CalcKit🩸 Period Calculator
When is your next period? Get your dates in seconds.
Enter the first day of your last period and your average cycle length (usually 21–35 days). The calculator predicts your next period, ovulation day and fertile window using the standard calendar method (ovulation ≈ 14 days before the next period).
Estimates only — real cycles vary. See a doctor for irregular cycles.
Frequently Asked Questions
How accurate is a period calculator?
It's an estimate based on averages. The calendar method assumes ovulation 14 days before your next period, which holds for regular cycles but not irregular ones.
What is a normal cycle length?
21–35 days is considered normal for adults, with 28 days the well-known average. Track a few months and use your personal average.
What is the fertile window?
The 5 days before ovulation plus ovulation day — sperm can survive up to 5 days, so conception is possible across all 6 days.
Can I use it with irregular periods?
You can, but predictions will be rough. Averaging your last 3 cycle lengths gives the best estimate; see a doctor if cycles vary wildly.