Period calculator — your next six cycles
Predict your next six periods and premenstrual weeks from one date and your cycle length, with a printable calendar. Private by construction.
- Runs in your browser
- Nothing you enter is sent
- Free, no account
Nothing you type here is sent anywhere.
The dates are used by JavaScript running inside this page and are discarded when you close the tab. There are no cookies, no analytics and no account. Storage on this device is off unless you switch it on below, and you can delete it in one click.
First day of one period to the first day of the next.
Ovulation to the next period. Far steadier than cycle length — 14 days if you don't know.
ovulation = first day of last period + (cycle length − luteal phase) ovulation = 25 Dec 2025 + (28 − 14) days ovulation = 25 Dec 2025 + 14 days = 8 Jan 2026 fertile window = ovulation − 5 days … ovulation day fertile window = 3 Jan … 8 Jan next period = first day of last period + cycle length next period = 25 Dec 2025 + 28 days = 22 Jan 2026
| Cycle | Period starts | Period ends | Fertile window | Ovulation | PMS week from |
|---|---|---|---|---|---|
| 1 | Thu 25 Dec 2025 | Mon 29 Dec 2025 | 3 Jan – 8 Jan | Thu 8 Jan 2026 | Thu 15 Jan 2026 |
| 2 | Thu 22 Jan 2026 | Mon 26 Jan 2026 | 31 Jan – 5 Feb | Thu 5 Feb 2026 | Thu 12 Feb 2026 |
| 3 | Thu 19 Feb 2026 | Mon 23 Feb 2026 | 28 Feb – 5 Mar | Thu 5 Mar 2026 | Thu 12 Mar 2026 |
| 4 | Thu 19 Mar 2026 | Mon 23 Mar 2026 | 28 Mar – 2 Apr | Thu 2 Apr 2026 | Thu 9 Apr 2026 |
| 5 | Thu 16 Apr 2026 | Mon 20 Apr 2026 | 25 Apr – 30 Apr | Thu 30 Apr 2026 | Thu 7 May 2026 |
| 6 | Thu 14 May 2026 | Mon 18 May 2026 | 23 May – 28 May | Thu 28 May 2026 | Thu 4 Jun 2026 |
Six cycles ahead, on one printable page
The arithmetic is simple — add your cycle length to the first day of the last period, and repeat — but doing it six times in your head, across month boundaries and a leap year, is where it stops being simple. This page projects six cycles, marks them on a calendar and prints cleanly, which is what you actually want when you are booking a holiday, a wedding, a race or a medical procedure.
How cycle length is measured
From the first day of bleeding in one period to the day before bleeding starts in the next. Not from the day bleeding stops. Getting this wrong shortens every prediction by the length of your period, which is typically four to six days — enough to make the whole projection useless.
If your cycles vary, use the average of the last three to six, and treat the prediction as a centre point with a few days either side rather than a date.
What counts as normal
| Measure | Typical range | Worth mentioning to a clinician |
|---|---|---|
| Cycle length, adults | 21–35 days | Consistently under 21 or over 35 |
| Cycle length, first years after menarche | 21–45 days | Still irregular after about three years |
| Period length | 2–7 days | Over 7 days, or a sudden change |
| Variation between cycles | Up to 7–9 days | Sudden new irregularity |
| Bleeding | Manageable with normal products | Soaking through hourly, or clots larger than a 10p coin |
Bleeding between periods, after sex, or after menopause is always worth getting checked, regardless of what any calendar says.
The premenstrual week
The amber band on the calendar is the seven days before the next period is due — the late luteal phase, when progesterone falls and premenstrual symptoms cluster. Around 3 in 4 people who menstruate report some premenstrual symptoms; a much smaller group meet the criteria for premenstrual dysphoric disorder, which is a recognised diagnosis with specific treatments and is worth raising with a clinician rather than enduring.
Symptoms that begin at ovulation rather than a week before, or that do not stop when bleeding starts, are usually something other than PMS.
Things that shift a cycle
- Illness, particularly with fever, in the first half of the cycle
- Significant travel across time zones, and sustained sleep disruption
- Sharp changes in training load or body weight
- Acute stress — usually delaying ovulation rather than the period itself
- Starting, stopping or missing hormonal contraception
- Thyroid conditions and PCOS
Almost all of these act by delaying ovulation. Because the luteal phase is fixed, a delayed ovulation delays the period by exactly the same number of days — which is why the fertile window is shown on this page even though it is a period calculator. It explains the timing.
A tracker that cannot leak
Period-tracking apps have been the subject of repeated data-sharing scandals and regulatory action, because the model is to collect intimate data on a server and find ways to make money from it. This page has no server. The dates are held in the page while you use it and discarded when you close the tab, unless you deliberately switch on local saving — in which case they are written to your own browser and nowhere else, with a delete button next to the switch. There is no account to create, nothing to sync, and no third party involved.
Questions
How is the next period date worked out?
By adding your average cycle length to the first day of your last period, then repeating. Cycle length is measured from the first day of one period to the day before the next starts — not from the day bleeding stops.
What counts as a normal cycle length?
Anything from 21 to 35 days in adults, and up to 45 days in the first few years after menarche. Variation of a few days month to month is normal; a sudden persistent change is worth mentioning to a clinician.
When does PMS usually start?
In the luteal phase, most commonly in the week before bleeding starts — which is the amber band on the calendar here. Symptoms typically ease within a day or two of the period beginning.
Why does this show a fertile window on a period calculator?
Because the same two inputs produce it, and knowing where ovulation falls explains why a period arrives when it does. If you are trying to conceive, the ovulation calculator presents the same engine from that angle.
Do you store my dates?
Only if you switch it on, and only in your own browser. There is no account, no sync and no server — a period tracker that cannot leak your data because it never has it. The delete button clears the local copy instantly.