How this calculator works
A decade is exactly ten years, so the conversion is a factor of ten in either direction. The page counts a year as 365.25 days, the Julian year, which averages the leap day over four years. That makes a year exactly 31,557,600 seconds and a decade exactly 315,576,000 seconds. A month is taken as one twelfth of a year, which is 30.4375 days on average, so months are a smooth average and not calendar months.
Formula
decades = years / 10. years = decades x 10. months = years x 12. In seconds, one Julian year is 365.25 x 86,400 = 31,557,600 s and one decade is 315,576,000 s.
Worked example
25 years is 2.5 decades and 300 months. 10 years is exactly 1 decade. 7 years is 0.7 decades and 84 months. 3 decades is 30 years and 360 months.
Advertisement
Frequently asked questions
Is a decade a period of ten calendar years?
Yes. A decade is any ten-year span, such as 2020 to 2029 inclusive, which is often called the 2020s. Because the page counts an average year, converting a decade to days gives 3,652.5 days rather than a whole number.
Why 365.25 days instead of 365?
Most years have 365 days and leap years have 366. Using 365.25 makes arithmetic consistent over long spans. It is an average, so a particular ten-year span can differ by a day or two.
How many months are in a decade?
120 by the page definition, because each year is counted as twelve months. Calendar months vary from 28 to 31 days, so an exact calendar conversion would need a start date.