How the pack year calculation works

What is a pack year?

A pack year is 20 cigarettes smoked every day for one year. That is 7,300 cigarettes (20 × 365). Ten a day for two years, or 40 a day for six months, is also one pack year.

What is the formula?

For every period entered, the calculator works through the same steps:

1. It converts the amount into cigarettes, using the figures in the table below. Cigarettes count as they are.
2. For amounts entered per week (loose tobacco and waterpipe sessions), it divides by 7 to give a daily figure.
3. It multiplies cigarettes a day by the number of years in that period.
4. It adds every period of every type together and divides the total by 20.
5. It rounds the answer to the nearest whole number.

What does each type of tobacco count as?

Where do the figures come from?

The figures for cigars, cigarillos, pipes and loose tobacco are conversion figures taught at the National Respiratory Training Centre at Warwick, England (now Education for Health). The References page says the original work behind them could not be found, but because large numbers of practice nurses had been trained using them, the authors decided to keep using them.

Two further sources were used to check that these figures were close to those of others working in this area: Wood DM et al, “‘Pack year’ smoking histories: what about patients who use loose tobacco?”, Tobacco Control 2005;14(2):141-142, which found that 12.5 g or half an ounce of loose tobacco was approximately equivalent to one packet of 20 cigarettes, and the 2003 Australian Gulf War veterans’ health study.

Spliffs, waterpipes and bidis have their own sources, shown in the table and explained on each type’s page.

How does a worked example add up?

Take someone who smoked 20 cigarettes a day for 20 years, then 10 a day for 5 years, and who also used 50 g of loose tobacco a week for 10 years.

592.86 ÷ 20 = 29.64. The calculator shows 30 pack years. Its breakdown shows each period in pack years: 20, 2.5 and 7.1.

How accurate is it?

It is an estimate. The References page says: “This is not an exact science and approximations have been made.” Each type of tobacco is counted with one figure for everyone, and FAQ 8 points out that the calculation also relies on an accurate history.

Pack years can also make different histories look the same. FAQ 2 notes that 5 cigarettes a day for 20 years and 20 a day for 5 years are both 5 pack years, yet the risk of smoking-related ill health is very much greater in the first case. FAQ 2 calls the measure “only a crude imprecise description of smoking related exposure” and recommends recording starting and stopping dates and years smoked as well. The calculator’s optional history step asks whether the person still smokes, when they stopped and how many years they smoked in total.

The calculator records combustible smoking products only, so vaping is not counted, and it does not record passive smoking (see the FAQs). Pack years are a record of past smoking, not a diagnosis. If you are worried about your health, speak to your GP practice or use NHS 111.