It is a common belief that 1 human year is equal to 7 dog years. That is not very accurate, since dogs reach adulthood within the first couple of years. The formula used is from a canine expert and is a bit more accurate (as accurate as one can judge these things).
The formula is: 10.5 dog years per human year for the first 2 years, then 4 dog years per human year for each year after.