Period & Ovulation Calculator
Predict fertile window and next menstrual period dates.
This period calculator (also an ovulation calculator) predicts your next menstrual period start date and your most fertile window, based on your last period date and typical cycle length. It works as a period cycle calculator and a menstrual period calculator in one.
Enter the start date of your last period and your average cycle length, and the calculator returns your predicted next period and fertile window.
Worked Calculation Examples
| Scenario | Result | Calculation Step |
|---|---|---|
| Last period started March 1, 28-day cycle | Next period: March 29 · Ovulation: March 15 · Fertile window: March 10-15 | Next period = March 1 + 28 days = March 29. Ovulation = March 29 − 14 days = March 15. Fertile window = March 15 − 5 days through March 15. |
| Last period started March 1, 32-day cycle | Next period: April 2 · Ovulation: March 19 · Fertile window: March 14-19 | Next period = March 1 + 32 days = April 2. Ovulation = April 2 − 14 days = March 19. A longer cycle shifts ovulation later, but the 14-day gap to the next period stays the same. |
How Ovulation Day Is Estimated
Ovulation typically occurs about 14 days before the next period starts, regardless of total cycle length - so in a standard 28-day cycle, ovulation falls around day 14, while in a 32-day cycle it would fall closer to day 18. This calculation relies on the luteal phase - the second half of the menstrual cycle, from ovulation to the next period - which reproductive research has found stays relatively consistent at 12-16 days regardless of overall cycle length, which is why ovulation is estimated by counting backward from the next expected period rather than forward from the last one.
The Fertile Window Explained
The fertile window spans roughly 5 days before ovulation plus the day of ovulation itself, since sperm can survive in the reproductive tract for several days while an egg is viable for about 24 hours after release - this gives about a 6-day window each cycle when conception is possible.
Common Mistakes When Tracking Cycles
The most common mistake is using a single unusual cycle's length as the average rather than tracking several cycles and using a typical or median length - cycle length can vary from month to month for many people, and a single outlier throws off the whole prediction. A second is counting forward 14 days from the last period to estimate ovulation instead of counting backward from the next expected period - these only give the same answer when the cycle is exactly 28 days. A third is assuming the fertile window is a single day rather than a roughly 6-day span.
Sources & References
The luteal phase length (12-16 days, averaging 14) used to estimate ovulation is a well-established finding in reproductive physiology research and is the basis for standard fertility awareness and cycle-tracking methods referenced by organizations such as ACOG and the NIH's National Institute of Child Health and Human Development.
How to Use the Period & Ovulation Calculator
- Open the Period & Ovulation Calculator and enter the values requested in the input fields.
- Check the units, percentages, dates, or time periods before reading the answer.
- Review the instant result and adjust any value to compare another scenario.
- Use the formula, example, and FAQs below to understand how the period calculator works.
Frequently Asked Questions
How is ovulation day calculated?
Ovulation is estimated to occur about 14 days before your next period starts, based on your typical cycle length and the start date of your last period.
What is the fertile window?
It spans roughly the 5 days before ovulation plus the day of ovulation itself, reflecting how long sperm can survive and the roughly 24-hour lifespan of a released egg.
Does this period calculator work with irregular cycles?
It works best with reasonably regular cycles, since predictions rely on your typical cycle length - irregular cycles make ovulation timing harder to predict precisely, and tracking multiple cycles over time can improve accuracy.
How do I use this period calculator?
Enter the known values, review the units or settings, and the calculator updates the result instantly. The formula and example on this page show how the answer is produced.
What does the Period & Ovulation Calculator calculate?
Predict fertile window and next menstrual period dates. It is designed for fast browser-based calculations without sign-up, downloads, or manual spreadsheet setup.
What formula does this period & ovulation use?
The formula is: Next Period = Last Period Start + Cycle Length; Ovulation Day = Next Period − 14 days. Last Period Start is the first day of your most recent period, and Cycle Length is the number of days from that start date to the start of your next period (commonly 21-35 days, averaging around 28). Adding Cycle Length to the start date projects the next period. Ovulation Day is then estimated by counting back 14 days from that projected next period - not forward from the last one - because the luteal phase (ovulation to next period) is more consistent across individuals than the follicular phase (period to ovulation). The fertile window spans roughly 5 days before ovulation through the day of ovulation itself.
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Last updated: September 29, 2026.