You cannot calculate your blood alcohol level because it depends on a large number of variables such as your age, weight, gender, general health and fitness, the amount of alcohol you've drunk, the type of alcohol you've drunk, and whether you've eaten (how much and how recently).
A small drink could put you over the limit. If you're stopped and your breath alcohol reading is between 30-150mcg per litre of breath you could be fined and given 50 demerit points. That means you are half way to being disqualified. You will receive a notice from NZTA that you have accumulated 50 demerit points and if this total exceeds 100 within a 2-year period you will be disqualified.
If you are caught with more than 150mcg of alcohol per litre of blood then you could be fined up to $2250 and sent to prison for up to 3 months and be disqualified from driving for at least 3 months, plus you'll still get 50 demerit points. If you are disqualified you may be able to apply for a special licence with zero alcohol or an alcohol interlock requirement.
If you blow over the limit you may be asked to provide an evidential blood or breath test. It is an offence to refuse consent.
Younger drivers tend to have less ability to cope with alcohol intake. Add to that their lack of experience and you can understand why the risk of having an accident is increased dramatically.
Read more about alcohol limits, including fines and penalties, in this article.