Weeks are the one calendar unit with a fixed length, which makes them the most reliable way to describe a medium-length span. Seven days is always seven days, so unlike months there is no ambiguity to resolve and the arithmetic is a straight division.
The calculator reports the result three ways: whole weeks with the remainder in days, the decimal equivalent, and the number of fortnights for anyone on a two-weekly pay or billing cycle. It also gives the ISO week number at each end, which is how most European businesses and manufacturing schedules refer to points in the year.
From 5 January 2026 to 7 September 2026 is 245 days, which is 35 whole weeks and 0 days, or 35.00 weeks expressed as a decimal.
The formula
A- Start date
B- End date
ΔD- Total days between them
W- Whole weeks — the integer part
R- Remainder days after those weeks
How it works, step by step
- Enter the start date.
- Enter the end date.
- The gauge shows the number of weeks as a decimal.
- The breakdown gives whole weeks plus days, fortnights and the ISO week at each end.
Worked examples
5 January 2026 to 7 September 2026
That is 245 days, so 35 whole weeks with 0 days left over, or 35.00 weeks as a decimal. On a fortnightly pay cycle it covers 17.50 pay periods.
A pregnancy in weeks
A full 40-week term is 280 days. Counting from 1 January 2026, week 40 is reached on 8 October 2026, which is why due dates are always quoted in weeks rather than months.
How to read your score
Frequently asked questions
Why use weeks instead of months?
Because a week never changes length. Months range from 28 to 31 days, so "two months" can differ by three days depending on where in the year it falls. Medicine, payroll and manufacturing all prefer weeks for exactly this reason.
What is an ISO week number?
A standard way of numbering the weeks of a year, defined in ISO 8601. Weeks run Monday to Sunday and week 1 is the one containing the first Thursday of January. Some years have 53 weeks.
Why is the decimal not a round number?
Because the two dates are not a whole number of weeks apart. The decimal part is the leftover days divided by seven — two extra days shows as 0.29 of a week.
How many weeks are in a year?
A common year of 365 days is 52.143 weeks and a leap year is 52.286, so neither divides evenly. That leftover day is why a given date moves forward one weekday each year, or two across a leap year.
Weeks reference
| Days | Whole weeks + days | Decimal weeks | Fortnights |
|---|---|---|---|
| 7 days | 1w 0d | 1.00 | 0.50 |
| 14 days | 2w 0d | 2.00 | 1.00 |
| 28 days | 4w 0d | 4.00 | 2.00 |
| 30 days | 4w 2d | 4.29 | 2.14 |
| 90 days | 12w 6d | 12.86 | 6.43 |
| 91 days | 13w 0d | 13.00 | 6.50 |
| 100 days | 14w 2d | 14.29 | 7.14 |
| 180 days | 25w 5d | 25.71 | 12.86 |
| 182 days | 26w 0d | 26.00 | 13.00 |
| 365 days | 52w 1d | 52.14 | 26.07 |
| 366 days | 52w 2d | 52.29 | 26.14 |
| 730 days | 104w 2d | 104.29 | 52.14 |
Note that 90 days is not exactly 13 weeks, and a year is not exactly 52 weeks.