What is Period Calculator?
A period calculator predicts your upcoming menstrual periods, fertile windows, and ovulation days based on your cycle history. Tracking your cycle helps you plan ahead, understand your body, and identify any irregularities early.
Our calculator provides a full 6-month visual calendar with color-coded days: red for period days, green for fertile days, and blue for ovulation. It also displays a detailed table with exact dates for each predicted cycle.
How It Works
Enter the first day of your last period, your average cycle length (21–35 days), and your period duration (2–8 days). The calculator predicts your next 6 periods by repeating your cycle length. It calculates ovulation as 14 days before each period start and marks the fertile window as 5 days before to 1 day after ovulation.
Formula
Next Period = Last Period Start + Cycle Length Ovulation Day = Period Start + (Cycle Length − 14) Fertile Window = Ovulation Day − 5 to Ovulation Day + 1 Period End = Period Start + Period Duration − 1
Formula Explained
The period prediction is based on your average cycle length. Ovulation is estimated using the luteal phase constant: ovulation occurs approximately 14 days before the next period regardless of cycle length. The fertile window spans 6 days because sperm can survive up to 5 days and the egg lives for 12–24 hours after ovulation.
Example
Last period: January 1 Cycle length: 28 days Period duration: 5 days Period: Jan 1–5 Ovulation: Jan 14 Fertile window: Jan 9–15 Next period: Jan 29–Feb 2
Tips & Best Practices
- ✓Track your cycle for 3–6 months to find your true average cycle length.
- ✓Cycle length can vary by 1–5 days month to month — this is normal.
- ✓Stress, travel, and illness can temporarily shift your cycle.
- ✓If your cycle is consistently outside 21–35 days, consult a healthcare provider.
- ✓Use the calendar view to plan around your predicted dates.
Common Use Cases
- •Planning vacations, events, and activities around your cycle
- •Tracking cycle regularity and identifying patterns
- •Predicting fertile windows for family planning
- •Preparing for period symptoms by knowing upcoming dates
- •Discussing cycle patterns with your healthcare provider