Health & medical New Zealand
Alcohol Limit Calculator NZ
New Zealand measures the driving limit two ways — micrograms per litre of breath and milligrams per 100 millilitres of blood — and applies a zero limit to drivers under 20. This estimates where a reading would sit against both.
Free, no signup Runs in your browser
Widmark's distribution ratio
One standard drink = 10 g of alcohol
Estimated blood alcohol
—Breath equivalent
—
Alcohol consumed
—
Hours to reach zero
—
Hours to your limit
—
Never use this to decide whether to drive. This is an estimate from a population formula and cannot tell you whether you are under the legal limit or safe to drive. Individual results vary widely with food, hydration, medication, fatigue and liver function, and the only measurement with legal standing is one taken by Police. Treat any result near a limit as over it, and do not use this page to decide whether to drive.
What is the legal alcohol limit for driving in NZ?
For drivers aged 20 and over, 250 micrograms of alcohol per litre of breath or 50 milligrams per 100 millilitres of blood. Drivers under 20 have a zero limit — 0 micrograms of breath alcohol and 0 milligrams of blood alcohol. Both figures describe the same threshold measured two different ways.
Two limits, one threshold
New Zealand expresses the driving limit in two units because roadside and evidential testing use two different methods.
Breath alcohol is measured in micrograms per litre of breath, and the limit for drivers 20 and over is 250 mcg/L. That is the figure a roadside device reports.
Blood alcohol is measured in milligrams per 100 millilitres, and the limit is 50 mg. That is what an evidential blood sample returns.
The two are set to describe the same level of impairment, which is why a driver near one is near the other. Neither is more lenient than the other.
The under-20 zero limit
Drivers under 20 have a zero alcohol limit in New Zealand. Not a lower limit — zero.
That means any detectable alcohol is an offence, and there is no allowance for a single drink, a drink several hours earlier, or a reading the model here would round to nothing. It applies regardless of licence stage.
The practical effect is that the calculation above stops being useful for an under-20 driver almost immediately. What matters is how long until the reading reaches zero, which is why that figure is shown separately.
Zero-limit categories also apply to some commercial driving. If you drive for work, check the conditions attached to your licence rather than assuming the general limit applies.
How this is calculated
The Widmark equation, then a conversion into the breath figure New Zealand roadside testing actually reports.
grams of alcohol = standard drinks × 10blood alcohol % = grams ÷ (body mass in grams × r) × 100r = 0.68 male, 0.55 femalecurrent = peak − (0.015 × hours elapsed)mg per 100 mL = blood alcohol % × 1000mcg per litre of breath ≈ mg per 100 mL × 5because the NZ limits of 50 mg and 250 mcg describe the same threshold
The breath-to-blood relationship is a legal convention rather than a physical constant. New Zealand's two limits — 250 micrograms per litre of breath and 50 milligrams per 100 millilitres of blood — are defined to describe the same threshold, which gives the factor of five used above.
An individual's actual breath-to-blood ratio varies with breathing pattern, temperature and how recently they drank. That is one reason an evidential blood test exists as an option alongside the breath test, and another reason this estimate should not be treated as a measurement.
A worked example: four standard drinks over two hours
A 75 kilogram driver aged over 20, four standard drinks, two hours after starting.
- Alcohol consumed — 4 × 10 g40 g
- Distribution volume — 75,000 × 0.6851,000 g
- Peak — 40 ÷ 51,000 × 1000.078%
- Less 2 hours of elimination−0.030
- Estimated blood alcohol0.048% = 48 mg
- Breath equivalent — × 5≈240 mcg/L
About 48 mg per 100 mL, or roughly 240 mcg per litre of breath — against limits of 50 and 250.
Two per cent under the limit, from a model whose own error band is far wider than two per cent. That is not a pass; it is a number that cannot distinguish over from under.
Why the morning after is the common offence
Alcohol clears at roughly 0.015 per hour of blood alcohol concentration, and nothing changes that rate. Not coffee, not food, not a shower, not sleep — those change how alert someone feels, which is a different thing entirely.
A driver who peaked at 0.15 needs about ten hours to reach zero. Stopping at 2 am puts that at midday. Feeling fine by 8 am is not evidence of a clear reading, and it is exactly why morning-after stops catch people who genuinely believed they had waited long enough.
If the figure above says several hours to zero, the honest reading is that the following morning is not automatically clear either.
What this calculator does not do
- A population average — individual variation is large in every direction
- Assumes instant absorption; food and drinking pace change the real curve
- Standard drink counts are usually underestimated, especially with home pours
- The breath conversion is a legal convention, not an individual physiological ratio
- Has no legal standing — only a Police test does
Common questions
What is the legal alcohol limit in NZ?
250 micrograms per litre of breath, or 50 milligrams per 100 millilitres of blood, for drivers aged 20 and over. Drivers under 20 have a zero limit.
Is New Zealand zero tolerance for drivers under 20?
Yes. Any detectable alcohol is an offence for a driver under 20, regardless of licence stage. There is no allowance for a single drink.
How many standard drinks can I have before driving?
There is no safe answer, which is why this page does not give one. The margin of error in any estimate is wider than the gap between under and over.
How long does it take for alcohol to leave your system?
About 0.015 of blood alcohol concentration per hour. A peak of 0.15 takes roughly ten hours to clear, and nothing speeds it up.
What is the difference between the breath and blood limits?
They are the same threshold measured two ways. 250 mcg/L of breath and 50 mg per 100 mL of blood are set to correspond, so a driver near one is near the other.
Where these numbers come from
- NZ Transport Agency Waka Kotahi — alcohol and driving Breath and blood alcohol limits, and the under-20 zero limit
- Health New Zealand — standard drinks One standard drink contains 10 g of alcohol
- Widmark equation — standard forensic and clinical method Distribution ratio and elimination rate
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
Related NZ calculators
kJ to calories
Kilojoules to calories and back, against the NZ label reference intake.
Open calculator Business & datesWorking days
Working days with NZ public holidays, Mondayisation and regional anniversary days.
Open calculator Power & utilitiesPower cost
What an appliance costs to run, at the rate on your own bill.
Open calculatorNew Zealand · Health & medical