How to Calculate Blood Alcohol Level

Co-authored by wikiHow Staff

Last Updated: April 5, 2021 References

Blood Alcohol Content, or BAC, is a measure of the ratio of alcohol in your blood. You can calculate your BAC a number of ways, but getting an absolute, accurate measurement is impossible without a blood test. Use a BAC chart or online calculator to give you a reasonable estimate of blood alcohol level. Because there are so many variables which contribute to your BAC, it's only possible to get an estimate without a professional test. Using your knowledge of your weight, the amount of alcohol you've consumed, and time can help you calculate your BAC and estimate when it's legally safe to drive.

Part 1

Part 1 of 3:

Calculating Your BAC Estimate

1
Understand what blood alcohol content (BAC) means. Blood alcohol content is a measurement of the amount of alcohol in your body. So, a BAC of 0.10% means 1 part alcohol for every 1000 parts of blood. It is often used for legal or medical purposes, like when determining if you've had too much to drink and drive. ^{[1]
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- A BAC of 0.08%, often the legal level, indicates that you have 80mg of alcohol per 100ml of blood in your body.
- Police often use a breathalyzer to test your BAC if you get pulled over under suspicion for driving while intoxicated. You can also get your own breathalyzer to make sure you are within the legal limit.
- If you don't have a breathalyzer handy, use a BAC chart or online calculator to work out when you are safe to drive. Calculating your BAC without a breathalyzer will require you to keep closer track of how much alcohol you consume.
2
Understand how different types of alcohol will affect your BAC. Different drinks have different percentages of alcohol content, and the size of a "standard" drink differs, as well. So a standard drink of beer is not only a different amount than a drink of whiskey, it also has a different alcohol content.^{[2]
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- One standard drink is considered one 12 fl oz beer; one 5 fl oz glass of wine; one 1.5 fl oz shot of hard liquor. Overall, a standard drink is a drink which contains 14 grams (0.49 oz) of pure alcohol. ^{[3]
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- Beer is usually 4% – 6% alcohol. This varies depending on the type of beer, so check the label to be sure. Some imports, malt liquors, and craft beers are stronger (8% – 12% or more).
- One standard drink of wine ( white, red, rose, and champagne or sparkling wine) is measured as 5 ounces.The average alcohol content of a 5-ounce glass of wine is around 12%.
- One standard drink of 80 proof hard liquor is measured as 1.5 ounces. One shot of liquor typically contains 40% alcohol. Be aware that some drinks are made with even stronger potency, like 151 proof rum or grain alcohol. Thus, your BAC may rise more quickly when drinking hard liquor in place of beer or wine.
- Some mixed drinks contain more than one type of alcohol and will contain more alcohol than a standard drink.
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3
Weigh yourself and factor in your gender and age. Your BAC depends on your gender, age, and your weight. These elements contribute to how quickly your body will metabolize alcohol.
- Alcohol is absorbed differently by men and women. Men's bodies typically contain more body water (61% vs 52%) and will be able to dilute the alcohol more.
- A 165-pound man who has had four standard beers in one hour will have an estimated BAC of 0.082%. If his 130-pound female friend decides to match him drink-for-drink during that hour, she would have a BAC of about 0.123%.
- Know your current weight. Your weight will affect what your estimated BAC is because typically if you are hydrated, the more you weigh the more water your body will have to dilute alcohol. This can slow absorption.
- Note that alcohol is absorbed through muscle tissue and not fat. The more body fat you have, the higher your BAC will be. So, a man who weighs 180 pounds but is in good shape will typically have a lower BAC than a man weighing 180 pounds who is out of shape.
- Your age will also have a factor. As we age our metabolism slows down and our bodies may not process alcohol as quickly.
4
Track your drinking. To calculate your estimated BAC, not only track how many drinks you've had, but what you've been drinking, and how fast you've been drinking.
- Whether you're serving yourself or drinking at a bar, the chances of you consuming a standard drink are lowered. Unless you are drinking from a can or pint glass, you're not always poured a standard drink with exact proportions. Oftentimes bartenders will eyeball the amount of liquor or wine that gets poured. Therefore, it's a good idea to count your drinks but round up to be safe.
- Notice how fast you are consuming alcohol. The faster you drink, the faster your BAC will rise. Because your body can only process a certain amount of alcohol at a time, the more you have in your stomach and bloodstream, the higher the BAC.
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Use your information to estimate your BAC. The most accurate way to calculate your BAC is by drawing blood. Other methods will give you a good estimate of where your BAC is, but shouldn't be relied on to be completely accurate.
- Use a Estimated BAC Charts to find where you are at. You can look up one online or print one out for wallet.
- The truth is that it's very hard to get an accurate measuring of your BAC without a breathalyzer, blood draw, or complicated math. Because everyone's bodies are different, variables such as food consumption, body type, hydration, fatigue, other ingredients in your drinks, etc., make it difficult to guess.
- You can also use an online BAC calculator to give you an estimate. If you've kept track of your intake and know your weight, this will give you a reasonable estimate.

