Weeks to Days Converter
Convert weeks to days — for pregnancy weeks, notice periods and sprint planning.
Updated
days
7.00d
1 wk = 7.00 d
In short
How many days is 40 weeks?
280 days. A week is exactly seven days, so weeks always give a whole number: 4 weeks is 28 days, 13 is 91, 52 is 364. The 280 is where pregnancy dating comes from, since the estimated due date is set 280 days after the first day of the last period.
This one is exact in both directions. What is not exact is any comparison with a month or a year, neither of which is a multiple of seven.
weeks to days — quick reference
| weeks (wk) | days (d) |
|---|---|
| 1 wk | 7.00 d |
| 2 wk | 14.00 d |
| 4 wk | 28.00 d |
| 6 wk | 42.00 d |
| 8 wk | 56.00 d |
| 12 wk | 84.00 d |
| 26 wk | 182.00 d |
| 52 wk | 364.00 d |
The formula, worked line by line
The multiplication is trivial and worth stating precisely anyway: days equals weeks times seven, exactly, with no average involved. That distinguishes this page from every conversion between a week and a month or a year, where a constant has to be chosen and defended.
“By convention, the estimated due date is 280 days after the first day of the last menstrual period.”
Notice what that sentence does and does not say. It fixes an interval and a starting point. It says nothing about conception, nothing about the length of anyone's cycle, and nothing about when a pregnancy is expected to end in practice — which is why the due date is an estimate with a name that says so.
days = weeks x 7
40 x 7 = 280 days
13 x 7 = 91 days
52 x 7 = 364 days - one day short of a common year
12 x 7 = 84 days, but 12 FMLA workweeks is not 84 days- Weeks entered
- 40
- Times seven
- 280 days
- As months, at 30.44 days
- 9.2 months
- As a share of a year
- 0.767
- Days
- 280 days
The month figure is why "nine months" and "forty weeks" are both used and neither is wrong. Nine mean months is 274 days; forty weeks is 280. The six-day gap is the month-length problem, not a disagreement about pregnancy.
Fractions of a week
Clinical notation writes them as sevenths: 39 3/7 weeks means thirty-nine weeks and three days, or 276 days. It is a mixed-radix notation rather than a decimal, and reading it as 39.3 weeks gives 275.1 days, which is the wrong day and occasionally the wrong side of a threshold.
Where the conversion stops being safe
Days and weeks convert exactly. Weeks and months do not, and weeks and years do not. So a chain like weeks to days to months carries an approximation it picked up at the second step, and the answer should be quoted with the month length it assumed rather than presented as exact.
- 40 weeks
- exact, by definition
- 280 days
- exact, 40 x 7
- 9.20 months
- approximate, at 30.436875 days
- 0.767 years
- approximate, at 365.2425 days
The first two rows are the same statement. The last two depend on a chosen average and should never be quoted to more precision than that average supports.
How to use the weeks to days converter
Multiply by seven. That is the entire calculation, and unlike almost every other conversion on this shelf it involves no average, no rounding and no choice of constant. A week has been exactly seven days for as long as the week has existed, in every calendar that uses one.
Which makes the interesting question not how to convert, but why so many things are counted in weeks in the first place. The answer is usually that the weekday matters. Anything measured in whole weeks keeps its weekday forever, and that property is worth more to a roster or a clinic than tidy round numbers would be.
280 days
Forty weeks
the estimated due date interval
91 days
Thirteen weeks
a quarter, counted in weeks
364 days
Fifty-two weeks
one day short of a year
Pregnancy is the clearest case. It is dated in weeks because the milestones that matter clinically are days apart, not months apart, and because a week count avoids the month-length problem entirely. Forty weeks is 280 days exactly; "nine months" is anywhere from 273 to 276, which is why the two never quite line up.
The same caution applies in reverse to deadlines counted in days. A filing period of 180 days is 25 weeks and 5 days, not six months and not 26 weeks. Statutes that count days mean days, and the mismatch with weeks is deliberate rather than sloppy drafting.
- 2 weeks, a fortnight
- 14 days
- 4 weeks, a four-weekly pay period
- 28 days
- 12 weeks, FMLA leave
- 84 days, but see the caveat above
- 13 weeks, a quarter
- 91 days
- 40 weeks, the due-date interval
- 280 days
- 52 weeks
- 364 days, a day short of the year
Every figure here is exact. The caveats are about what the weeks mean, never about the multiplication.
