BAC Calculator

Last updated: April 2026

Unit system

Sex

Body weight

Number of drinks

drinks

Drink size

Alcohol by volume

Hours since first drink

hrs

Estimated BAC

0.024%

Low estimated BAC

Breakdown

Alcohol consumed28g / 0.99 oz

Raw BAC before metabolism0.039%

Estimated metabolism deduction0.015%

Safety warning

This BAC estimate is not legal or medical advice. Do not use it to decide whether you can drive. Alcohol impairment varies by person and situation.

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What is BAC?

BAC means Blood Alcohol Concentration. It estimates the amount of alcohol in the bloodstream.

How is BAC estimated?

This calculator uses a simplified Widmark formula based on pure alcohol consumed, body weight, sex, and estimated metabolism over time.

Why BAC estimates vary

BAC can vary because of body size, sex, food intake, metabolism, medication, health, drink strength, and measurement accuracy.

Can I use this to decide if I can drive?

No. This calculator must not be used to decide whether driving is safe or legal.

About this calculator

The BAC Calculator estimates blood alcohol concentration from sex, body weight, drink volume, alcohol percentage, number of drinks, and time elapsed. It is designed to explain the Widmark-style calculation, not to decide whether someone is safe or legal to drive. Use this expanded guide when you need more than a quick result. It explains the assumptions behind the BAC Calculator, the records to gather, and the decisions the estimate can support. It is especially useful for adults learning how drink amount, alcohol strength, body weight, sex, and time can affect an estimated BAC. The strongest use of the page is scenario comparison: change one input at a time, compare the output, and keep a note of which assumption changed.

BAC estimate method

The calculator estimates pure alcohol consumed, converts it to ounces, divides it by body weight and a Widmark distribution ratio, then subtracts an estimated metabolism deduction of 0.015 BAC per hour since the first drink. The calculator result depends on the quality of the inputs and on the rule set or formula selected in the calculator above. For practical use, treat the output as a structured estimate: start with the core inputs, review the main outputs, then test the decision points that matter most to your situation. Key decisions include why drink count alone is not enough, how serving size and ABV change alcohol grams, why driving decisions should not rely on estimates.

alcohol grams = drinks x volume ml x ABV x 0.789
raw BAC = (alcohol ounces x 5.14) / (weight lb x Widmark ratio)
estimated BAC = max(0, raw BAC - 0.015 x hours elapsed)
better estimate = accurate inputs + correct rule set + realistic assumptions
scenario difference = revised result - original result

How to use the BAC calculator

Choose metric or imperial weight units.
Enter sex because the calculator uses different Widmark ratios.
Enter drink count, serving size, and alcohol percentage.
Enter hours since the first drink, not just since the last drink.
Read the result as an estimate only and do not use it to make driving decisions.
Gather the main inputs first: sex, body weight, drink count.
Check supporting records such as drink labels and serving sizes before relying on a final number.
Enter one realistic scenario first, using conservative assumptions where the future is uncertain.
Review the main outputs: estimated BAC, alcohol grams, raw BAC before metabolism.
Run at least one alternative scenario so you can see which input changes the answer most.
Compare the result with GOV.UK drink-drive limit guidance or the relevant contract, bill, statement, or professional document.
Keep the calculation date and assumptions with your notes so you can revisit the estimate when rates, rules, or circumstances change.

Worked example

Two standard beers

Input: Male, 180lb, two 355ml drinks at 5%, one hour elapsed

Calculation: The calculator estimates alcohol grams, raw BAC, then subtracts 0.015 for one hour.

Result: Estimated BAC is about 0.024 using the calculator method.

Higher ABV scenario

Input: A user changes beer strength from 4% to 6%.

Calculation: Pure alcohol ml increases in proportion to ABV.

Result: Estimated BAC rises even though drink count is unchanged.

Time elapsed scenario

Input: The same drinks are compared after 1 hour and after 3 hours.

Calculation: The calculator subtracts 0.015 BAC per elapsed hour.

Result: The later estimate is lower, but still only an estimate and not proof of safety.

Why BAC estimates are uncertain

Real alcohol absorption varies with food, drinking speed, body composition, medication, fatigue, health, and drink strength. Two people entering the same drink count can have different impairment and legal risk.

What to check before relying on the result

A useful BAC Calculator result starts with the same evidence you would use if you were checking the answer manually. The calculator can organise the arithmetic, but it cannot know whether a payslip is final, a bill is estimated, a quote excludes fees, or a personal circumstance has changed since the last statement.

