Period Calculator

What Is a Period Calculator?

Two numbers describe a period and they measure different things. Swapping them moves every date the tool produces, so it is worth being precise about which is which before you type anything.

A Cycle Length and a Period Length Measure Different Things

Cycle length is the gap between the start of one period and the start of the next — first day to first day, bleeding days included. Period length is only the days you actually bleed. The World Health Organization puts the cycle at an average of 21 to 35 days, and the NHS says a period “can last between 2 and 7 days, but it will usually last for about 5 days.”

Inside the calculator the two fields do different jobs, and you can watch the split by changing one at a time. With a last period of August 17, 2026 and a 28-day cycle, the headline reads “Monday, September 14, 2026” whether you enter a 2-day period or a 7-day one. Only the second line moves: “Expected to last about 2 days (through Sep 15)” becomes “Expected to last about 7 days (through Sep 20)”.

Day 1 Is the Date You Type, Whatever You Meant by It

The US Office on Women’s Health states the convention plainly: “A menstrual cycle begins with the first day of your period, or menstruation.” Every day number on this page counts from there, which means day 1 is the start date itself and not the day after it.

The calculator has no opinion about what qualifies as a start. It takes the date in the box and adds to it, so a light spotting day logged as day 1 shifts the entire set. Entering August 16 instead of August 17 with the same 28-day cycle returns “Sunday, September 13, 2026” rather than “Monday, September 14, 2026”, and the ovulation estimate slides from Aug 31 back to Aug 30. Choose the first day of real flow and apply the same rule every month, because consistency matters more here than the definition does.

What Each of the Three Fields Controls

Three inputs, three separate jobs:

• First day of your last period — the anchor. Move it one day and all four output dates move one day with it.
• Average cycle length — the multiplier. It sets the gap to the next period, and the same gap is applied twice more for the two periods after that.
• Average period length — the end date, and nothing else. It changes the “through” date on the second line and leaves the three start dates untouched.

None of these boxes carries a minimum or maximum attribute, and the form is submitted with browser validation switched off, so nothing stops you typing 200 into the cycle field. The refusal comes from the formula, which replaces the result with an amber “Check your inputs” notice reading “Enter a cycle length between 15 and 60 days”.

What It Cannot Know

It cannot detect a pregnancy, confirm that you ovulated, or notice that you have been ill, sleeping badly, or flying between time zones. It projects the average you stated forward by one, two, and three cycles, and that is the whole of its knowledge.

The arithmetic itself runs in your browser rather than on a server. Both number boxes start empty, so the result panel holds its dash until you fill them in and press Calculate, and it will not update again as you type — press Calculate after any change. Like every page on this site, it also loads analytics and advertising tags.

For the fertile half of the same cycle, worked through in far more detail than this page has room for, use the Ovulation Calculator.

How Do You Calculate Your Next Period?

Four short pieces of arithmetic produce every date the panel shows, and none of them needs more than a calendar. Doing them by hand once makes the output far easier to judge.

The Period Formula

Next period     = day 1 of last period + cycle length
Second period   = day 1 of last period + (2 × cycle length)
Third period    = day 1 of last period + (3 × cycle length)
Bleeding ends   = next period start + period length − 1
Ovulation       ≈ next period start − 14 days
Fertile window  ≈ ovulation − 5 days  through  ovulation

Two of those lines deserve a note. The minus one on the end date is inclusive counting — a 5-day period that starts on September 14 covers the 14th through the 18th, not the 19th. And the 14 in the ovulation line is the luteal phase, the stretch between ovulation and the next bleed, held fixed at the textbook figure. The NHS is more careful about it than the formula is: ovulation, it says, happens “around 10 to 16 days before the next period.”

Calculating It Step by Step

By hand, in six moves:

• Write down the first day of your last period. That date is cycle day 1.
• Add your average cycle length in days. The result is the expected start of the next period.
• Add your period length to that start date and subtract one day for the expected last day of bleeding.
• Add two cycle lengths, then three, to the original day 1 for the two periods after that.
• Subtract 14 days from the next period’s start date for the estimated ovulation day.
• Count back five more days from ovulation to open the fertile window, which closes on ovulation day itself.

The calculator rounds both numbers to whole days before it starts, so a fractional average is not rejected — it is quietly rounded. Type 28.67 and the tool works with 29; type 28.4 and it works with 28. From an August 17, 2026 start those two produce “Tuesday, September 15, 2026” and “Monday, September 14, 2026” respectively.

