Days to Weeks, Months, and Years — Time Conversion Guide

Most people have a rough sense of how days relate to weeks and months. But when you actually need a precise number — for a contract term, a billing period, a project deadline, or an age calculation — "roughly four weeks" is not enough. The exact conversion depends on which month, whether there is a leap year, and what the question is actually asking.

The Time Converter handles days, weeks, months, hours, and seconds in one place. This article covers the conversions in detail, explains where the numbers come from, and flags the edge cases worth knowing about.

Days to Weeks

This one is clean:

1 week = 7 days

weeks = days ÷ 7

So 30 days is 4 weeks and 2 days (4.286 weeks). 100 days is 14 weeks and 2 days (14.286 weeks).

A standard calendar year has 52 weeks and 1 day (or 2 days in a leap year). This is why the same calendar date shifts by one day each year — and by two days the year after a leap year.

Days to Months

Here is where it gets less tidy. Months are not a fixed number of days. They range from 28 to 31 days depending on the month and whether it is a leap year.

For general purposes, two averages are commonly used:

• 365.25 ÷ 12 = 30.4375 days — the average month length based on the mean Gregorian year
• 365 ÷ 12 ≈ 30.417 days — a simpler approximation

For most conversions, using 30.44 days per month is accurate enough.

months = days ÷ 30.4375

The imprecision matters in certain contexts. A "6-month contract" is not 180 days — it is 6 calendar months, which could be anywhere from 181 to 184 days depending on which months are involved. June 1 to December 1 is exactly 183 days. January 1 to July 1 is 181 days (or 182 in a leap year).

When precision matters — in a legal document, a lease, a subscription term — specify calendar months, not a day count. When you need a quick estimate, use 30.44 days.

Days to Years

1 year = 365 days (common year) 1 year = 366 days (leap year) 1 year = 365.25 days (average, accounting for leap years)

years = days ÷ 365.25

For calendar math, you also need to know where in the year you are, because a "year" from March 15 to March 15 next year is 365 or 366 days depending on whether there is a February 29 in between.

Note that 1,461 days = exactly 4 years (including one leap year): 365 × 3 + 366 = 1,461.

How Many Days in Each Month

For exact conversions over a specific period:

The mnemonic for 30-day months: April, June, September, November. All others have 31, except February.

When You Need Exact Days vs Approximate Months

The difference between calendar months and day counts matters in several practical situations.

Loan and mortgage terms: A 30-year mortgage is 360 monthly payments, not 10,950 days. If the lender charges interest daily, the calculation uses the actual number of days in each month.

Subscription billing: "Monthly" billing is usually calendar month to calendar month. If you sign up on January 31, your next billing date is February 28 (or 29) — one calendar month later, regardless of the day count.

Employment and notice periods: A 3-month notice period in a UK contract means calendar months. If you give notice on October 31, the period ends January 31 — 92 days, not 90.

Age calculations: How old is someone born on January 1, 2000, on their birthday in 2025? 25 years. How many days old are they? 25 × 365 + 7 leap days (2000, 2004, 2008, 2012, 2016, 2020, 2024) = 9,131 days.

Prescription and medication schedules: "Take for 14 days" is unambiguous. "Take for two weeks" is the same thing — but "take for two months" varies by which months.

Converting Larger Units Back to Days

Sometimes you need to go in the other direction:

The 100-year count (36,524) accounts for the Gregorian calendar rule that century years are not leap years unless divisible by 400. So 1900 had 365 days, but 2000 had 366.

Working Days vs Calendar Days

One more distinction worth flagging: working days (business days) and calendar days are not the same. A "5 business day" processing period is a week of calendar time, but a "10 business day" period is two calendar weeks — assuming no public holidays.

A common approximation: there are roughly 260 working days in a year (52 weeks × 5 days). Subtract public holidays — typically 8–11 in the UK, 10 in the US — and you get about 250 working days per year.

If you need to calculate deadlines precisely, the Time Converter converts between days, hours, weeks, and larger units. For date arithmetic that factors in weekdays and holidays, a dedicated date calculator will give more reliable results than a manual unit conversion.