BAC Calculator
Estimate blood alcohol content based on drinks, weight, and time. Never drive impaired.
Understanding the Calculation
Percentage calculations involve:
- Base Value: The original or total amount
- Percentage Rate: The proportion expressed as a percent
- Result: The calculated value based on your inputs
Common Formulas
- Finding a percentage: (Part ÷ Whole) × 100 = %
- Finding the part: Whole × (% ÷ 100) = Part
- Percentage change: ((New - Old) ÷ Old) × 100 = % Change
Practical Applications
Use percentage calculations for:
- Financial planning and budgeting
- Academic performance tracking
- Business metrics and KPIs
- Health and fitness goals
- Shopping and discount calculations
How Blood Alcohol Concentration Is Estimated
The Widmark formula estimates BAC from drinks, body weight and elapsed time:
``
BAC = (standard drinks × 5.14 ÷ (weight in lb × r)) − (0.015 × hours)
`
where r is 0.73 for men and 0.66 for women, reflecting differences in body water.
| BAC | Typical effects |
|---|---|
| 0.02% | Mild relaxation; some loss of judgement |
| 0.05% | Impaired coordination; legal limit in much of Europe |
| 0.08% | US legal limit for driving |
| 0.15% | Major impairment; vomiting likely |
| 0.30%+ | Loss of consciousness; potentially fatal |
A US standard drink is 14 g of pure alcohol: 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of
40% spirits. Restaurant pours routinely exceed these.Any estimate is unreliable for deciding whether to drive. Food, medication, sex, body
composition, drinking rate and individual metabolism all move the result, and elimination is
roughly 0.015%/hour with nothing — coffee, cold showers, food — able to speed it up. The only
safe number before driving is zero.
The Four Percentage Questions
Almost every percentage problem is one of these, and mixing them up is the most common
arithmetic error there is:
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × (X ÷ 100) | 15% of 80 = 12 |
| X is what % of Y? | (X ÷ Y) × 100 | 12 is 15% of 80 |
| X is Y% of what? | X ÷ (Y ÷ 100) | 12 is 15% of 80 |
| % change from X to Y | ((Y − X) ÷ X) × 100 | 80 → 92 is +15% |
Percentage Points Are Not Percent
A rate moving from 4% to 5% is one percentage point, and a 25% increase. Both
statements are true and they are not interchangeable — headlines routinely use the smaller
one when reporting a rise and the larger one when reporting a fall.
Increases and Decreases Do Not Cancel
| Start | Change | Result |
|---|---|---|
| 100 | +50%, then −50% | 75 |
| 100 | −50%, then +50% | 75 |
| 100 | −20%, then +25% | 100 |
The reason is that the second percentage applies to a different base. To reverse a −20% you
need +25%, not +20%. This is why a stock that falls 50% must double to break even, and why
"we cut costs 30% then grew 30%" leaves you 9% down.Reversing a Percentage
To find the price before a 20% discount, divide by 0.8 — do not add 20%:
`
before = after ÷ (1 − discount)
$80 ÷ 0.8 = $100 ✓
$80 × 1.2 = $96 ✗
``
The same asymmetry applies to tax, tips, markups and platform fees.