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Learn more →Period Calculator — next 6 periods & cycle phase
Enter the first day of your last period, your usual cycle length, and how many days your period lasts. The calculator shows which cycle day and phase you're in today, your next period, the fertile window of each cycle, and your next six periods.
Cycle day 8
Phase Follicular
Next periodJune 27, 2026
Your next 6 periods
| Period | Fertile window | Ovulation |
|---|---|---|
| Jun 27 – Jul 1 | Jul 6 – Jul 12 | Jul 11 |
| Jul 25 – Jul 29 | Aug 3 – Aug 9 | Aug 8 |
| Aug 22 – Aug 26 | Aug 31 – Sep 6 | Sep 5 |
| Sep 19 – Sep 23 | Sep 28 – Oct 4 | Oct 3 |
| Oct 17 – Oct 21 | Oct 26 – Nov 1 | Oct 31 |
| Nov 14 – Nov 18 | Nov 23 – Nov 29 | Nov 28 |
Predictions assume regular cycles. If yours vary by more than a week, track a few cycles for a better estimate — and use this for planning, not as contraception.
How it works
How the menstrual cycle is counted
The cycle is counted from the first day of one period to the first day of the next. Day 1 is the first day of bleeding. An average cycle is 28 days, but anywhere from 21 to 35 days is normal, and your own length can shift a little from month to month.
Knowing your cycle length is what lets a calculator predict the next period: it simply adds your cycle length to the start of the last one, and repeats. Tracking the length of your period (the bleeding days) lets it show the whole period, not just the start date.
The four phases
Menstrual phase: bleeding, roughly days 1 to 5. Follicular phase: from the end of bleeding until ovulation, when an egg matures. Ovulation: the egg is released, usually about 14 days before the next period. Luteal phase: from ovulation to the next period, often when premenstrual symptoms appear.
Because ovulation is tied to the next period (about 14 days before it), longer cycles mean later ovulation, not a longer luteal phase. That's why the fertile window in this calculator moves with your cycle length.
When predictions are less reliable
This calculator assumes fairly regular cycles. Irregular cycles — common in the years after a first period, around perimenopause, with PCOS, thyroid issues, significant stress, or weight change — make any prediction approximate. If your cycle length varies by more than 7 to 9 days, treat the dates as rough.
Predicted fertile windows are for awareness, not birth control: the calendar method has a high failure rate. For contraception, use a medically reliable method; to conceive, ovulation tests or temperature tracking are more precise.
Frequently asked questions
›How does it predict my next period?
It adds your cycle length to the first day of your last period, then repeats for the following cycles. Accuracy depends on how regular your cycles are.
›What is 'cycle day'?
The number of days since your current period started, counting the first day of bleeding as day 1. It resets each time a new period begins.
›Can I use the fertile window for birth control?
No. The calendar (rhythm) method has a high failure rate — around 24% per year with typical use. Use a medically reliable method for contraception.
›Why does ovulation move when I change cycle length?
Ovulation happens about 14 days before the next period, so a longer cycle pushes ovulation later. The luteal phase stays roughly the same length.
›What counts as a normal cycle length?
Anywhere from 21 to 35 days is medically normal. Consistently shorter, longer, or very irregular cycles are worth discussing with a clinician.
›My cycles are irregular — is this useful?
It still gives a rough forecast, but predictions are less reliable. Tracking several cycles, or using a period-tracking app that learns your pattern, will do better.
›Does period length affect the prediction?
It doesn't change the next start date, but it lets the calculator show the full span of each period and export accurate calendar events.
›Is my data stored?
No. Everything is calculated in your browser; nothing is uploaded or saved on a server.
Sources & methodology
Recommended services
DisclosureRelated tools
This tool is for general information only and is not medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about your health.
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