Work out a due date from a last period
The 280 days only becomes useful once it is anchored to a date. That page applies the interval and shows the trimester boundaries alongside it.
Open the due date calculator →Weeks converted to days, with what each span is normally used for. Every row is exact, because a week is exactly seven days.
| Weeks | Days | What it usually marks |
|---|---|---|
| 1 week | 7 days | a week of notice |
| 2 weeks | 14 days | a fortnight |
| 3 weeks | 21 days | |
| 4 weeks | 28 days | a four-weekly pay period |
| 6 weeks | 42 days | |
| 8 weeks | 56 days | |
| 12 weeks | 84 days | FMLA leave, counted in workweeks |
| 13 weeks | 91 days | a quarter of a 364-day year |
| 26 weeks | 182 days | half of 52 weeks |
| 39 weeks | 273 days | full term begins |
| 40 weeks | 280 days | the estimated due date |
| 52 weeks | 364 days | one day short of a year |
| 104 weeks | 728 days | two years of whole weeks |
Why pregnancy is counted in weeks, and from when
ACOG sets the estimated due date at 280 days after the first day of the last menstrual period, which is where the familiar forty weeks comes from. The convention assumes a 28-day cycle with ovulation around day fourteen, so the count deliberately starts about two weeks before conception could have happened.
That starting point is the single most misunderstood part of the figure. A pregnancy described as forty weeks is not forty weeks of embryonic development; it is forty weeks from a date that precedes conception. Anyone comparing the number against a conception date will be about two weeks out, every time.
- Full term begins
- 39 weeks 0 days = 273 days
- The estimated due date
- 40 weeks 0 days = 280 days
- Full term ends
- 40 weeks 6 days = 286 days
- Post-term begins
- 42 weeks 0 days = 294 days
- The full-term window
- 273 to 286 days
The 39-to-41 week terminology is from ACOG Committee Opinion 579, which is a different document from the due-date one. It replaced the older habit of calling anything from 37 weeks "term" — 37 0/7 to 38 6/7 is early term.
The due date itself is not fixed by the arithmetic alone. ACOG treats first-trimester ultrasound as the most accurate dating method and recommends redating when it disagrees with the last-period date by more than five days before 9 0/7 weeks, or more than seven days from 9 0/7 through 13 6/7 weeks.
When the law counts in weeks on purpose
Statutory drafting picks its unit deliberately, and weeks are chosen when the entitlement should track a working pattern rather than a calendar. UK statutory notice under the Employment Rights Act 1996 is one week for each year of continuous employment from two years onward, capped at twelve weeks. It is not drafted as 84 days, and substituting days changes the termination date.
Do
- Use weeks whenever the source document uses weeks
- Convert to days for planning and comparison only
- Check whether a statutory week means a calendar week or the employee's working week
- Anchor pregnancy weeks to the last menstrual period, not conception
Don't
- Assume twelve workweeks of leave equals 84 calendar days
- Restate a contractual notice period in a different unit than the contract uses
- Treat four weeks and one month as interchangeable in a payroll run
- Convert a 180-day filing deadline into "about six months"
US FMLA works the same way from the other direction. The entitlement is twelve workweeks in a twelve-month period, so what a week is worth depends on the employee's schedule; where hours vary too much to determine, the regulations use the weekly average of hours scheduled over the previous twelve months.
Every weeks value, worked out
65 common weeks figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.
Questions people ask
Sources
Where the constants and formulas on this page come from. Each line names the figure it backs.
By convention the estimated due date is 280 days after the first day of the last menstrual period, and first-trimester ultrasound is the most accurate dating method.
Committee Opinion 700 — Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists
Full term is defined as 39 0/7 through 40 6/7 weeks of gestation, with 37 0/7 to 38 6/7 weeks classed as early term.
Committee Opinion 579 — Definition of Term Pregnancy — American College of Obstetricians and Gynecologists
Statutory minimum notice is one week for each year of continuous employment from two years, capped at twelve weeks.
Employment Rights Act 1996, section 86 — UK Government, legislation.gov.uk
FMLA leave is an entitlement to twelve workweeks rather than a fixed number of calendar days, and irregular schedules are averaged over the preceding twelve months.
Fact Sheet 28I — Calculation of Leave under the FMLA — US Department of Labor, Wage and Hour Division
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