Period Calculator
Result
Next period starts
- Days until it starts
- 5 days
- Cycle day today
- 24 days
- Cycle length you entered
- 28 days
- Current period ends
- September 14, 2026
- Next period ends
- October 12, 2026
A period calculator answers one question exactly, and refuses to answer the neighbouring ones. Give it the first day of your last period, how long your menstrual cycle length usually runs, and how many days your period length lasts, and it returns the next period date as a calendar date, the number of days remaining until it, which cycle day you are on today, and the end dates of both the current period and the next one. That is the whole page, and the restraint is the point. A period calendar built this way is arithmetic on a straight line rather than a forecast: the cycle is projected forward at one fixed length, which is what makes the answer reproducible and checkable against a paper diary. It also means the page keeps working when your last period was months ago, because the projection steps over the whole cycles that have already gone by and reports where you are in the current one instead of printing a date that has already passed. What it will not do is guess at ovulation or a fertile window. Those turn on the length of the luteal phase, which varies between people and between cycles, and they get their own page rather than a second answer here.
Where a cycle length sits against the usual range
| Cycle length | Range | What it means |
|---|---|---|
| Shorter than usual | Fewer than 21 days from the first day of one period to the first day of the next | Cycles this short still usually involve ovulation, but they leave less time between one bleed and the next. ACOG counts anything under 21 days as abnormal, which is a statement about when to ask a clinician rather than a diagnosis in itself — a single short cycle after an illness or a stretch of poor sleep is not the same finding as a pattern that has held for months |
| In the usual range | 21 to 35 days | The range ACOG gives for a normal cycle. Almost everything between three and five weeks belongs here, and 28 days — the figure most people have heard — is the middle of it rather than the rule. Two cycles of 24 and 33 days both land in this band, which is worth remembering before reading a friend's cycle length as evidence of anything |
| Longer than usual | More than 35 days | ACOG counts cycles longer than 35 days as abnormal. Cycles in this range are common in the years after periods begin and again in the years before menopause, when ovulation becomes less regular; they are also a normal consequence of some forms of hormonal contraception. A cycle that has moved into this range and stayed there is worth raising with a clinician rather than averaging into a number |
The three bands are the ones the badge above is drawn from, and the boundaries are the published ones: 21 and 35 days, from the ACOG guidance cited below. Read the range column against the cycle length you entered rather than against the calendar — a cycle that sits just inside a band is not safer than one just outside it, and the bands are a way of reading a number rather than a verdict on a person. Two things the table cannot tell you. It says nothing about whether a single cycle is unusual for you, which is the question that actually matters clinically: a 33-day cycle is inside the usual range for everybody and can still be a change worth mentioning if your own cycles have always been 27. And it says nothing about the bleeding itself, which is assessed on its own length, heaviness and timing.
Formula
cycle day = ((days since the first day of your last period) mod cycle length) + 1; next period date = the current cycle's start + cycle length; next period end = next period date + period length − 1
- last period start
- Day one of the cycle: the first day you bled, not the day the bleeding stopped. Both ACOG and MedlinePlus count a cycle from the first day of one period to the first day of the next, so this field is the anchor every other number is measured from — enter the day it ended and the whole projection shifts forward by the length of your period
- cycle length
- Your usual number of days from one first day to the next. 28 is the figure everyone has heard, but it is the middle of a range rather than the rule: ACOG puts a normal cycle at 21 to 35 days (days)
- period length
- How many days the bleeding itself lasts. It only moves the two end dates; the next period date does not depend on it. MedlinePlus describes a usual period as three to seven days (days)
- mod
- The remainder after division. It is what makes the page work when your last period was months ago: the cycles that have already finished are discarded and what is left is how far into the current one you are
- cycle day
- Where you are now, counted from day one of the current cycle. Its range is always 1 up to your cycle length, and it is the one number on the page that moves every day without you changing anything (days)
Use it when you know the first day of your last period and want the next one dated — to book something around it, to know whether a trip will land on it, or simply to see how regular you have been without opening an app. Use it together with a written record rather than instead of one: the accuracy of the projection is exactly the accuracy of the cycle length you type in. For the reverse question — how many days between two dates, or what date is a given number of days away — the date calculator on this site takes the same arithmetic without the cycle framing.
