personal cycle forecast · Generated Your 6-month forecast
Predicted period days, fertile window and ovulation — based on your average cycle.
Upcoming important dates — next 12 cycles
The direct answers first
Next period = first day of your last period + your average cycle length. From the same two inputs, the calculator estimates ovulation (about 14 days before the next period) and the 6-day fertile window ending on ovulation day. Everything runs in your browser — no account, nothing uploaded. Full formulas and their limits: methodology.
How to read your results
The calculator returns four answers, each with a different level of certainty. Your next period date is a projection from your own inputs — treat it as the center of a window of a few days. Estimated ovulation assumes a 14-day luteal phase and carries the same caveat. The fertile window marks the 6 days when conception is possible — useful for planning, never for contraception. And “Today” shows your cycle day with an estimated phase label — a model reading, not a measurement.
The projected start of your next period, plus a countdown and the next six cycles — shown in the calculator above.
The center estimate and its realistic range — and how to confirm instead of estimate.
Ovulation calculator →The 6 days ending on ovulation day, and what each day’s probability looks like.
Ovulation & fertile window →Where today sits in your cycle — and what a phase label can and can’t tell you.
Cycle phase calculator →Count it right: from the start, not the end
The most common counting question: do you count from the start of your period or the end? Always from the start. Day 1 is the first day of full flow — spotting before real flow does not count — and your cycle length is the number of days from one day 1 to the next day 1.
Do
- Count day 1 = first day of full flowSpotting before real flow is not day 1.
- Count start-of-period to start-of-periodThat gap in days is your cycle length.
- Not sure if you are a 28 or a 30?The helper above averages your past start dates, or use the cycle length calculator for the full breakdown.
Don’t
- Count from the day your period endedMakes your cycle look ~a week shorter — every prediction lands early.
- Count pre-period spotting as day 1Stretches your apparent cycle and shifts every prediction late.
- Default to 28 without checking28 is the textbook figure; your real average is probably not 28.
How the calculation works
The math follows the standard calendar method: next period = day 1 of your last period + your average cycle length, and each future cycle is projected the same way. Ovulation is estimated 14 days before each predicted period, because the luteal phase (the stretch after ovulation) is the stable half of the cycle — stable, not fixed: real-world data on 600,000+ cycles puts it at 10–15 days. The fertile window is the 6 days ending on ovulation day (Wilcox 1995).
| Event | Falls on | How it is derived |
|---|---|---|
| Last period starts | Day 0 | Your anchor input (first day of full flow) |
| Estimated ovulation | Day 14 | Predicted next period − 14 |
| Fertile window | Days 9–14 | The 6 days ending on ovulation day |
| Next period expected | Day 28 | Day 0 + your average cycle length |
| Period after that | Day 56 | Same projection repeated cycle by cycle |
| Your cycle length | Next period falls about | Estimated ovulation (next period − 14) |
|---|---|---|
| 21 days | 3 weeks after your last period started | Day 7 |
| 24 days | About 3.5 weeks later | Day 10 |
| 28 days | About 4 weeks later | Day 14 |
| 30 days | About 4.3 weeks later | Day 16 |
| 35 days | 5 weeks later | Day 21 |
You may also see this kind of tool called a menstrual calculator, a menstrual cycle calculator, a next period calculator or a period date calculator — different names, same method and same inputs. Whatever you call it, what matters is a good anchor date and an honest cycle length.
What a normal cycle looks like
A cycle is counted from day 1 of one period to day 1 of the next — and “regular” has a real definition: your shortest and longest cycles over a year staying within about 7–9 days of each other (FIGO). Outside these ranges isn’t automatically dangerous, but it is the point where a logged history and a clinician conversation beat any calculator. Your own numbers: the cycle length calculator averages your past dates and shows the spread.
Which period tool do you need?
That is this page — the calculator above shows the predicted date with a live countdown and the next six cycles.
Back to the calculator ↑See exactly how many days late you are and what each count usually means.
Late period calculator →Single dates will keep failing you — predict an honest range instead.
Irregular period calculator →Log start dates on this device; your real average builds itself.
Period tracker →A printable 12-month view of predicted periods and fertile windows.
Period calendar →Ovulation timing and the 6-day fertile window, with the method and its limits explained.
Ovulation & fertile window →Your cycle day and the estimated phase — and what a phase label can and can’t tell you.
Cycle phase calculator →What a prediction can and can't tell you
A prediction is a window, not a fixed day. Even in regular cycles the luteal phase runs 10–15 days, so ovulation — and the expected period date — can shift by several days with stress, illness, travel or medication changes. A calendar estimate cannot confirm that ovulation actually happened, and it must never be used for contraception.
If your predicted date has already passed, the reading changes: the calculator shows how many days late you are, and the late period calculator explains what each day count usually means. Accuracy improves the most when predictions run on your own logged average rather than the generic 28 — log each period as it arrives and the calculator offers your personal number.
