Period Calculator
Predict your next period, fertile window, and ovulation date based on cycle history.
A period calculator predicts your next menstrual period and fertile window based on your cycle history.
Examples
28-day cycle starting January 1
Frequently Asked Questions
How accurate are period predictions?
What are the phases of the menstrual cycle?
Quick Tips
- •Log your period start dates consistently for at least 3 cycles to improve prediction accuracy.
- •Note any unusual symptoms or changes in flow, as these can indicate cycle irregularities.
- •Always confirm results with your healthcare provider if you have concerns about your cycle.
A period calculator predicts your next menstrual period and fertile window based on your cycle history.
How to Use This Calculator
Enter the first day of your last period, your average cycle length, and how many days your period typically lasts. The calculator predicts your next 6 periods, ovulation dates, and fertile windows, and shows your current cycle phase.
Understanding the Formula
Next period = Last period + Cycle length. Ovulation estimated at cycle day (Cycle length - 14). Fertile window is 5 days before to 1 day after ovulation.
Examples
28-day cycle starting January 1
Next period: January 29. Ovulation: January 15 (day 14). Fertile window: January 10-16.
Frequently Asked Questions
How accurate are period predictions?
Predictions are based on your average cycle length and assume regularity. Factors like stress, illness, travel, and hormonal changes can cause cycles to vary. Track several months for better predictions.
What are the phases of the menstrual cycle?
The four phases are: Menstrual (days 1-5, bleeding), Follicular (days 1-13, egg development), Ovulation (day 14, egg release), and Luteal (days 15-28, uterine lining thickens). Actual timing varies by individual.
Assumptions & Limitations
- Based on average cycle lengths; individual cycles may vary due to stress, illness, or hormonal changes.
- Assumes a regular menstrual cycle — predictions are less accurate for irregular cycles.
- Ovulation estimates are based on an average 14-day luteal phase, which varies between individuals.