Week Number Calculator

Find the ISO or US week number of any date, the dates of any week, and this week's number.

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Week numbering

Date

Gives the week number of any date and the first and last day of any week, in the two numbering systems in common use: ISO 8601, used across Europe and much of Asia and by most planning software, and the US system that Excel's WEEKNUM uses by default.

How weeks are numbered

ISO 8601 weeks run from Monday to Sunday, and week 1 is the week containing 4 January — the week with the year's first Thursday. Every week has seven days, so the first days of January can belong to the last week of the year before, and the last days of December to week 1 of the next. The year a week belongs to is its week-year, written with the week and the weekday as 2026-W40-3 (Wednesday of week 40). A year has 52 or 53 weeks.

US weeks run from Sunday to Saturday, and week 1 is the week containing 1 January. The weeks at the start and end of the year stop at its edges, so they can be shorter than seven days, and every date belongs to its own calendar year. A year has 53 weeks, or 54 when a leap year starts on a Saturday.

Examples

29 December 2025

In ISO numbering this Monday is in week 1 of 2026, which runs 29 December 2025 – 4 January 2026. In US numbering it is week 53 of 2025.

1 January 2027

This Friday is in ISO week 53 of 2026 (2026-W53-5), because that week's Thursday is 31 December 2026. In US numbering it is week 1 of 2027, which has 2 days: 1–2 January 2027.

A 54th US week: 31 December 2028

2028 is a leap year that starts on a Saturday, so 1 January is a week of its own, and 31 December, a Sunday, is US week 54, a week of 1 day.

Week numbers for 2026 and 2027

ISO weeks of 2026

Week	Monday	Sunday
1	29 December 2025	4 January
2	5 January	11 January
3	12 January	18 January
4	19 January	25 January
5	26 January	1 February
6	2 February	8 February
7	9 February	15 February
8	16 February	22 February
9	23 February	1 March
10	2 March	8 March
11	9 March	15 March
12	16 March	22 March
13	23 March	29 March
14	30 March	5 April
15	6 April	12 April
16	13 April	19 April
17	20 April	26 April
18	27 April	3 May
19	4 May	10 May
20	11 May	17 May
21	18 May	24 May
22	25 May	31 May
23	1 June	7 June
24	8 June	14 June
25	15 June	21 June
26	22 June	28 June
27	29 June	5 July
28	6 July	12 July
29	13 July	19 July
30	20 July	26 July
31	27 July	2 August
32	3 August	9 August
33	10 August	16 August
34	17 August	23 August
35	24 August	30 August
36	31 August	6 September
37	7 September	13 September
38	14 September	20 September
39	21 September	27 September
40	28 September	4 October
41	5 October	11 October
42	12 October	18 October
43	19 October	25 October
44	26 October	1 November
45	2 November	8 November
46	9 November	15 November
47	16 November	22 November
48	23 November	29 November
49	30 November	6 December
50	7 December	13 December
51	14 December	20 December
52	21 December	27 December
53	28 December	3 January 2027

ISO weeks of 2027

Week	Monday	Sunday
1	4 January	10 January
2	11 January	17 January
3	18 January	24 January
4	25 January	31 January
5	1 February	7 February
6	8 February	14 February
7	15 February	21 February
8	22 February	28 February
9	1 March	7 March
10	8 March	14 March
11	15 March	21 March
12	22 March	28 March
13	29 March	4 April
14	5 April	11 April
15	12 April	18 April
16	19 April	25 April
17	26 April	2 May
18	3 May	9 May
19	10 May	16 May
20	17 May	23 May
21	24 May	30 May
22	31 May	6 June
23	7 June	13 June
24	14 June	20 June
25	21 June	27 June
26	28 June	4 July
27	5 July	11 July
28	12 July	18 July
29	19 July	25 July
30	26 July	1 August
31	2 August	8 August
32	9 August	15 August
33	16 August	22 August
34	23 August	29 August
35	30 August	5 September
36	6 September	12 September
37	13 September	19 September
38	20 September	26 September
39	27 September	3 October
40	4 October	10 October
41	11 October	17 October
42	18 October	24 October
43	25 October	31 October
44	1 November	7 November
45	8 November	14 November
46	15 November	21 November
47	22 November	28 November
48	29 November	5 December
49	6 December	12 December
50	13 December	19 December
51	20 December	26 December
52	27 December	2 January 2028

Frequently asked questions

In Excel and Google Sheets, ISOWEEKNUM(date) gives the ISO week and WEEKNUM(date) the US week. WEEKNUM(date, 2) starts weeks on Monday but still counts week 1 from 1 January, which is neither system: it differs from ISO around the new year.

Those that start on a Thursday, and leap years that start on a Wednesday — about one year in six. The next are 2026, 2032, 2037, 2043 and 2048.