Private by design
Most cycle tools ask you to hand intimate health data to a server — and app-based trackers have repeatedly made headlines, and drawn regulator action, for sharing exactly this kind of data with third parties. We deliberately do not. Every calculation happens in your browser; logged periods and preferences are stored only on your device, and closing this site sends nothing to us.
That makes this page safe to use on any device, anywhere, without an account — and it is the promise we consider non-negotiable. Details are in our privacy policy.
How it works
Enter your last period
Pick the first day of your most recent period — the day full bleeding started, not spotting.
Set your cycle length
Not sure? Use the built-in helper: enter a few past start dates and we compute your personal average.
Get instant answers
Next period date, ovulation day, fertile window and a 6-month forecast — plus a heads-up if you're running late.
Log it, and it gets personal
When your period actually starts, come back and tap "Log this period". After two or three logs we learn your real cycle length — not the textbook 28 days — and your predictions start using it.
Frequently asked questions
Is my period late?
If today is past your predicted start date, the calculator shows how many days late you are, based on your average cycle. A single late cycle is common and can be caused by stress, travel, illness, weight change or coming off birth control. If there is any chance of pregnancy and your period is about a week late, take a pregnancy test — the pregnancy test calculator covers timing and what results mean. If your period has been absent for three months or more, that is missed-period territory — the missed period calculator explains the thresholds, and a clinician visit is due.
How do you calculate my next period?
Your next period is estimated by adding your average cycle length to the first day of your last period. For example, if your last period started on August 1 and your cycle is 28 days, your next period is expected around August 29. Each future cycle is projected the same way.
How is ovulation calculated?
Ovulation is estimated as 14 days before your predicted next period. This luteal-phase method is used because the second half of the cycle is far more stable in length than the first half. The fertile window covers the five days before ovulation plus ovulation day itself, since sperm can survive up to five days.
What if my cycles are irregular?
Switch to irregular cycles mode and enter your shortest and longest typical cycle lengths. Instead of single dates you get predicted ranges that widen further out. If your cycles vary by more than 7–9 days, or are consistently shorter than 24 or longer than 38 days, it is worth discussing with a clinician. See our period calculator for irregular periods.
How accurate is a period calculator?
A prediction is a window, not a fixed day. Even in regular cycles the luteal phase runs 10–15 days, so ovulation — and the expected period date — can shift by several days. Accuracy depends above all on how consistent your cycles are, and estimates can move with stress, illness, travel and medication changes.
Do you store my data?
No. Every calculation runs locally in your browser. Your inputs are remembered on this device only using local storage, and nothing is ever sent to a server. And if you want no digital footprint at all, the printable period tracker is a free PDF you keep on paper — nothing to store, nothing to leak.
What happens when I tap "Log this period"?
The start date you entered is saved on this device only. Log each period as it arrives; once two or more periods are logged, the calculator works out your personal average cycle length and offers a one-tap "Use it" button, switching predictions from the generic 28-day guess to a number based on your own cycles.
Where is my period history stored? Can I delete it?
Your history lives only in this browser on this device using local storage. There is no account and nothing is sent to a server. "Clear history" removes every logged date with one tap, and clearing browser site data removes it too.
Can I use this as contraception?
No. Calendar-based predictions are not reliable enough to prevent pregnancy. If you need contraception, speak with a health care provider about proven methods.
What is a normal cycle length?
A typical adult cycle runs 24 to 38 days (FIGO), measured from the first day of one period to the first day of the next; for teens, 21 to 45 days counts as normal (ACOG). Cycles are considered regular when the difference between your shortest and longest cycle over a year stays within about 7–9 days (FIGO). See the NHS overview of periods for the clinical picture.
Is a menstrual calculator different from a period calculator?
No. Menstrual calculator, menstrual cycle calculator, next period calculator, period date calculator and period tracker are all names for the same kind of tool: enter your last period's first day and your cycle length, get your next period date plus ovulation and fertile-window estimates. This page covers all of those searches with one calculator.
Is my period counted from the start or the end?
From the start — always. Day 1 is the first day of full flow, not spotting, and your cycle length is the count of days from one day 1 to the next. Counting from the day your period ended makes your cycle look about a week shorter than it is, so every prediction lands early.
How do I know if my cycle is 28 days or 30 days?
Count the days from the first day of one period to the first day of the next, across at least 3 cycles, and average them. The helper above does this from your past start dates, and the cycle length calculator shows your average, shortest and longest. Only default to 28 if you have no data — most people are not exactly 28.
What is the 7-2-1 rule for periods?
A screening rule of thumb for heavy menstrual bleeding, not a prediction method: bleeding that lasts more than 7 days, soaking a pad or tampon every 2 hours or less, or clots larger than about 1 inch. Any one of the three is worth mentioning to a clinician — especially if it is new for you.