The number of weeks between two dates is found by dividing the total number of days by 7:
Example: From January 1 to March 31, 2026 = 89 days = 89 ÷ 7 = 12 weeks and 5 days.
To find a future date from a number of weeks: Target date = Start date + (Weeks × 7) days. For example, 16 weeks from March 1 = March 1 + 112 days = June 21.
The Gregorian calendar has a predictable relationship with weeks: a standard year has 52 weeks and 1 day (365 = 52 × 7 + 1). A leap year has 52 weeks and 2 days (366 = 52 × 7 + 2). This is why your birthday shifts one day of the week each year (or two after a leap year) — it can never stay on the same day without a 5- or 6-year gap.
ISO 8601 defines a standardized week numbering system where Week 1 is the week containing the first Thursday of the year. This means January 1 can fall in the last week of the previous year (if Jan 1 is Friday, Saturday, or Sunday). Most European countries and business calendars use ISO week numbering.
| Duration | Days | Weeks | Common Context |
|---|---|---|---|
| 1 month (avg) | 30.44 | 4.35 | Monthly planning |
| 6 weeks | 42 | 6 | Post-surgery recovery, Couch to 5K start |
| 1 quarter | 91 | 13 | Business/fiscal quarter, college semester |
| 6 months | 182–184 | 26 | Half-year, probation periods |
| 40 weeks | 280 | 40 | Full-term pregnancy |
| 1 year | 365/366 | 52.14/52.29 | Annual planning |
| 18 months | 547–548 | 78 | Product milestones, toddler age |
| 2 years | 730/731 | 104 | Biennial reviews, visa periods |
Running training plans in weeks:
| Event | Beginner Plan | Intermediate Plan | Advanced Plan |
|---|---|---|---|
| 5K | 6–8 weeks (Couch to 5K) | 8–10 weeks | 6–8 weeks |
| 10K | 10–12 weeks | 8–10 weeks | 8 weeks |
| Half Marathon | 12–16 weeks | 10–12 weeks | 8–10 weeks |
| Marathon | 18–20 weeks | 16–18 weeks | 12–16 weeks |
| Ultra Marathon (50K+) | 20–24 weeks | 16–20 weeks | 12–16 weeks |
Today is March 15, 2026. Your marathon is October 11, 2026. How many weeks of training do you have?
Last menstrual period (LMP) was January 5, 2026. When is the due date (40 weeks)?
A company's fiscal year runs October 1 to September 30. How many working weeks?
| Factor | Weeks Calculator | Days Calculator |
|---|---|---|
| Best for | Pregnancy, training plans, sprints, semesters | Countdowns, legal deadlines, age calculation |
| Unit | Weeks + remaining days | Total days |
| Precision needed | Weekly planning | Day-exact deadlines |
| Common question | "How many weeks until race day?" | "How many days until Christmas?" |
| Output | 12 weeks and 3 days | 87 days |
Use weeks when the context naturally thinks in weeks (training plans, pregnancy, work sprints). Use days when exact day counts matter (legal deadlines, age calculations, countdowns). The date difference calculator provides both formats, while the days until calculator focuses on countdown-style calculations. For precise age calculations, use years, months, and days together.
A standard year has 52 weeks and 1 day (365 ÷ 7 = 52.143 weeks). A leap year has 52 weeks and 2 days. For practical planning, use 52 weeks. ISO 8601 occasionally assigns 53 weeks to a year when January 1 falls on a Thursday (or Thursday/Friday in leap years).
90 days = 12 weeks and 6 days (90 ÷ 7 = 12.857 weeks). This is very close to 3 calendar months (91.3 days on average). The 90-day period is commonly used for probation periods, fitness challenges, and quarterly business goals.
Count the days from the first day of your last menstrual period (LMP) to today, then divide by 7. For example, if your LMP was 70 days ago, you are 10 weeks pregnant (10 weeks + 0 days). Medical professionals write this as "10+0." Pregnancy is always counted from LMP, not from conception (which occurs approximately 2 weeks later).
Approximately 26 weeks (182–184 days depending on which months). The exact count varies because months have 28–31 days. For precision, calculate the actual days between your specific dates rather than using averages.
Beginner plans: 16–20 weeks. Intermediate: 12–16 weeks. Advanced: 12–16 weeks (with higher starting mileage). The most popular plans (Hal Higdon, Jack Daniels, Pfitzinger) are typically 18 weeks. You should be able to comfortably run 25–30 km/week before starting most beginner plans.
52 total weeks minus holidays and vacation: typically 48–50 working weeks in the US (2 weeks PTO + holidays) or 45–47 in Europe (4–6 weeks PTO + holidays). At 5 days/week, this equals 240–250 working days in the US or 225–235 in Europe.
ISO 8601 numbers weeks 01–52 (or 53), with Week 01 being the week containing the first Thursday of the year. Weeks start on Monday. This means January 1–3 can fall in the previous year's last week. Most European countries, business calendars, and programming languages use ISO week numbering.
A typical US college semester is 15–16 weeks of classes plus 1–2 weeks of finals. Fall semester: ~17 weeks (late August to mid-December). Spring semester: ~17 weeks (mid-January to early May). Summer break: ~12–14 weeks. Winter break: ~4–5 weeks. Quarter systems use 10-week terms.
In ISO week numbering, yes. A year has 53 weeks when January 1 is a Thursday (common year) or when January 1 is Wednesday or Thursday (leap year). This happens roughly every 5–6 years. Examples: 2004, 2009, 2015, 2020, 2026. In standard counting, every year has exactly 52 weeks plus 1–2 extra days.