Weeks to years calculator, and back again.

A weeks to years calculator turns a number of weeks into years. The answer depends on how long you call a year, because a year is not a whole number of weeks. A common year of 365 days is 52 weeks and 1 day, and a leap year of 366 days is 52 weeks and 2 days. So there is no single right figure, and this calculator lets you choose the rule. It also converts the other way, from years to weeks, and from days and months.

How to use the calculator

1. Type the number in Amount. Decimals work, such as 18.5.
2. Pick the unit under Convert from: Weeks, Days, Months or Years. The big card then gives the answer in years, or in weeks if you start from years.
3. Choose Count a year as: 365.2425 days, 365 days or 52 weeks. It starts on the average year.
4. Leave the Start date on today, or pick another day, to see the date you would reach and the years, months and days between the two. Press Today to put it back.
5. Use Copy result for one sentence or Copy link to keep the numbers in a link. The chips under the boxes fill in common values.

The three ways to count a year, side by side

Take 100 weeks, which is 700 days. Counting a year as 52 weeks, 100 divided by 52 is 1.9231 years, or 1 year and 48 weeks. Counting 365 days, 700 divided by 365 is 1.9178 years, which is 1 year and 335 days. Counting the average of 365.2425 days, 700 divided by 365.2425 is 1.9165 years. The three answers are close, but the gap grows with the length. At 1,040 weeks, the 52-week rule says exactly 20 years and the average year says 19.9320.

Which should you use? For a quick answer, 52 weeks is the simplest rule. For a pay or contract period that runs by the calendar, use the start date, which counts real months. For a long span, the average year is the fairest.

From years to weeks

Multiply by 52.1775 for the average year. 1 year is 52.1775 weeks, 5 years is 260.8875 weeks (260 weeks and 6 days), and 10 years is 521.775 weeks. With the simple rule, 5 years is 260 weeks and 10 years is 520. The 400 year Gregorian cycle has 146,097 days, which is exactly 20,871 weeks, so the average of 52.1775 weeks per year is exact over that cycle.

Weeks to months

A month is a twelfth of the year you chose. For the average year that is 30.4369 days, or 4.3481 weeks. So 26 weeks is 182 days divided by 30.4369, which is 5.98 months, and 18 months is 18 times 4.3481, which is 78.27 weeks. Real calendar months run from 28 to 31 days, which is why the start date answer can differ a little from the average.

The real date, from a start date

The start date answer ignores averages. It adds the days, or the calendar months, to the date you pick. 100 weeks from 1 January 2026 is 700 days later. 365 days takes you to 1 January 2027, and 335 more days is 2 December 2027. As calendar units that is 1 year, 11 months and 1 day. It is a Thursday, the same weekday as the start, because 700 is a multiple of 7.

Weeks in each year, and 53 week years

The table under the result lists 8 years. Each has 52 full weeks plus 1 day, or 2 days in a leap year. The ISO weeks column counts weeks the way ISO week numbering does, where a year has 52 or 53 weeks. 2020, 2026, 2032 and 2037 are 53 week years, as this calculator works them out. The what week is it page explains the numbering.

Common problems

• A different answer elsewhere. Most sites divide by 52, 52.14 or 52.18 without saying which. Here the rule is named on the card.
• Pregnancy or age in weeks. This is plain calendar maths. It is not medical advice, and it does not estimate a due date.
• A date past the year 9999. The start date answer stops there and says so.

For the gap between two exact dates, use the date calculator or days between dates. To see how old someone is in weeks and days, try the age calculator.