Worked examples
A 28-day cycle starting on 1 January
- The cycle runs 1 January to 28 January, and 29 January is day one of the next one
- 14 days have passed since 1 January, and 14 ÷ 28 leaves a remainder of 14, so today is cycle day 14 + 1 = 15
- 14 days remain until 29 January
- The period that started on 1 January covers 5 days and ends on 5 January — the start day counts, so it is +4 rather than +5
- The next period runs 29 January to 2 February by the same rule
The combination the page opens with. The one figure worth pausing on is the cycle day: it is 15, not 14, because the first day of bleeding is day one of the cycle. Counting from zero is the single most common way to be a day out, and it is also why the range of this output is 1 to your cycle length rather than 0 to cycle length minus one.
A 30-day cycle going back to 20 December
- 20 December plus 30 days is 19 January — the addition crosses the end of the year
- 26 days have passed, and 26 mod 30 is 26, so today is cycle day 27
- 4 days remain until 19 January
- The period that began on 20 December ran 6 days and ended on 25 December
- The next one runs 19 January to 24 January
The case that shows the two date conventions on the same screen. The cycle crossed from one year into the next and the period did not, and both are printed as ordinary calendar dates. A page that computed the end of a period as start plus length rather than start plus length minus one would be a day out here, and a day out on this output is the kind of error a reader notices immediately and cannot reconcile with the arithmetic they did themselves.
A 36-day cycle
- 1 January plus 36 days is 6 February
- 14 days have passed, so today is cycle day 15 — the same cycle day as the first example, because the elapsed time is the same
- 22 days remain until 6 February
- The current period ended on 5 January and the next runs 6 February to 10 February
The same inputs as the first example apart from the cycle length, and it makes the point that the cycle day is not a measure of how far through a month you are. Both examples sit on day 15; one is halfway through a 28-day cycle and the other is not yet halfway through a 36-day one. This is also the case where the badge below the result changes to the long-cycle band, and 36 is one day past the threshold — at 35 the answer is the usual range.
A last period three months ago
- 75 days have passed since 1 November
- 75 ÷ 28 is 2 whole cycles with a remainder of 19, so the current cycle began 19 days ago and today is cycle day 20
- Adding 28 days to that cycle's start gives 24 January
- That start is 27 December, and 5 days from it ends on 31 December
- The next period runs 24 January to 28 January
The example that explains why the page asks for a start date rather than telling you to come back with a recent one. The cycles between November and now have already happened, so they are skipped; what is left is where you stand in the cycle you are actually in. Read against a wall calendar, this is also the answer to a question people ask about this kind of arithmetic — whether a cycle-tracking calculation can go wrong by counting from the wrong month. It can, if the whole cycles are not removed, and the figure that gives it away is a cycle day larger than the cycle length.
Limitations
This page projects a line, and the line is only as good as the assumption behind it: that every cycle is the same length. Real cycles move. ACOG's own guidance singles out variation of 7 to 8 days or more between cycles as one of the things that makes a cycle irregular, which is a normal amount of drift for many people and enough to move a projected date by a week. So read the next period date as the centre of a window rather than a fixed appointment, and read the cycle day as an estimate that gets less reliable the further the current cycle has run. Second, the page's idea of today is the calendar date in UTC, not in your own time zone. For most of the day those agree; for the hours around midnight in the Americas and in Asia they can be a day apart, and a cycle day that is one out is the result. Third, the two end dates assume the bleeding lasts exactly as long as you entered. Periods that are shorter or longer than usual are common and the arithmetic will not flag them, which is why the field has an upper bound of 14 days even though MedlinePlus describes a usual period as three to seven: a longer one is entered, not rejected. Finally, nothing here is a diagnosis or a substitute for one. The bands below describe where a cycle length sits against the range ACOG publishes; they are a way of reading the number you typed, not an assessment of your health. Cycles that are consistently shorter than 21 days or longer than 35, bleeding that lasts more than 7 days, bleeding between periods, or a cycle that has changed and stayed changed are all things to raise with a clinician rather than to adjust on a web page.
