# Week Calculator

URL: https://speedycalculator.com/calculators/everyday-life/week-calculator/
Last updated: 2026-10-25

> The week calculator counts the weeks between two dates (as full weeks and days and as decimal weeks), shows the ISO week number of each date, and adds a number of weeks to a date.

## Inputs

- Start date
- End date
- Weeks to add to the start date

## Results

- Days between
- Full weeks
- and days
- Decimal weeks
- ISO week of start date
- ISO week of end date
- Start date + weeks

## Formula

```
Weeks between = days ÷ 7 (full weeks + remaining days)
ISO week = week (Monday–Sunday) containing the date's Thursday
Date + n weeks = date + 7n days
```

## Explanation

Weeks are the natural unit for pregnancies, projects, school terms and training plans. ISO week numbers (1–52 or 53) start on Monday, and week 1 is the week containing the year's first Thursday – used widely in business and in Europe.

### How to use the Week Calculator

- **Start date**
- **End date**
- **Weeks to add to the start date**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter two dates, and optionally a number of weeks to add to the start date.

### Tips

- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so "one month" is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.

### Common mistakes to avoid

- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).

### Inputs explained

FieldWhat to enterStart dateA dateEnd dateA dateWeeks to add to the start dateA number

### What each result means

- **Days between** = [End date] - [Start date]
- **Full weeks** = sign([Days between])×floor(abs([Days between])÷7)
- **and days** = sign([Days between])×mod(abs([Days between]), 7)
- **Decimal weeks** = [Days between]÷7
- **ISO week of start date** = (floor((dayofyear(([Start date]) - mod(weekday([Start date]) + 6, 7) + 3) - 1)÷7) + 1)
- **ISO week of end date** = (floor((dayofyear(([End date]) - mod(weekday([End date]) + 6, 7) + 3) - 1)÷7) + 1)
- **Start date + weeks** = [Start date] + 7×[Weeks to add to the start date]

### Example values

How the main result changes when **Weeks to add to the start date** changes and everything else stays at the example values above:

Weeks to add to the start dateFull weeks6149141214151418142414

## Worked example

Inputs — Start date: 2026-10-25; End date: 2027-02-02; Weeks to add to the start date: 12.

Results — Days between = 100; **Full weeks = 14**; and days = 2; Decimal weeks = 14.29; ISO week of start date = 43; ISO week of end date = 5; Start date + weeks = Sun, January 17, 2027.

## FAQ

**How many weeks between two dates?**
Divide the days between them by 7.

**How many weeks are in a year?**
52 weeks and 1 day (2 in a leap year); ISO years have 52 or 53 weeks.

**What week number is it?**
See "ISO week of start date" (with today as the start date).

**What formula does the Week Calculator use?**
Weeks between = days ÷ 7 (full weeks + remaining days).