Calculate standard drinks, estimate BAC progression, and understand alcohol metabolism timing.
2026-05-24
Note: These are estimates (±20% variation normal). Never use to decide if safe to drive—always use designated driver, taxi, or public transport.
What is a standard drink in NZ?
One standard drink contains exactly 10ml of pure alcohol (ethanol), equivalent to a 330ml beer (5%), 150ml wine glass (13%), or 40ml spirits (40%).
Why does this calculator need drinking duration?
BAC depends on when you drank, not just what. Peak BAC occurs ~30min after finishing drinks. Your body eliminates ~0.015% BAC per hour, but the exact timeline depends on your weight, gender, food, liver function—varies ±20% individually. This calculator is an estimate, not a guarantee.
What is BAC and what are safe limits?
Blood alcohol concentration (BAC) is the percentage of alcohol in your blood. NZ legal driving limit is 0.05%. Impairment affects judgment/coordination well below this. Never drive after drinking alcohol.
How much alcohol does your body process per hour?
Average: ~0.015% BAC per hour (roughly 1 standard drink/hour). This varies by body weight, gender, food, liver health, and genetics. Food intake can reduce absorption significantly.
Per drink
Total drinks consumed
How long you spent drinking (e.g., 60 = 1 hour)
0 = just finished drinking, 60 = 1 hour ago
Total Standard Drinks
1.65
(1 × 1.65 per drink)
Total Pure Alcohol
13.0g
Peak BAC
27.350%
Occurs ~1h 30m after starting
Current BAC
27.350%
✗ Above legal limit (0.05%)
Time to Milestones
BAC is the percentage of pure alcohol in your bloodstream. It's measured as grams of alcohol per 100ml of blood. The NZ legal driving limit is 0.05% for drivers over 20 years old (0.03% for drivers under 20). Importantly, impairment occurs well below the legal limit: at 0.02-0.03%, judgment and vision are already affected; at 0.04-0.06%, reaction time and muscle coordination decline significantly. BAC is not directly proportional to drinks consumed—body weight, gender, food intake, drinking speed, and metabolism all matter significantly.
This calculator uses the Widmark formula, a well-established model for estimating BAC from alcohol consumption and personal factors:
Peak BAC occurs approximately 30 minutes after you finish drinking all your drinks (accounting for ~15-30min absorption lag). This calculator shows when peak BAC would occur (e.g., if you drink 3 drinks over 1 hour, peak BAC is ~1.5 hours from when you started). Current BAC is calculated as peak BAC minus alcohol eliminated through metabolism over the hours since drinking stopped.
The Widmark formula is an average model. Real-world BAC varies significantly due to factors this calculator cannot measure:
Conclusion: This calculator provides an average estimate based on typical conditions. Your actual BAC could easily be 15-20% higher or lower. Never use these estimates to decide if it's safe to drive. The safest choice is always: If you've consumed any alcohol, use a taxi, designated driver, or public transport.
The body metabolizes approximately 0.015% BAC per hour on average. This translates to roughly one standard drink (10ml pure alcohol) per hour, but this is an approximation. Some people metabolize faster (up to 0.02%/hour), others slower (down to 0.01%/hour). You cannot speed up metabolism by drinking water, coffee, or exercising—your liver processes alcohol at its own rate. Alcohol remains in your system for hours after BAC drops below 0.05%: breath tests detect it for 12-24 hours, urine tests for 24-48 hours, and blood tests for up to 48 hours.
BAC depends on when you drank, not just how much. If you drank 3 drinks over 3 hours with food, your peak BAC will be lower than drinking 3 drinks in 30 minutes on an empty stomach—because your body was metabolizing while you were still drinking. This calculator accounts for this by showing peak BAC (worst-case impairment) and current BAC (accounting for hours of metabolism). The drinking duration input is crucial for accuracy.
Scenario 1: Rapid Friday Night (5 drinks in 90 minutes)
75kg male: 3 pints of beer (570ml × 5%) plus 2 vodka shots (40ml × 40% each) over 1.5 hours.
⚠️ Peak 0.107% is well above legal limit (0.05%), representing severe impairment. Even 3 hours later at 0.052%, still above legal limit. Not safe to drive for ~6+ hours total. This scenario shows how rapid drinking creates dangerous peak BAC.
Scenario 2: Slow Sunday Session (3 drinks over 3 hours with food)
65kg woman: 3 glasses of wine (150ml × 13% each) consumed slowly with dinner over 3 hours. Ate well before and during.
✓ Slow drinking + food reduces peak BAC to 0.063% (still above legal limit of 0.05%, but much lower than rapid scenario). After 2 more hours at 0.033%, below legal limit. Slower pace + food significantly mitigates peak impairment. Note: Food cannot be accounted for by this calculator—actual BAC may be 10-20% lower.