Worked Example: A 28-Day Cycle Starting August 17

Inputs: last period began Monday, August 17, 2026; average cycle 28 days; average period 5 days.

• August 17 + 28 days = September 14. August has 31 days, so 14 of the 28 fall in August and the remaining 14 land in September.
• September 14 + 5 − 1 = September 18, the expected last day of bleeding.
• August 17 + 56 days = October 12, and August 17 + 84 days = November 9.
• September 14 − 14 days = August 31, the ovulation estimate — cycle day 15, counting August 17 as day 1.
• August 31 − 5 days = August 26, so the fertile window runs August 26 through August 31.

Panel headline

Monday, September 14, 2026

Underneath the headline the three breakdown rows read, word for word: “Expected to last about 5 days (through Sep 18)”, “Following periods: Oct 12 and Nov 9”, and “Estimated ovulation: Aug 31, fertile window Aug 26–Aug 31”.

The whole projection covers 84 days. Nothing in it looks past the average you stated, which is why the November date is a planning aid rather than a diary entry.

Worked Example: A 21-Day Cycle From the Same Date

Same August 17 anchor and the same 5-day period, but a 21-day average cycle instead of 28.

The panel returns “Monday, September 7, 2026”, followed by “Expected to last about 5 days (through Sep 11)”, “Following periods: Sep 28 and Oct 19”, and “Estimated ovulation: Aug 24, fertile window Aug 19–Aug 24”.

Seven days off the cycle length compounds three times over. Three cycles at 21 days span 63 days and finish on October 19; three at 28 days span 84 days and finish on November 9 — a three-week gap by the third prediction, out of a one-week difference in the average.

The fertile window is the more interesting change. August 19 is cycle day 3, so at this cycle length the window opens while a 5-day period is still running.

What the Calculator Refuses, and What It Rounds

Three guard messages exist. Each one replaces the result panel with the amber “Check your inputs” notice and prints its own line:

• Date left empty: “Enter the first day of your last period”.
• Cycle length below 15 or above 60 after rounding: “Enter a cycle length between 15 and 60 days”. A typed 14 is refused, 14.5 rounds up to 15 and passes; 60.4 rounds down to 60 and passes, 60.5 rounds to 61 and does not.
• Period length below 1 or above 15 after rounding: “Enter a period length between 1 and 15 days”. Zero is refused, 15 is accepted, 16 is not.

A blank cycle field triggers the cycle-length message rather than a “fill this in” prompt, because an empty box reads as zero and zero is below the floor. Nothing prevents a start date in the future or one from several years ago — both are accepted and added to exactly as typed, which has a consequence covered under the limits below.

Period Chart by Cycle Length

Where the key days fall across the cycle lengths people most often enter. Day 1 is the first day of your last period, so a 28-day cycle ends on day 28 and the next one opens on day 29.

Key Days by Cycle Length

All figures are cycle days, and the bleeding-end column assumes a 5-day period:

Two patterns run down the columns. Ovulation always lands on cycle day (length − 13), and the fertile window always opens on cycle day (length − 18). Both are fixed distances from the end of the cycle rather than from its start, which is exactly what a fixed 14-day luteal phase produces.

The day-15 figure for a 28-day cycle deserves a pause, because charts elsewhere commonly say day 14 — the NHS itself writes that women with “a regular, 28-day cycle are likely to be fertile around day 14 of their menstrual cycle.” That is a counting convention rather than a disagreement: a date 14 days after day 1 is day 15. Either way the difference is smaller than the genuine spread in ovulation timing.

The Same Chart as Calendar Dates

Identical rows with a real anchor — a period that began Monday, August 17, 2026, with 5 days of bleeding.

The spread widens as you read to the right. Across these seven cycle lengths the first predicted period covers a two-week span, September 7 to September 21. By the third prediction those same seven inputs are spread from October 19 to November 30 — six weeks apart, from inputs that differ by at most fourteen days.

That fanning-out is the practical argument for getting your average right rather than reaching for 28. One day of error in the input is one day of error in September and three days of error in November.

Period Length Moves One Date and No Others

Holding the August 17 start and the 28-day cycle fixed and changing only the bleeding days:

That is the whole of its influence — the ovulation line and the two following periods do not shift by a day. If you only want start dates, a rough period length costs you nothing. If you are packing for a trip, it is the number that decides whether the last day falls before you fly.

How to Read Your Result

The panel gives one headline and three supporting rows. They answer different questions and they do not carry equal confidence.

