Estimate your current BAC from body weight, sex, drinks, and time, then compare it with common thresholds and rough wait times.
How to use our BAC Calculator
- Choose Sex used for body-water constant, Weight unit, and enter Body weight and Time since first drink (hours).
- Pick Drink entry mode. In preset mode, enter Beer drinks (12 fl oz at 5% ABV), Wine drinks (5 fl oz at 12% ABV), and Spirits drinks (1.5 fl oz at 40% ABV). In custom mode, enter Number of custom drinks, Custom drink size (fl oz), and Custom drink alcohol by volume (percent).
- Leave Alcohol elimination rate (BAC % per hour) at 0.015 unless you want a sensitivity check using a different estimate.
- Click Calculate, then first review Pure alcohol consumed and US standard drinks. If those look too high or too low for what you had, fix the drink entries before trusting Estimated current BAC.
- Use Estimated current BAC, Estimated impairment range, Compared with 0.08% driving limit, and the two time outputs as rough planning estimates only if no more alcohol is consumed.
Definitions
Estimated current BAC: Your estimated blood alcohol concentration now, shown as a percent of alcohol in the blood.
Estimated impairment range: A plain-language caution band based on the current BAC estimate.
Compared with 0.08% driving limit: A simple comparison with the common US adult driving limit. Laws can differ by situation.
Pure alcohol consumed: The total grams of ethanol from all drinks entered.
US standard drinks: Total alcohol divided by 14 grams per drink [1].
Estimated peak BAC before time reduction: The BAC estimate before subtracting alcohol eliminated over time.
Alcohol elimination rate (BAC % per hour): The estimated amount BAC drops each hour in the formula.
Sex used for body-water constant: The Widmark method choice that sets the body-water constant used in the estimate.
Common mistakes and quick fixes
Mistake: Entering Time since first drink (hours) from the last drink instead of the first one.
Fix: Count from when drinking started, because the calculator subtracts alcohol over the full time window.
Mistake: Using preset counts in Drink entry mode set to custom, or filling custom fields while preset mode is selected.
Fix: Use only the fields for the selected Drink entry mode ; the hidden mode's drink inputs are ignored.
Mistake: Typing bottle size, can pack size, or total container size into Custom drink size (fl oz) when it does not match one drink.
Fix: Enter the size of one actual drink and use Number of custom drinks for how many you had.
Mistake: Confusing proof with ABV in Custom drink alcohol by volume (percent) .
Fix: Enter ABV percent, not proof. In the US, proof is about double the ABV, so 80 proof is about 40% ABV.
Mistake: Ignoring Pure alcohol consumed and US standard drinks and jumping straight to Estimated current BAC .
Fix: Check those two outputs first to catch wrong drink sizes, counts, or strength before using the BAC estimate.
Mistake: Changing Alcohol elimination rate (BAC % per hour) to 0 or a negative number.
Fix: Enter a value greater than 0, or keep the default 0.015 so the time-to-threshold results can be calculated.
Limitations & Key Assumptions / Boundary Conditions
- This tool gives an educational estimate, not a direct measurement. Real BAC can differ because of food, drinking pace, medications, health, body composition, and individual alcohol processing.
- The formula uses fixed Widmark body-water constants: 0.68 for male and 0.55 for female. These are population averages, not exact personal values.
- Time since first drink (hours) assumes alcohol was available for absorption over that broad period. Very fast drinking or drinks taken much later in the period can make the estimate less realistic.
- Alcohol elimination rate (BAC % per hour) is only a rough assumption. Changing it can meaningfully change Time until below 0.08% and Time until 0.00%.
- In preset mode, standard drink examples are used. In custom mode, accuracy depends on entering the correct Custom drink size (fl oz) and Custom drink alcohol by volume (percent).
- If the math gives a negative current BAC, the calculator shows 0.00% because BAC cannot be below zero, but Estimated peak BAC before time reduction may still be above zero.
- The 0.08% comparison is general US context only. It does not account for stricter rules for underage drivers, commercial drivers, probation terms, or state-specific laws.
Methodology
How the estimate is calculated
The calculator first converts the drinks you entered into grams of pure alcohol. It then estimates peak BAC with the Widmark method and subtracts alcohol eliminated over time.
ethanol_g = volume_mL x (abv_percent / 100) x 0.789
Here, 0.789 g/mL is the density of ethanol. For custom drinks, fluid ounces are converted to milliliters with 29.5735 mL per US fl oz.
standard_drinks_us = total_ethanol_g / 14
This converts total ethanol into US standard drinks [2].
bac_peak_percent = (total_ethanol_g / (weight_g x r)) x 100
The body-water constant r is 0.68 for male and 0.55 for female in this calculator.
bac_current_percent = bac_peak_percent - elim_rate x hours
If this result is below zero, the displayed Estimated current BAC is set to 0.00% because BAC cannot be negative.
time_to_target_h = max(0, (bac_current_percent - target_bac_percent) / elim_rate)
This is used for both Time until below 0.08% and Time until 0.00%. If you are already below a target, the time shown is 0 hours.
Preset drink values used
Quick preset drinks use these standard entries:
beer = 12 fl oz at 5% ABV, wine = 5 fl oz at 12% ABV, and spirits = 1.5 fl oz at 40% ABV. Those examples are close to one US standard drink each [1].
Mini example
Suppose a male weighing 180 lb enters 2 beers, 0 wine, 0 spirits, 2.5 hours since first drink, and an elimination rate of 0.015.
total_ethanol_g = 28 g
standard_drinks_us = 28 / 14 = 2
weight_g = 180 x 0.45359237 x 1000 = 81646.6 g
bac_peak_percent = (28 / (81646.6 x 0.68)) x 100 = 0.0504%
bac_current_percent = 0.0504 - (0.015 x 2.5) = 0.0129%
time_to_0.00 = 0.0129 / 0.015 = 0.86 hours
So the estimate is about 0.0129% now, with about 0.86 hours to reach 0.00% if no more alcohol is consumed.
Assumptions that affect results
This method assumes a fixed body-water constant and a steady elimination rate. Real BAC can differ from the estimate because BAC depends on more than weight, sex constant, drink amount, and time [2]. Use the result as a rough educational estimate, not a safety or legal decision tool.