ISO 8601 week numbers

The ISO week date system numbers the weeks of the year from 1 to 52 or 53. Two rules define it: every week starts on Monday, and week 1 is the week containing the year's first Thursday (equivalently, the week containing January 4th).

Because of that, the first few days of January sometimes belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. A year has 53 ISO weeks when it starts on a Thursday, or on a Wednesday in a leap year — otherwise 52.

Example: Monday 30 December 2024 falls in ISO week 1 of 2025, because that week's Thursday (2 Jan 2025) is in 2025.

Want the whole year at a glance? See the week number calendar for 2026 — every week with its Monday–Sunday dates, with 2025 and 2027 a click away — or check how many weeks are in a year for the 52-vs-53 rule and a year-by-year table. The four calendar quarters (Q1–Q4) are broken down separately. And if you are wondering whether this year has a 29 February, see is 2026 a leap year.

Which dates a week number covers

A week number on its own — "week 23", "CW 40" — only means something once you know the dates behind it. The Dates in a week card above turns it around: type a year and an ISO week number and it gives you that week's Monday–Sunday range. Handy when a shipping schedule, payroll run, or sprint plan is written as a calendar week and you need the actual days.

Example: ISO week 23 of 2026 runs Monday 1 June to Sunday 7 June 2026. Ask for a week that doesn't exist (most years stop at 52) and the card tells you the year's highest week number instead.

Going the other direction — date to week number — is the What week is… card, and the 2026 week calendar lays every week and its dates out together.

Days and business days between two dates

Calendar days between two dates is simply the count of dates from the start to the end. Business days count only Monday to Friday, excluding weekends. This site does not subtract public holidays, since those vary by country and region — if your jurisdiction's holidays matter, subtract them yourself.

business days ≈ full weeks × 5 + leftover weekday count

Useful for delivery estimates, invoice due dates ("net 30"), notice periods, and project planning.

For the full how-to — the counting formula, worked examples, and a month-by-month table of business days in 2026 — see counting business days between two dates. If you only need the gap from today to a future date, the how many days until a date countdown handles that, working days included.

Adding or subtracting days from a date

Sometimes you don't want the gap between two dates — you want the date that lands a set number of days from a known one. "What date is 90 days from today?" or "60 days before the deadline?" The Add / subtract days tool above takes a start date and a day count and gives you the resulting date, its weekday, and the ISO week it falls in.

result = start date ± number of days

Counting is calendar days, weekends included — handy for "net 30" invoice dates, 90-day return windows, notice periods, and trial end dates. For working-days-only math, use the Add business days tool below.

Adding business days (working days) to a date

Many deadlines are counted in working days, not calendar days — "ships within 5 business days", "respond within 10 working days", "settlement is 2 business days after the trade". The Add business days tool above steps forward (or back) from a start date one weekday at a time, skipping every Saturday and Sunday, and gives you the resulting date, its weekday, and the ISO week it falls in.

result = start date, then step ± over Mon–Fri only

If your start date lands on a weekend, the first step jumps to the nearest weekday, so the count never "wastes" a day on a Saturday. Business days here means Monday to Friday and does not subtract public holidays — for a wider working-days picture across the year, see working days in 2026.

What day of the week a date falls on

"What day of the week was I born?" or "what day is Christmas on this year?" The Day of the week tool above takes any date and names the weekday — and tells you whether it's a weekday or a weekend, which day of the year it is, and the ISO week it sits in.

Handy for picking a meeting date, checking which birthday lands on a weekend, settling a "what day was it?" argument, or planning around a public holiday. Past, present, or far-future dates all work the same way.

Hours between two times (timesheets & shifts)

The Hours between times tool above takes a start and end time and tells you how long that span is — both as hours and minutes (8h 30m) and as a decimal (8.5 hours), the form payroll and invoices usually want. Enter an optional break in minutes and it's subtracted from the total.

decimal hours = total minutes ÷ 60 (e.g. 510 min = 8.5 h)

To turn minutes into decimal hours by hand: 15 min = 0.25, 30 min = 0.5, 45 min = 0.75. If the end time is earlier than the start, the span is treated as crossing midnight (a 22:00 → 06:00 night shift is 8 hours, not minus 16). Multiply the decimal hours by your rate to price a job, or add up a week of these for a timesheet total.

