This period calculator predicts your next three periods, your ovulation day and your fertile window from the first day of your last period and your usual cycle length.
Predictions assume regular cycles. This calendar method is an estimate — it is not a reliable form of contraception, and if you conceive, our Due Date Calculator takes over from there.
How Cycle Prediction Works
A menstrual cycle is counted from the first day of one period to the first day of the next. Two anchor facts drive every prediction tool: the next period arrives one cycle-length after the last one began, and ovulation occurs about 14 days before the next period (the luteal phase is fairly constant, even when the first half of the cycle varies). The fertile window spans the five days before ovulation plus ovulation day itself, because sperm can survive up to five days while the egg lasts about a day. This calculator is part of our Health Calculators collection, with a deeper ovulation-focused tool at the Ovulation Calculator.
Worked Example (Verified)
Last period began October 1, 2026, with a 28-day cycle:
- Next period: October 29, 2026
- Ovulation: October 15, 2026 (28 − 14 days after the start)
- Fertile window: October 10–16, 2026
- Following periods: November 26 and December 24, 2026
How Accurate Are Period Predictions?
- Regular cycles (varying by a day or two): predictions are typically accurate within a couple of days.
- Variable cycles: only 10–15% of people have textbook 28-day regularity; stress, travel, illness, perimenopause and conditions like PCOS all shift timing — mostly by moving ovulation, since the first half of the cycle is the flexible half.
- Tracking beats guessing: log a few cycles and use your average length rather than 28. Basal temperature and ovulation tests pin ovulation far better than calendar math.
- Not contraception: calendar predictions alone have substantial real-world failure rates for pregnancy prevention. Treat this as planning information, not protection.
Related Tools
- Ovulation Calculator — conception-focused detail
- Due Date Calculator — from a positive test onward
- Health Calculators — the full collection
Frequently Asked Questions
How do I predict my next period?
Add your average cycle length to the first day of your last period. With a 28-day cycle and a period starting October 1, expect the next around October 29.
When do I ovulate?
About 14 days before your next period — day 14 of a 28-day cycle, day 17 of a 31-day cycle. This calculator works it out from your cycle length.
What is the fertile window?
The five days before ovulation plus ovulation day — the days when conception is possible, thanks to sperm survival of up to five days.
Is a 28-day cycle normal?
It is the textbook average; healthy cycles range roughly 21–35 days. What matters more is whether yours is consistent for you.
Why is my period late if the calculator says it is due?
Ovulation likely happened later than usual (stress, travel, illness) — the period follows ovulation by ~14 days, so a delay usually started two weeks earlier. If pregnancy is possible, test; if lateness is a pattern, see a clinician.
Is this period calculator free?
Yes — free, instant, no sign-up, and your dates stay in your browser.