A decade is 10 years — 120 months and 3,652 or 3,653 days (two or three of the years are leap years). A century is 100 years and a millennium 1,000 years.

10 years

1 decade = 10 years

The same amount in every unit
Unit	Amount
Milliseconds	315,569,520,000
Seconds	315,569,520
Minutes	5,259,492
Hours	87,658.2
Days	3,652.425
Weeks	521.775
Months	120
Years	10
Centuries	0.1

A decade is 10 years; on the average year that is 3,652.425 days. A year is 365.2425 days on average (Gregorian calendar: 97 leap years every 400 years). A common year has 365 days (52 weeks and 1 day); a leap year has 366 (52 weeks and 2 days).

How it’s calculated

Every unit is converted through seconds: 1 minute = 60 s, 1 hour = 3,600 s, 1 day = 86,400 s, 1 week = 604,800 s. Months and years use the Gregorian average: 1 year = 365.2425 days = 31,556,952 s, 1 month = 1/12 year = 2,629,746 s.

Worked example with your numbers

1 decade × 315,569,520 seconds per decade = 315,569,520 seconds.
315,569,520 seconds ÷ 31,556,952 seconds per year = 10 years.

Quick answer

A decade is 10 years — 120 months and 3,652 or 3,653 days (two or three of the years are leap years).

Decade: 10 years
Century: 100 years
Millennium: 1,000 years
Decade in days: 3,652–3,653

Every time unit in seconds, days and years
Unit	Seconds	Days	Years
1 millisecond	0.001	0.00000001157	0.000000000032
1 second	1	0.00001157	0.00000003169
1 minute	60	0.0006944	0.000001901
1 hour	3,600	0.041667	0.0001141
1 day	86,400	1	0.00273791
1 week	604,800	7	0.01916535
1 month	2,629,746	30.436875	0.08333333
1 year	31,556,952	365.2425	1
1 decade	315,569,520	3,652.425	10
1 century	3,155,695,200	36,524.25	100

1 decade = 10 years; 1 century = 100 years; 1 millennium = 1,000 years. Months and years use the Gregorian average (365.2425 days a year).

How it works

In everyday use the 2020s run from 2020 through 2029; counting from year 1, a strict decade runs from a year ending in 1 to one ending in 0.

Frequently asked questions

How many years are in a century?

100 years.