Weeks, Months and Years Calculator

Weeks, months and years without the guesswork

How many weeks is 18 months? What week of the year is it? How many months between March and November? These sound like simple questions, but months have different lengths and years do not split into a whole number of weeks, so the answers are easy to get slightly wrong.

This week calculator does three things. It counts the weeks, months and years between two dates. It converts a length of time between days, weeks, months and years. And it gives the week number of any date, along with the day of the year and the quarter.

How to use it

1. Choose Between two dates, Convert a length of time, or Week number of a date.
2. For two dates, pick the start and the end.
3. To convert, type an amount and choose its unit.
4. For a week number, pick the date.
5. Read the answer, with the other units listed underneath.

The conversions used

1 week = 7 days
1 year = 365.2425 days on average
1 month = 365.2425 ÷ 12 = 30.436875 days on average

The average year of 365.2425 days comes from the leap year rule: three leap years in every four centuries are skipped. Dividing by 12 gives the average month. These averages are right for converting lengths of time. Between two real dates, the calculator counts whole calendar months instead, then the days left over.

Week numbers follow the ISO 8601 standard used across most of the world: weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. Some years end with a week 53.

Worked example: how many weeks is 18 months?

• 18 months × 30.436875 days = 547.86 days
• 547.86 ÷ 7 = 78.27 weeks
• 547.86 ÷ 365.2425 = 1.5 years

18 months is about 78.27 weeks, or 1.5 years. A quick estimate of 4 weeks a month would give 72 weeks, nearly a month and a half short, which is why the average month length matters.

For a real date example, 1 March 2026 to 15 November 2026 is 259 days: exactly 37 weeks, or 8 months and 14 days on the calendar.

Handy conversions

A year is 52 weeks and one or two days. That leftover day is why a date falls on a different weekday each year, and why some years have a week 53.

Mix-ups to avoid

• Using 4 weeks for a month. A month is about 4.35 weeks, so the error grows quickly over several months.
• Mixing week numbering systems. The US often counts the week containing 1 January as week 1, starting on Sunday. ISO weeks start on Monday and can differ by one around New Year.
• Expecting the decimal months between dates to be exact. Calendar months vary, so the calculator gives whole months plus days for real dates.
• For plain day counts between dates, the date calculator adds weekday counts and lets you add or subtract dates.

Frequently asked questions

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