Frequently asked questions
- How accurate is a calculated next period date?
- It is exactly as accurate as the cycle length you enter, and no more. If your cycles have all been 28 days for years, the projection will land on the right day most months. If they range from 26 to 31, then a projection made at 28 is the middle of a five-day window and should be read as one. The honest way to use it is to type in the average of your last few cycles rather than the one you remember best, and then to treat the answer as the centre of a window. This page will not tell you the window's width, because it has not seen your history — if you have a record of the last six cycles, the spread between them is the number that matters.
- Why does this page not show my fertile window?
- Because a fertile window cannot be projected from a cycle length alone. It depends on when ovulation happens, and ovulation is dated backwards from the next period by the length of the luteal phase — a phase that is fairly stable within one person and varies considerably between people, and which shifts with age, illness and disrupted sleep. Printing a window here would mean printing a five-day span as though it were a fact, when the underlying variable is one this page does not take as input. That calculation has its own page on this site, with its own explanation of what it can and cannot claim.
- Should the cycle length be an average?
- Yes, and it is worth taking a moment over. Add up the number of days from the first day of one period to the first day of the next, across your last three to six cycles, and divide. Do not count from the end of one period to the start of the next — that skips the days of bleeding and will come out several days short. If your cycles vary by more than a week between the shortest and the longest, a single average is a poor summary and the projection will be wrong in both directions at different times; that pattern is what clinicians mean by irregular periods, and it is worth describing to one rather than averaging away.
- How long should a period last?
- Three to seven days covers most people, and ACOG describes a period as usually lasting up to 7 days. Bleeding that regularly runs longer than that is not just inconvenient — it is one of the patterns clinicians look for, because prolonged bleeding is a common route to iron deficiency. This page asks for the length only so it can print the two end dates; it does not judge the number, and it will accept anything from 1 to 14 days. If the figure you are typing in is consistently at the top of that range, that is information about your health rather than about your arithmetic, and it belongs in a conversation with a clinician.
- What does it mean if my cycles are irregular?
- It means the projection on this page is less useful to you than it is to someone whose cycles are steady, and that is all it means on its own. Cycles commonly vary with age — they are often long and uneven in the first years after they begin, and they shorten and then spread out again in the years before menopause. They also respond to weight change, intense training, illness, travel across time zones and disrupted sleep. What matters clinically is not that a cycle varies but the pattern: cycles that are consistently shorter than 21 days or longer than 35, cycles that stop, or a stable pattern that changes and stays changed. Each of those is a reason to see someone, and none of them is something a calendar can settle.
- Can I use the result to plan around a holiday or a wedding?
- For planning a date you can move, yes — and the useful habit is to plan around the window rather than the date. If your cycles vary by three or four days, then a projection that lands on the 12th means the 9th to the 15th, and booking something for the 12th without allowing for that is where this kind of arithmetic lets people down. For anything you cannot move, the answer is not a better calculation, it is a form of contraception or management that does not depend on predicting the date at all. A projected period date is a reasonable planning tool and a poor basis for a decision you cannot revisit.
References
- Abnormal Uterine Bleeding — Frequently Asked Questions — American College of Obstetricians and Gynecologists (ACOG), FAQ095, reviewed August 2025 — the source of this page's 21-to-35-day normal range and of the 7-day figure for a usual period
- Menstruation — MedlinePlus, U.S. National Library of Medicine — defines a cycle as counted from the first day of one period to the first day of the next, which is the convention every field on this page assumes, and gives three to seven days as the usual length of a period