Article — Days to Weeks Converter
Days to weeks: convert days into weeks (and weeks plus days)
A week is exactly 7 days. To go from days to weeks, divide by 7. To go the other way, multiply by 7. 30 days is 4.29 weeks, or 4 weeks and 2 days — the same value in two formats. The factor never changes; only the remainder does. The number people most often want is one of three: 30 days = 4 weeks 2 days, 90 days = 12 weeks 6 days, or 280 days = 40 weeks (full-term pregnancy).
The calculator above runs the conversion instantly in both directions. The article below covers where the 7-day week came from (Babylonian astronomy, ~2300 BCE), why 4 weeks is not 1 month, how pregnancy weeks work, and the ISO 8601 standard that quietly governs week numbers in software, payroll, and international logistics.
Why a week has 7 days
The 7-day week is not astronomical in the way a year or a day is. The Earth completes a rotation in 24 hours, and orbits the Sun in 365.25 days — both facts that any culture could measure independently. A week is a human invention layered on top of those, and it traces back to Babylonian astronomers around 2300 BCE.
The Babylonians divided the roughly 28-day lunar cycle into four phases of seven days. Each day was assigned to one of the seven naked-eye celestial objects: the Sun, the Moon, and the five visible planets — Mars, Mercury, Jupiter, Venus, and Saturn. The names survive: Sunday (Sun), Monday (Moon), Saturday (Saturn) in English; Lundi (Moon), Mardi (Mars), Mercredi (Mercury) in French. Judaism formalized the 7-day week in the creation narrative; Roman emperor Constantine made it the official week of the empire in 321 CE. Despite the fact that 7 doesn't divide evenly into 365 (52 × 7 = 364), the system has stayed in continuous use for around 4,300 years.
The French Revolutionary calendar tried to abolish the 7-day week in 1793, replacing it with a 10-day "decade." Workers got one day off in ten instead of one in seven. The experiment lasted 12 years before Napoleon scrapped it and reinstated the Babylonian week in 1806. The Soviet Union ran a similar experiment with 5-day and then 6-day weeks from 1929 to 1940, also abandoned. The 7-day week has so far survived every attempt to replace it.
The days to weeks formula
To convert days to weeks, divide by 7. To go the other direction, multiply by 7. The factor is exact, by definition. There is no measurement involved.
days ÷ 7 = weeks (decimal)weeks × 7 = daysfloor(days ÷ 7) = whole weeksdays mod 7 = leftover daysThe decimal version (4.29 weeks) and the mixed version (4 weeks 2 days) describe the same span. Engineers and statisticians usually want decimals; pregnant patients, project managers, and HR forms usually want the mixed form. The calculator shows both.
Decimal weeks vs. "weeks and days"
30 days converts to 4.285714... weeks, which rounds to 4.29 in two decimals. The mixed form drops the decimal: 30 ÷ 7 = 4 remainder 2, so "4 weeks and 2 days." Both are correct. The mixed form is preferred when the leftover days matter — pregnancy charts, sprint planning, paid-leave accruals — because "0.29 weeks" obscures the fact that two real calendar days remain.
To convert decimal back to mixed: keep the integer part as whole weeks, multiply the decimal by 7 to get leftover days. 12.86 weeks → 12 weeks and (0.86 × 7) = 6.02 days, so 12 weeks 6 days. The math survives the round trip exactly if you use enough decimal places.
Days, weeks, and months are not the same
This is the conversion error people make most often. 4 weeks is 28 days. A month is 28-31 days, with an average of 30.44 (365.25 ÷ 12). The average month is therefore 4.35 weeks, not 4. Treating them as equivalent undercounts most months by 2-3 days.
US apartment leases and many consumer contracts use "30 days notice" rather than "one month." The two are not identical. A 30-day notice given on January 15 ends February 14. A "one month" notice given on January 15 ends February 15 — the calendar-day equivalent. The 1-day gap can matter for rent prorating or termination dates. Always read the exact wording.
Some examples of where the 28 vs. 30-31 confusion shows up: monthly subscriptions billed every 30 days drift earlier each year; loan terms quoted in "months" use the lender's specific calendar convention; project schedules estimated in "X months" tend to be 4-8% short when delivered in weeks.
Pregnancy weeks: the 40-week clock
Obstetric weeks count from the first day of the last menstrual period (LMP), not from conception. Full-term pregnancy is 40 weeks (280 days) from LMP. Conception happens around week 2 by this clock, which is why a patient at "4 weeks pregnant" is carrying an embryo that is about 2 weeks old.
The framework was formalized by Franz Karl Naegele in 1812: estimated due date = first day of LMP + 280 days. Modern ultrasound dating is more accurate in early pregnancy (especially before 14 weeks), but the LMP-based week number remains the standard reference in clinical practice. The full-term range is 37-42 weeks; only about 5% of babies are born on the exact due date.
- Day 14 (week 2) = ovulation and conception, in a typical 28-day cycle
- Day 28 (week 4) = first missed period; positive pregnancy tests possible from ~10 days post-conception
- Day 84 (week 12) = end of first trimester, miscarriage risk drops sharply
- Day 168 (week 24) = viability threshold with modern NICU care
- Day 259 (week 37) = full term begins
- Day 280 (week 40) = estimated due date (Naegele's rule)
- Day 294 (week 42) = post-term; induction usually considered
ISO 8601 weeks and year boundaries
ISO 8601 is the international standard for date and time formatting. It defines weeks as starting on Monday and ending on Sunday — different from the US convention of Sunday-to-Saturday. Week 1 of an ISO year is the week containing the first Thursday of January.
The implications are subtle. Because Week 1 always contains January 4 and the first Thursday, an ISO year can have either 52 or 53 weeks. December 29-31 sometimes belongs to Week 01 of the next ISO year; January 1-3 sometimes belongs to Week 52 or 53 of the previous ISO year. The format is YYYY-Www-D — for example, 2026-W19-3 means Wednesday of Week 19, 2026.
If your payroll or shift-scheduling software shows a week number that doesn't match your wall calendar, it is almost certainly using ISO week numbering. The ISO standard is the default in most European countries and in many global IT and logistics systems. The US Sunday-start convention is built into a smaller share of business software.
Work, leave, and project sprints
The day-to-week conversion shows up everywhere in employment and project work. US "two weeks notice" is 14 days, often 10 working days. UK statutory leave is 28 days, including bank holidays — 4 weeks exactly. EU minimum paid leave under the Working Time Directive is 20 working days, often described as "4 weeks." Software teams using Scrum typically run 14-day sprints, occasionally 7 or 21.
FMLA leave in the US is "up to 12 weeks" — 84 days. A 90-day probationary period is 12.86 weeks, slightly longer than the 12 weeks people sometimes round to. The NFL regular season is 18 weeks = 126 days. Each of these uses the same 7-day factor; the difference is whether the number gets quoted as days, weeks, or weeks-and-days.
Common day-to-week mistakes
Calling 30 days "4 weeks." 30 days is 4.29 weeks. The 2 leftover days matter for due dates, lease end dates, and refund windows. Use the exact form when the date matters.
Confusing pregnancy weeks with embryo age. "4 weeks pregnant" by clinical convention means 4 weeks since LMP, not 4 weeks since conception. The embryo is approximately 2 weeks old at that point.
Assuming a year has exactly 52 weeks. A year has 52 weeks and 1 day (or 2 in leap years). Yearly subscriptions billed weekly drift by one full week roughly every 6-7 years.
Using US week numbering against ISO software. If a team in Berlin says "Week 19" and a team in New York says "Week 19," they may be referring to different 7-day spans. Always specify the week's start date when communicating across regions.