Period / Menstrual Cycle Calculator
Predict your next periods, fertile window, and ovulation date
About the Period Calculator
This calculator predicts future menstrual period dates, ovulation days, and fertile windows based on your last period start date and average cycle length. It is a practical tool for menstrual health tracking, family planning awareness, and identifying patterns in your cycle. The predictions are generated using established cycle arithmetic and the standard luteal-phase model.
How to Use
- Enter the start date of your most recent period.
- Input your average cycle length (typically 21–35 days; 28 is default).
- Set your typical period duration in days.
- Choose how many future cycles to predict and click "Calculate".
Formula / Methodology
Ovulation ≈ Period Start + (Cycle Length − 14)
(Luteal phase is ~14 days before next period)
Fertile Window:
Start = Ovulation − 5 days
End = Ovulation + 1 day
The luteal phase (from ovulation to next period) is relatively fixed at around 14 days for most women, making it possible to back-calculate ovulation from cycle length. The fertile window accounts for sperm survival (up to 5 days) before ovulation.
Understanding Your Results
Not a Contraception Tool
Cycle lengths vary naturally due to stress, illness, hormonal changes, and other factors. These predictions are estimates and cannot account for all individual variation. This calculator is not a reliable method of contraception. Consult a gynaecologist for fertility treatment, irregular cycles, or any contraception guidance.