Smoking, the No. 1 cause of preventable death in the United States, causes 130,000 lung cancer deaths each year.

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How long and how much you smoked, and how long it's been since the last puff, all make a difference in the risk of getting lung cancer.

Scientists now have a formula that certain smokers and ex-smokers can use to calculate that risk.

The formula, published in Wednesday's issue of the Journal of the National Cancer Institute, shows a wide variation in risk, from a low of less than 1 percent chance in the next 10 years to a high of 16 percent in the same time period.

But the formula only works for people older than 50 who smoked at least half a pack a day for at least 25 years, because it's based on a study that tracked cancer development in just those people.

"The risk assessment tool should help physicians and patients balance the possible risks and benefits of screening," said lead researcher Dr. Peter Bach, of Memorial Sloan-Kettering Cancer Center in New York.

The researchers looked at data from more than 18,000 patients who were current or former smokers.

According to the new formula, a 51-year-old woman who smoked one pack per day for 29 years but stopped smoking nine years earlier had a 0.8 percent risk of getting lung cancer in the next 10 years.

Meanwhile, a 68-year-old man who smoked two packs a day for the past 50 years and continued to smoke had a 15 percent chance of developing the disease in the same time period.