Divide by 365.25, not 365
A calendar year is 365 days, but one year in four is a leap year with 366. Averaged over that cycle a year runs 365.25 days, which is the figure to divide by. Using a flat 365 looks close for one year but drifts — over 100 years it overshoots by about 24 days.
Common day counts in years
| Days | Years (÷ 365.25) | In plain terms |
|---|---|---|
| 365 | 0.999 | One year |
| 730 | 1.998 | Two years |
| 1,000 | 2.738 | ~2 years 9 months |
| 1,825 | 4.993 | Five years |
| 3,650 | 9.986 | A decade |
| 10,000 | 27.38 | ~27 years 4 months |
| 36,500 | 99.86 | Nearly a century |
Round decades read a hair under a whole number because 3,650 days is short of ten actual years by the leap days — 10 years is really 3,652.5 days.
A few things worth knowing
- Months average 30.44 days. To split a remainder into months, divide by 365.25 ÷ 12 = 30.44, not 30 or 31, since real months vary in length.
- The Gregorian year is 365.2425. Skipping three leap years every 400 (1700, 1800, 1900 — but not 2000) trims the average slightly below 365.25. The difference only matters over many centuries.
- Leap years are not every fourth year. A century year is a leap year only if it divides by 400. So 2000 was one; 1900 and 2100 are not.
Common questions
How do I convert days to years?
Divide the days by 365.25, the average length of a year once leap days are counted. 1,825 days divided by 365.25 is 4.99, so almost exactly 5 years.
Should I divide by 365 or 365.25?
Use 365.25 for anything spanning several years. Plain 365 ignores leap days, and over a century that gap adds up to about 24 days. 365 is only right for a single non-leap year.
How many years is 1,000 days?
About 2.74 years, or 2 years and roughly 9 months. Exactly, 1,000 divided by 365.25 is 2.7379.
