Convert days to weeks by dividing by 7, with tables for pregnancy weeks, notice periods and why a month is not four weeks.

The formula

weeks = days ÷ 7

The remainder usually matters more than the decimal. 100 days is 14.29 weeks, which is better read as 14 weeks and 2 days — multiply the fractional part by 7 to get the days.

Quick reference

Days	Weeks	As weeks + days
7 d	1	1 w 0 d
10 d	1.43	1 w 3 d
14 d	2	2 w 0 d
30 d	4.29	4 w 2 d
60 d	8.57	8 w 4 d
90 d	12.86	12 w 6 d
100 d	14.29	14 w 2 d
365 d	52.14	52 w 1 d

A month is not four weeks

Four weeks is 28 days. Only February is 28 days. Every other month is 30 or 31, so the approximation is out by two or three days each time and by more than a month across a year:

	Days	Weeks
4 weeks	28	4
Average month	30.44	4.35
12 × 4 weeks	336	48
Year	365	52.14

A year is 52 weeks and 1 day, not 48. This is why a “four-weekly” pay cycle produces thirteen payments a year rather than twelve, and why the two are not interchangeable in a contract.

Pregnancy

Gestational age is quoted in weeks and days, counted from the first day of the last menstrual period rather than from conception:

	Weeks	Days
End of first trimester	13 w	91 d
End of second trimester	27 w	189 d
Full term begins	39 w	273 d
Due date	40 w	280 d
Post-term	42 w	294 d

A due date is 280 days after the last period, which is why it is nine months and a week rather than nine months exactly.

Notice and statutory periods

Legal and contractual periods are written in whichever unit the drafter preferred, and converting between them is the usual reason for this page:

Period	Days	Weeks
2 weeks	14	2
30 days	30	4.29
1 month	28–31	4–4.43
90 days	90	12.86
3 months	89–92	12.71–13.14
6 months	181–184	25.86–26.29

A 90-day period and a three-month period are not the same length, and which one applies depends on the wording, not on the intent.

Estimating in your head

Divide by 7, or take a tenth and add a bit under half again: 100 ÷ 10 is 10, plus 4 is 14 weeks, against a true 14.29.

Days to weeks