Scenario 3: After-Work Single Beer (Check Timing)
70kg male: 1 standard beer (330ml, 5%) consumed quickly. Started drinking now (0 minutes since drinking finished).
✓ Single beer yields peak 0.027% (well below legal limit). Safe to drive after ~2 hours. Recovery time: ~1.5-2 hours to full clearance. This demonstrates a low-risk scenario, though mild impairment affects judgment even at 0.027%.
Why does drinking duration matter for BAC?
BAC is the result of absorption minus elimination. If you drink 5 beers in 30 minutes, your body hasn't had time to metabolize and peak BAC is very high. If you drink the same 5 beers over 5 hours, your body metabolizes ~1 drink per hour as you're drinking, so peak BAC is much lower. Drinking duration directly affects peak impairment.
What is peak BAC and when does it occur?
Peak BAC is the highest blood alcohol level you reach during a drinking episode, representing worst-case impairment. It typically occurs 15-30 minutes after your last drink (accounting for absorption lag). This calculator estimates peak BAC occurs ~30 min after your drinking session ends. Peak BAC determines whether you've crossed legal/safety thresholds, even if your current BAC is lower.
Why is this estimate accurate to ±20%?
The Widmark formula uses average constants for gender and elimination rate, but real people vary significantly. Food slows absorption by 20-30%; lack of food accelerates it. Liver function varies by health/genetics; some people metabolize 30% faster than average. Medications can inhibit metabolism. The formula cannot account for these, so ±20% variation is normal and expected.
Can I speed up alcohol metabolism with water or exercise?
No. Your liver processes alcohol at a fixed rate (~0.015% BAC/hour on average), and nothing speeds this up. Water hydration is good for general health and may reduce hangover severity, but it does not increase alcohol elimination. Exercise, coffee, and cold showers do not speed metabolism—only time works.
Why do women have higher BAC than men from the same drinks?
Women typically have lower body water percentage than men (55-60% vs. 60-65%), so the same amount of alcohol is distributed in less water, resulting in higher concentration. Additionally, women's bodies produce less gastric alcohol dehydrogenase (enzyme that breaks down alcohol before absorption), so more alcohol reaches the bloodstream. These factors combine to create higher BAC and often longer duration of effects.
Does weight affect BAC calculation?
Yes, significantly. Body weight is in the denominator of the Widmark formula: a 50kg person has double the BAC of a 100kg person from identical consumption. However, it's specifically body *water* that matters, not total weight. Muscle has higher water content than fat, so two people of the same weight may have different BAC if their body composition differs. This calculator uses weight as a proxy.
What is NZ's legal driving limit and when is it safe?
Legal limit: 0.05% for drivers over 20 years old, 0.03% for drivers under 20. However, legal ≠ safe. Impairment affects judgment at 0.01-0.02%, coordination at 0.03-0.05%, and severe impairment occurs above 0.08%. The safest choice: don't drive after any alcohol consumption. Use a taxi, designated driver, or public transport. These estimates cannot guarantee safety for driving.
How do I know if I'm 'too impaired to drive'?
If you've consumed alcohol at all, do not drive. There's no safe threshold. Impairment begins well below legal limits and affects judgment/coordination in ways you may not notice. This calculator cannot assess your individual impairment. The only safe approach: if you've consumed alcohol in the last few hours, use alternative transport. Period.
How does food affect BAC?
Food significantly slows alcohol absorption. Peak BAC on a full stomach can be 20-30% lower than on an empty stomach because alcohol reaches the bloodstream more gradually. Fatty foods are most effective at slowing absorption. This calculator cannot account for food, so if you drank with a meal, your actual BAC is likely 10-20% lower than the estimate. Eating before/during drinking is always recommended for harm reduction.
Why do different countries use different 'standard drinks'?
Historically, countries chose standard drink sizes based on local drinking cultures and preferences. NZ & Australia: 10ml pure alcohol. UK: 8ml (smaller). USA: 14ml (larger). Canada: 13.6ml. There's no 'correct' size—they're just reference points. When reading health guidelines from other countries, convert to NZ standards to compare (1 US standard drink ≈ 1.4 NZ standard drinks).
What tolerance and why doesn't it matter for this calculator?
Tolerance is when regular drinkers 'feel' less drunk at the same BAC because their brain adapts. However, impairment is unchanged—your judgment, coordination, and reaction time are equally affected at 0.08% whether you're a heavy drinker or not. Tolerance is psychological; the danger is the same. This calculator ignores tolerance (correctly) because BAC impairment is objective.
NZ safe drinking guidelines: what are they?
Ministry of Health: max 3 standard drinks/day for women, 4 for men. Long-term: max 15/week for women, 20/week for men. Pregnant people and under-15s should not drink. These limits are based on health risks (liver, cancer, heart disease). Exceeding them increases health risks significantly. This calculator tracks standard drinks to help monitor consumption against these benchmarks (see total standard drinks count).