estimate your next period, ovulation day & fertile window
Privacy-first: your entries stay on your device only. No account, no cloud sync, no tracking.
Your cycle details
Your Cycle Estimate
Next period—
Estimated ovulation—
Fertile window—
Your cycle calendar
Sun
Mon
Tue
Wed
Thu
Fri
Sat
Period
Fertile window
Ovulation
Cycle history
Log past period start dates and this tracker will calculate your actual average cycle length from real data, instead of a guess.
Why track your cycle?
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Predict your cycle
Know when your next period will start so you're never caught off guard.
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Identify ovulation
Track your fertile window for family planning, in either direction.
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Understand patterns
Spot changes in cycle length over time that are worth discussing with a doctor.
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Complete privacy
Your dates stay on your device. No cloud storage, no data collection.
How it works
Enter the first day of your last period. That's the one date this calculator actually needs.
Set your average cycle and period length. 28-day cycles and 5-day periods are typical defaults, but adjust to match your own pattern if you know it.
We estimate your next period by adding your cycle length to your last period's start date.
We estimate ovulation as roughly 14 days before your next period, with a 6-day fertile window (5 days before ovulation through 1 day after) reflecting sperm and egg viability.
Your calendar shows the pattern projected across the next few cycles.
Important: Ovulation estimates aren't exact — actual timing can shift by several days each cycle. This isn't a method of birth control. If you have irregular cycles, very heavy bleeding, severe pain, or think you might be pregnant, talk to a clinician.
Frequently asked questions
It's a solid estimate for people with fairly regular cycles, but natural variation of a few days is common even in "regular" cycles — treat the dates as a likely window rather than an exact prediction.
You can, but accuracy drops significantly with irregular cycles since the math assumes a consistent cycle length. If your cycles vary a lot, consider tracking symptoms alongside this estimate, or talk to a doctor.
The fertile window is estimated as 5 days before ovulation through 1 day after, reflecting how long sperm can survive and the egg's shorter viable window.
Typically around 14 days before your next period starts — not 14 days after your last period began, which is a common misconception for cycles that aren't exactly 28 days.
Yes — all your dates are saved locally on your own device, not on a server or cloud account, matching the privacy-first approach across this site.
Yes — log a few past period start dates in "Cycle history," and the tracker calculates your actual average cycle length from the gaps between them, which is often more accurate than guessing a round number like 28 days.
Three or more logged start dates give a reasonable average. More cycles (ideally 6+) give a better sense of how much your cycle length naturally varies month to month.