It depends on how much hair is analysed rather than on a fixed period. Scalp hair grows at roughly a centimetre a month, so a segment covers approximately that many months of growth, subject to individual growth rate and the laboratory's segmentation protocol.
No. It shows that whatever was present was below the laboratory's cut-off at the moment the sample was taken. Dilution, timing and the choice of threshold can all produce a negative result after drinking.
The NHS alcohol support page sets out what a GP conversation involves and signposts community services, Drinkline on 0300 123 1110 and self-help groups. Alcohol Change UK is a charity offering free information and routes to support. Outside the UK, your national health authority lists the equivalent services.
This page is an information tool for general educational use only. It is not medical or legal advice, it makes no prediction about any individual's test result, and it cannot tell anyone when it is safe or legal to drive or operate machinery. Every figure shown is a published population range that depends on the laboratory's cut-off and method. If a test result affects your health, employment or legal position, speak to a qualified clinician or an appropriate professional.