Before making a decision, compare the calculator result with the source document that controls the real outcome. For this topic, that usually means checking GOV.UK drink-drive limit guidance. If there is a difference between the calculator and an official statement, contract, assessment, or professional advice, treat the official document as the stronger source.

drink labels: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
serving sizes: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
time drinking started: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
body weight unit: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.

Inputs that usually change the answer

The most important input is not always the largest number on the form. Sometimes a date, threshold, percentage, eligibility flag, or timing assumption changes the result more than the headline amount. This is why scenario testing is more useful than a single calculation.

Input	Why it matters	What to double-check
sex	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
body weight	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
drink count	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
drink volume	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
alcohol percentage	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.

How to interpret the output

The output should be read as a decision aid, not just a number. For BAC Calculator, the useful question is often what the result means for timing, affordability, eligibility, comparison, or next steps.

estimated BAC: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
alcohol grams: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
raw BAC before metabolism: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
metabolism deduction: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
status label: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.

Scenarios worth comparing

A single estimate is a snapshot. A better approach is to save a base case, then adjust one assumption at a time. This shows whether the result is stable or whether a small change in timing, rate, usage, income, or cost creates a very different answer.

Scenario	Change one assumption	What the comparison shows
Base case	Use the best current evidence.	Shows the result you would expect if nothing important changes.
Conservative case	Use lower income, higher cost, slower growth, or less favourable timing.	Shows whether the decision still works with less optimistic assumptions.
Improved case	Use the realistic upside, such as lower cost, better rate, higher usage, or stronger evidence.	Shows the potential benefit without treating it as guaranteed.

Common mistakes and edge cases

Most errors come from using the right formula with the wrong assumption. Dates can be counted differently, rates can change, official thresholds can move, and real bills or contracts often include conditions that a simple calculator cannot infer automatically.

Alcohol impairment can occur below legal limits.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
BAC can keep rising after the last drink.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Legal limits differ by jurisdiction.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Medication, food, illness, and fatigue are not modelled.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.

Next steps after calculating

Once you have a result, write down the key assumptions and compare them with GOV.UK drink-drive limit guidance. If the number affects a deadline, tax return, benefit claim, employment issue, medical question, finance agreement, or major purchase, use the calculator as preparation for a more formal check.

For lower-stakes use, the next step may simply be comparing two or three scenarios. For higher-stakes use, the next step should be checking the official guidance, speaking to the relevant organisation, or getting qualified advice before acting.

BAC edge cases

A drink poured at home may contain more alcohol than a standard serving.
BAC can continue rising after the last drink while alcohol is still being absorbed.
The legal drink-drive limit is not the same in every UK jurisdiction.
Alcohol impairment can occur below legal limits.
BAC can keep rising after the last drink.
Legal limits differ by jurisdiction.
Medication, food, illness, and fatigue are not modelled.

Limitations

This calculator is general information only and is not medical, legal, or driving advice. Do not use it to decide whether to drive, operate machinery, or assess legal compliance. This is general information only and is not medical, legal, or driving advice. The calculator is designed to support understanding and planning, but it cannot verify documents, predict future rule changes, or account for every exception. Use it as an estimate and check the official source before acting where the result matters.

Check GOV.UK drink-drive guidance and local law where relevant.
Only proper testing can measure actual BAC.
If there is any doubt, do not drive.
Check GOV.UK drink-drive limit guidance for current rules, rates, definitions, and eligibility where relevant.
Do not rely on a single scenario where income, costs, dates, rates, usage, or health circumstances may change.
Keep records of the inputs used so that the estimate can be reviewed later.

Frequently asked questions

Can this prove I am under the legal limit?

No. It is an estimate and cannot prove legal status or safety.

Why does time reduce the estimate?

The calculator subtracts a fixed metabolism estimate, but real metabolism varies.

Does food change BAC?

Food can slow absorption, but the calculator does not model meals or absorption timing in detail.

Why is the result rounded to three decimals?

BAC estimates are conventionally shown with small decimals, but rounding should not imply precision.

Can coffee or a shower reduce BAC?

No. Time is the main factor; this calculator does not model quick fixes because they do not remove alcohol from blood.

Does the UK use BAC percentage?

UK law is usually expressed in legal limits for breath, blood, or urine, so check official guidance.

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What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.

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