Working out an age (years, months and days)

An age is the time elapsed from a date of birth to today, expressed the way people actually say it: whole years, then leftover months, then leftover days. The Age / how old tool above also gives you the totals — your age in months, weeks and days — and counts down the days to your next birthday.

age = years, then months, then days remaining

Borrowing matters here: if today's day-of-month is earlier than your birth day-of-month, the tool borrows a month (using that month's real length), so it never tells you "0 months, −5 days". Leap-day birthdays roll to 1 March in common years. Pop in a future date instead and it becomes a plain days-until countdown.

Percent change

Percent change measures how much a value grew or shrank relative to its starting point. A positive result is an increase, a negative one is a decrease.

percent change = (new − old) ÷ |old| × 100

From 80 to 100 is a +25% change; from 100 to 80 is −20%. Note the two are not symmetric, because the denominator (the starting value) differs.

Percent of (the part as a share of the whole)

This answers "what percentage is X of Y?" — the share one number represents of another.

percent of = part ÷ whole × 100

15 out of 60 is 25%. Common for test scores, completion rates, and "X of the budget is spent".

Adding or removing a percentage (VAT, markup, discount)

Adding a percentage grosses a number up — used to add sales tax or VAT to a net price, or a markup to a cost.

gross = net × (1 + rate ÷ 100)

Removing a percentage goes the other way: it finds the net amount hidden inside a gross figure (a tax-inclusive price). This is not the same as simply subtracting the percentage.

net = gross ÷ (1 + rate ÷ 100)

A €120 price that includes 20% VAT contains €100 net and €20 VAT — because 120 ÷ 1.20 = 100, not 120 − 20%.

Margin vs. markup

These two are easy to confuse because the same sale produces different percentages depending on which you use. Both start from cost and selling price.

• Markup is profit as a percentage of cost.
• Margin is profit as a percentage of price.

markup = (price − cost) ÷ cost × 100 margin = (price − cost) ÷ price × 100

Buy at 50, sell at 100: that's a 100% markup but only a 50% margin. Margin is always the smaller of the two.

Compound annual growth rate (CAGR)

CAGR is the single steady yearly rate that would take a starting value to an ending value over a number of years, smoothing out the ups and downs in between.

CAGR = (end ÷ start) ^ (1 ÷ years) − 1

Growing from 1,000 to 2,000 over 3 years is a CAGR of about 26% per year — not 33%, because growth compounds on itself each year. The CAGR card above does this for you: enter the start value, end value, and number of years.

What ISO week number is it right now?

The current ISO week is shown right at the top of this page for today's date. ISO weeks always run Monday to Sunday, and week 1 is the week containing the year's first Thursday (the week that holds January 4th).

Want the full year laid out? See the 2026 week-number calendar.

What dates are in a given week number?

Pop a year and a week number into the Dates in a week card above and it hands back that week's Monday-to-Sunday range. Ask for a week the year doesn't have and it tells you the highest week number instead.

ISO week 23 of 2026 runs Mon 1 June – Sun 7 June 2026.

How many weeks are in a year?

Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or on a Wednesday in a leap year — otherwise 52.

There's a year-by-year breakdown on the weeks in a year page.

How do I count business days between two dates?

Use the days-between calculator above. Enter a start and end date and it returns the total calendar days and the number of business days — Monday to Friday, with weekends excluded.

For the wider context, see working days in 2026.

How do I add business days to a date?

Use the Add business days card above. Enter a start date and how many working days to add (or subtract) and it counts forward over Monday–Friday only, skipping weekends, then shows the resulting date and weekday. It's the tool for "5 business days from today" or a "within 10 working days" deadline. Note it doesn't subtract public holidays.

What day of the week was I born?

Use the Day of the week card above — type in your date of birth and it names the weekday instantly, along with whether it was a weekday or a weekend. It works for any date, past or future, so you can also check which day a future birthday or anniversary lands on.

How do I work out hours between two times?

Use the Hours between times card above: enter a start and end time and it gives you the span as hours-and-minutes and as decimal hours (so 09:00 to 17:30 is 8h 30m, or 8.5 hours). Add a break in minutes and it's taken off the total. An end time earlier than the start is read as an overnight shift.

How do I calculate percent change?

Percent change is (new − old) ÷ old × 100. Going from 80 to 100 is a +25% change; going back from 100 to 80 is −20%. The percent-change card handles both directions for you.

What's the difference between margin and markup?

Markup is profit as a share of cost; margin is profit as a share of the selling price. A 50% markup on a cost of 100 gives a price of 150 — which is a 33% margin. The margin ↔ markup card converts between the two.

Is anything I type sent to a server?

No. Every calculation runs entirely in your browser. Nothing you enter is uploaded, stored, or shared with anyone.