Ovulation & Menstrual Cycle Calculator
Estimate the six-day fertile window, likely ovulation day, and next expected period — or switch to Cycle Tracker mode for a multi-month forecast of periods, fertile windows, and ovulation dates.
Last updated: 2026-03-27
Ovulation & menstrual cycle calculator
Enter your values
Estimate your fertile window and ovulation day, or switch to Cycle Tracker mode for a multi-month forecast of periods and fertile windows.
Cycle Estimate
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Enter your last period date and average cycle length. Use Single mode for the next fertile window, or Cycle Tracker for a multi-month forecast.
Calculation History(0)
Example calculations
Tap an example to prefill the calculator with sample values.
Typical 28-day cycle
LMP on March 1 with a 28-day cycle
A common planning estimate for a fairly regular cycle.
Result: The fertile window lands about two weeks before the next expected period.
Longer cycle
32-day cycle with a 6-day period
Useful when ovulation tends to happen later in the cycle.
Result: The fertile window shifts later because the expected next period shifts later too.
6-month forecast
Plan ahead with a 28-day cycle over 6 months
Use Cycle Tracker mode to see upcoming periods, fertile windows, and ovulation dates for the next 6 cycles.
Result: A 6-cycle forecast with predicted period dates, fertile windows, and ovulation days through August.
How the fertility estimate works
The calculator treats the first day of your last period as cycle day 1, adds your average cycle length to estimate the next period, and then works backward 14 days to estimate ovulation. The 14-day luteal phase is the standard assumption used in clinical fertility planning.
The fertile window is shown as the five days before ovulation plus ovulation day itself — reflecting the typical lifespan of sperm. In Cycle Tracker mode, the same logic repeats across multiple future cycles to produce a forecast calendar.
This is a planning estimate only, not a diagnosis, contraception method, or substitute for professional medical advice. Cycle timing varies — these projections assume consistent cycle length.
Menstrual cycle & fertility FAQs
Use these answers to interpret cycle-based estimates carefully, especially if your cycle timing is irregular.
How is ovulation estimated here?
The calculator estimates ovulation as roughly 14 days before the next expected period (the standard luteal phase assumption), then marks the six-day fertile window that includes the five days before ovulation and ovulation day itself.
What if my cycles are irregular?
This tool works best for planning around fairly regular cycles (21-35 days). If your cycle length varies meaningfully month to month, treat the results as rough guidance rather than a precise medical prediction.
How do I calculate my luteal phase?
The luteal phase runs from ovulation to the start of your next period. For most people it is around 14 days. This calculator assumes a 14-day luteal phase and derives ovulation from that. The follicular phase (period to ovulation) is the variable part of the cycle.
What is a normal cycle length?
Most cycles fall between 21 and 35 days, with 28 days as the textbook average. Cycles shorter than 21 days or longer than 35 days may warrant a conversation with a healthcare provider.
Can I use the cycle tracker for contraception?
No. Calendar-based fertile window estimates are not reliable enough for contraception. Ovulation can shift unpredictably. This calculator is a planning tool, not a medical device.
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