Part 2

Part 2 of 3:

Considering Other Factors that Affect BAC

1
Keep tabs on the time. Time will have an effect on your BAC. Your BAC won't lower faster if you space out your drinking. But it also won't be as high as if you drank several drinks in a short time period. However, as your body processes the alcohol over a period of time, your BAC will lower.
- For example, if you have three 12 fl oz beer drinks in one hour, are male, and weigh around 200 pounds, your estimated BAC is roughly around 0.044%. If you were to space those same four drinks out over three hours, your BAC would be around 0.010%. ^{[4]
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- As for sobering up, the general rule is to subtract 0.015% from your total BAC for each hour that passes since your last drink. This is not, however, exact. If you are unsure, err on the side of caution and call a cab if you're on your way home for the night.
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Be aware of the other factors that affect BAC. ^{[5]
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Research source} In addition to age and gender, your BAC can be affected by the following BAC variables:^{[6]
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- Body type
- Fat/muscle content
- Metabolism
- Emotional state
- Diabetes
- Alcohol intolerance
- Hereditary factors
- Other ingredients. Water and juice mixed with alcohol may slow absorption for a lower BAC. Carbonated beverages like tonic water and energy drinks speed up absorption for a higher BAC.
- Most online BAC charts and calculators assume that you are not drinking on an empty stomach or have any prevailing conditions that would further affect your BAC. These variables are why it is so hard to calculate your BAC without a professional test or breathalyzer.
3
Understand that certain things don't affect your BAC. While some people believe that certain substances, foods, or actions can help you sober up, many of these techniques don't actually affect your BAC.
- Note that drinking a glass of water or coffee does not dilute the blood and sober you up. Water can slow the absorption of new alcohol but cannot reduce the effects of alcohol already in your blood. Caffeine may make you feel more awake but doesn't have any effect on your BAC.
- Food will only prevent your BAC from rising quickly if it's already in your system before consuming alcohol.
- The type of alcohol you consume won't affect your BAC. One type of alcohol doesn't affect your BAC more than another. Your BAC is affected by the quantity of alcohol, not the type.
- Your tolerance doesn't affect your BAC, just how you feel. BAC is measured by the quantity of alcohol in your blood. Even if you don't feel drunk, you may be over the legal limit.
4
Don't rely on only one method for calculating your BAC. Using breathalyzers and calculating your BAC by keeping track of alcohol consumption over time, your weight, and reference charts are great ways to stay responsible and get an estimate. However, relying on only one method to calculate BAC, or using any of the methods as a concrete calculation is not advised.
- Learn to judge your level of sobriety by how you feel, but don't rely on it. Signs of impairment vary with the individual. Even at a BAC level of 0.02%, you may experience some loss in judgment, an altered mood, and a decline in your ability to multitask.
- If you think you might be too intoxicated to drive, then play it safe and don't.
- Keep a BAC chart on your smartphone or in your wallet to use as a reference.

Part 3

Part 3 of 3:

Practicing Responsible Drinking Habits

1
Always drink responsibly. Learn to pace your drinks and drink wisely; focus most on enjoying your time among friends. That way you can keep yourself, and anyone around you safe and sound.
- Alternate between alcoholic and non-alcoholic drinks. A glass of water or club soda and lime won't sober you up faster, but will help to keep you BAC at a manageable level.
- Eat before you drink. Don't drink on an empty stomach as this will not only allow alcohol to be absorbed faster but can make you sick.
- Stay hydrated. Alcohol dehydrates your body and can cause you to feel hungover the next day.
2
Plan ahead. If you're going somewhere to drink make sure that you plan a safe way to get home. The more intoxicated you are the more likely you are to make bad decisions.
- Assign a designated driver or ask a friend to pick you up at a certain time or when you call.
- Use a taxi, Uber, or Lyft. If you know that you'll be too intoxicated to drive, plan to take a cab home and budget for it.
- Make arrangements to stay at a friend's place.
- Leave your car keys at home or with someone you trust.
3
Have backup travel plans if you go over the legal limit. If you do drive somewhere and drink too much, or find yourself in an uncomfortable situation, it's good to have a backup plan.
- Have a friend or family member you know you can call to pick you up.
- Keep extra cash on you in a place separate from the rest of your money. This extra cash can be used to get you a cab if you need. By separating it, you prevent yourself from spending it.
4
Use your estimated BAC as a reference guide for impairment. A DUI or accident is a horrible, expensive experience. Learn to enjoy drinking less while socializing more.
- A BAC chart, calculator, or breathalyzer are all good tools to help you stay safe when drinking. However, it's impossible to calculate your absolute BAC without a blood test. If you feel like you're in danger of over drinking or unable to drive, don't.
- Learn to read your own stages of inebriation, and learn your limits. It's not always easy to cut yourself off. Ask your friends to look out for you, stay hydrated, and take it slow.

Warnings

Be extra vigilant during the holiday season and on celebratory occasions.

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If you miscalculate your BAC, you may be over the drink drive limit. It's best not to drink and drive at all.

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