What Each Line Means

Reading the 28-day example from top to bottom:

• The headline is the next expected start date with its weekday spelled out — “Monday, September 14, 2026”. It is the sturdiest number on the page: one cycle length added to a date you supplied.
• “Expected to last about 5 days (through Sep 18)” is the bleeding window, counted inclusively from that start date.
• “Following periods: Oct 12 and Nov 9” are the two after it, spaced one cycle apart each. Any error in your average is doubled in the first and tripled in the second.
• “Estimated ovulation: Aug 31, fertile window Aug 26–Aug 31” is derived backwards from the predicted period rather than forwards from the date you entered, which is why it moves whenever your cycle length does.

The copy button beneath the panel takes all four lines at once, which is the quickest way to move them into a calendar app.

How Wide the Window Actually Is

A cycle length is an average of numbers that were never identical. A 2019 analysis in npj Digital Medicine covering 612,613 ovulatory cycles logged by 124,648 users of the Natural Cycles app put the mean cycle length at 29.3 days rather than 28 — worth knowing before you accept the default that most charts start from.

The effect on this page is direct and undamped. From an August 17 start, entering 26 predicts Saturday, September 12, and entering 30 predicts Wednesday, September 16: four days of difference in the average, four days of difference in the answer. Understating the average by a single day costs one day on the first prediction and three on the third — 28 returns “Following periods: Oct 12 and Nov 9” while 29 returns “Following periods: Oct 14 and Nov 12”.

So the practical reading is a multiplier, not a fixed margin. Whatever spread your own cycles show, the second prediction carries twice it and the third carries three times it. If your last three cycles ranged over four days, the November date is a twelve-day zone rather than a Monday.

When the Date Arrives and Nothing Does

A period a couple of days late against a calendar prediction is inside the noise. The projection assumes this cycle matches your average exactly, and ovulation running late for any reason pushes the bleed after it by the same number of days.

If pregnancy is possible, the NHS is specific about timing: “Pregnancy tests are most reliable from the first day of your missed period”, and if you do not know when the period was due, “do the test at least 21 days after you last had unprotected sex.”

If periods stop altogether rather than running late, the NHS advises seeing a GP after three consecutive missed periods, and ACOG lists “not having a period for 3 to 6 months” among the bleeding patterns that warrant investigation.

Planning With the Second and Third Dates

Three cycles is close to a quarter of a year — 84 days on a 28-day average, 90 on a 30-day one. That is far enough ahead to book travel around, and far enough that the error has had three cycles to accumulate.

The useful move is to plan against the earliest plausible date rather than the printed one. If the third prediction says November 9 and your cycles have run two days short before, treat the start of November as the zone. Supplies packed for the earliest plausible date are never wrong; supplies packed for the printed date sometimes are.

Limits: When This Prediction Does Not Apply

Calendar prediction has failure modes that are structural rather than unlucky. These are the ones that change what the numbers mean rather than merely shifting them.

The 14-Day Luteal Phase Is a Convention, Not a Constant

The ovulation and fertile-window lines rest entirely on one assumption: that exactly 14 days separate ovulation from the next period. The NHS puts the real figure at “around 10 to 16 days before the next period.” The npj Digital Medicine analysis measured it directly and reported a mean luteal phase of 12.4 days, with a 95% confidence interval of 7 to 17 days.

That same study measured the other half of the cycle — a mean follicular phase of 16.9 days, 95% CI 10 to 30 — which is why the convention survives at all. The luteal stretch really is the steadier of the two, just not steady enough to be treated as a constant. In practice the ovulation date on this page can be several days out even in a cycle whose period date lands exactly.

It Adds One Cycle — It Does Not Roll Forward to Today

The formula adds one, two, and three cycle lengths to whatever date sits in the box. It never compares that date against today, so a stale anchor produces stale answers with no warning. Enter January 1, 2026 with a 28-day cycle and the headline is “Thursday, January 29, 2026” — a date long gone, presented in exactly the same style as a future one.

Always enter the most recent period whose start date you actually know. If you are catching up after a gap in tracking, add cycle lengths yourself until you reach the present month, then use that date as the anchor and accept that the uncertainty has grown with every cycle you skipped.

Very Short Cycles Break the Fertile-Window Arithmetic

The fertile window is drawn six days wide ending on ovulation day, and ovulation is placed 14 days before the next period. On a short cycle those two rules collide with the period itself. At 23 days with 5 days of bleeding the window opens on cycle day 5, the last day of the bleed; at 21 days it opens on day 3, with three fertile days inside the period.

