Paste a column of dates and get the MMWR epiweek, ISO week, epi-year, and both week start/end dates for every row in one paste-ready table — free, instant, no signup, and nothing uploaded: all processing happens in your browser.
Accepts YYYY-MM-DD and MM/DD/YYYY. Nothing is uploaded; dates are processed in your browser only.
How to use
- Paste your dates — one per line, in
YYYY-MM-DDorMM/DD/YYYYformat — into the box on the left. The result table appears as you type. - Check the table — each row shows the MMWR epiweek (epi-year and week), the MMWR week start and end (Sunday to Saturday), plus the ISO week (year and week) with its Monday-to-Sunday start and end dates.
- Copy or download — click “Copy for Excel / Sheets” to paste the table straight into a spreadsheet, or “Download CSV” to save it as a file.
Worked example
Click “Load example” in the tool above and it fills in these three year-boundary dates:
| Date | MMWR | MMWR start | MMWR end | ISO | ISO start | ISO end |
|---|---|---|---|---|---|---|
| 2018-12-30 | 2019-W01 | 2018-12-30 | 2019-01-05 | 2018-W52 | 2018-12-24 | 2018-12-30 |
| 2024-12-29 | 2025-W01 | 2024-12-29 | 2025-01-04 | 2024-W52 | 2024-12-23 | 2024-12-29 |
| 2025-01-01 | 2025-W01 | 2024-12-29 | 2025-01-04 | 2025-W01 | 2024-12-30 | 2025-01-05 |
Why do the year-boundary dates differ? 30 December 2018 is a Sunday. MMWR weeks start on Sunday, so it opens the MMWR week that contains 4 January 2019 — making it MMWR 2019-W01. ISO weeks start on Monday, so that Sunday still belongs to the final ISO week of 2018 (ISO 2018-W52, 24–30 December). The same logic puts 29 December 2024 (also a Sunday) into MMWR 2025-W01 while ISO counts it in 2024-W52. 1 January 2025 falls in week 1 of 2025 under both systems, but the week windows differ: the MMWR week runs 29 December to 4 January (Sunday start), while the ISO week runs 30 December to 5 January (Monday start).
MMWR vs ISO weeks
MMWR (Morbidity and Mortality Weekly Report) epiweeks, used by the CDC for disease surveillance, run Sunday to Saturday. MMWR week 1 is the week containing 4 January that has at least four days in the new year. ISO 8601 weeks run Monday to Sunday, and ISO week 1 is the week containing the first Thursday of the year. Both systems use 52 or 53 weeks per year, but because their weeks start on different days, dates near 1 January often land in different week numbers and even different week-years.
Formulas for Sheets and Excel
To compute these by hand with a date in cell A2:
- ISO week:
=ISOWEEKNUM(A2) - ISO year:
=YEAR(A2-WEEKDAY(A2,2)+4) - MMWR year:
=YEAR(A2-WEEKDAY(A2,1)+4) - MMWR week 1 start:
=DATE(MMWRyear,1,4)-WEEKDAY(DATE(MMWRyear,1,4),1)+1 - MMWR week number:
=INT((A2-week1start)/7)+1
Note: Google Sheets has no native MMWR function — that is exactly why this converter exists. The tool above runs the same logic for hundreds of dates at once, with no formulas to write.
Common mistakes
- Using
WEEKNUMas if it were an MMWR week — Excel’s default week numbering is a different system. - Assuming 1 January is always week 1 — dates near the year boundary can belong to week 52/53 of the previous year or week 1 of the next.
- Mixing Sunday and Monday week starts — MMWR weeks start Sunday, ISO weeks start Monday, and swapping them shifts every boundary.
Frequently asked questions
What is an MMWR epiweek and how is it different from an ISO week?
An MMWR epiweek is the CDC’s epidemiological week: weeks run Sunday to Saturday, and week 1 is the first week with at least four days in the new calendar year. ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the first Thursday. Both use 52 or 53 weeks, but they assign year boundaries and week numbers differently.
Why is 30 December 2018 in MMWR week 1 of 2019 but ISO week 52 of 2018?
30 December 2018 is a Sunday. MMWR weeks start on Sunday, so it opens the MMWR week containing 4 January 2019 — week 1 of the 2019 MMWR year. ISO weeks start on Monday, so that Sunday still belongs to the last ISO week of 2018 (ISO 2018-W52, 24–30 December).
How do I convert a date to an epiweek in Google Sheets?
Sheets has no native MMWR function. With the date in A2, compute the MMWR year with =YEAR(A2-WEEKDAY(A2,1)+4), the start of week 1 with =DATE(y,1,4)-WEEKDAY(DATE(y,1,4),1)+1 (using the MMWR year as y), then the week number with =INT((A2-start)/7)+1. Or paste your column into the converter above and copy the result back.
How do I convert a date to an epiweek in Excel?
Use the same formula set as in Sheets: MMWR year =YEAR(A2-WEEKDAY(A2,1)+4), week-1 start =DATE(y,1,4)-WEEKDAY(DATE(y,1,4),1)+1, week number =INT((A2-start)/7)+1. Or convert a whole column at once with the free tool above and paste the results into Excel.
Can I convert a whole column of dates to epiweeks at once and paste the result back?
Yes. Paste the whole column (one date per line) into the converter, then click “Copy for Excel / Sheets” and paste into your spreadsheet. The columns arrive as tab-separated text, with each row including the date, MMWR week, MMWR start/end, and ISO week with start/end.