At 19 days the window reaches the anchor itself, and below that it leaves the cycle entirely. A 19-day cycle from an August 17 start returns “Estimated ovulation: Aug 22, fertile window Aug 17–Aug 22” — opening on the very date entered as day 1. An 18-day cycle opens it on August 16, one day before that. A 15-day cycle returns “Estimated ovulation: Aug 18, fertile window Aug 13–Aug 18”, four days before the anchor, with ovulation itself landing on cycle day 2 in the middle of the bleed. The guard accepts 15, so the page will show you that; showing it is not endorsing it.

Two further things make the true window wider than the drawn one. The NHS puts sperm survival in the fallopian tubes at “up to 7 days after sex”, while this tool opens its window only 5 days before ovulation. And the window hangs off a predicted date, so a period that arrives four days late takes the whole window with it.

Hormonal Contraception, Pregnancy, and the Years Around Menopause

Combined pills, patches, rings, hormonal coils and injections set bleeding timing themselves. Whatever pattern appears on them is a property of the method rather than of a cycle this arithmetic describes, so the three inputs have nothing meaningful to hold on to.

Age moves the number slowly but measurably: the npj Digital Medicine analysis found mean cycle length falling by 0.18 days per year of age between 25 and 45. MedlinePlus puts menopause “most often” between ages 45 and 55, and the years leading into it are precisely when cycle length stops behaving like an average. Pregnancy, breastfeeding, large weight changes and hard training blocks interrupt the pattern in the same way.

This Is Not a Contraceptive Method

Nothing here measures a hormone, a temperature, or a cervical sign. It is a projection of your own past average, and the ovulation line inside it is a subtraction from a date that has not happened yet. Reading the fertile window in reverse — as a set of days to avoid — stacks the luteal-phase assumption on the cycle-length assumption on a sperm-survival figure this tool draws narrower than the NHS does.

For contraception, use a method designed for it. For conception timing, treat the fertile window here as a starting point that ovulation tests or basal body temperature can sharpen considerably.

Irregular Cycles and When to See a Provider

Two respected authorities publish two different normal ranges, and the gap between them is worth knowing before you decide your own cycle is unusual.

What Counts as Normal, and Who Says So

ACOG states that “the normal length of the menstrual cycle is typically between 21 and 35 days” and that “a normal menstrual period generally lasts up to 7 days.” The WHO fact sheet gives the same 21-to-35-day span. The US Office on Women’s Health draws the line differently: periods “are still ‘regular’ if they usually come every 24 to 38 days,” and it advises talking to a doctor about cycles longer than 38 days or shorter than 24.

A 22-day cycle therefore sits inside ACOG’s normal range and outside the Office on Women’s Health’s — a genuine difference of convention, not a rounding artifact. It is a good reason to describe your pattern to a clinician rather than score it against whichever chart you found first.

Averaging Three Cycles Instead of Guessing

If your cycles vary, the number to enter is the mean of the last three, each measured first day to first day. Cycles of 26, 31 and 29 days average 28.67, which the calculator rounds to 29 and predicts from — “Tuesday, September 15, 2026” from an August 17 anchor.

Keep those three cycles recent. Cycle length drifts with age and with most of the disruptions listed above, so an average assembled from last year describes last year rather than this one. Note the shortest and longest of the three as well: the gap between them is the width you should mentally add to every date the panel prints.

The Patterns Worth a Clinic Visit

ACOG lists the following as abnormal uterine bleeding:

• Bleeding or spotting between periods.
• Bleeding or spotting after sex.
• Heavy bleeding during your period.
• Bleeding that soaks through one or more tampons or pads every hour.
• Bleeding that lasts more than 7 days.
• Menstrual cycles that are longer than 35 days or shorter than 21 days.
• Irregular periods in which cycle length varies by more than 7 to 9 days.
• Not having a period for 3 to 6 months.

The NHS adds a shorter trigger for one of those: see a GP if you have missed three consecutive periods, and also for bleeding between periods, bleeding after sex, or any bleeding after menopause. On pain, the WHO reports that more than two out of three women and girls have pain with menstrual bleeding — common, certainly, but pain that stops you working or sleeping is still worth raising.

The 7-to-9-day variation figure is the one that decides whether this calculator is useful to you at all. Inside it, a calendar projection is worth reading and acting on. Outside it, the arithmetic still runs and still prints a weekday, but what it prints is a guess in formal clothing.