Period Calculator: Next Period, Fertile Window & Ovulation — A Practical Guide
1) How this calculator works
Enter the start date of your most recent period, your average cycle length (the number of days from day‑1 of one period to day‑1 of the next), and how many days your period usually lasts. The calculator projects your next period by simply adding the cycle length to the last start date. Ovulation is estimated as roughly luteal length days before your next period; when not provided, a common assumption is around 14 days. Your most fertile days are typically the five days before ovulation plus ovulation day itself.
2) Cycle basics: follicular, ovulation, luteal
The menstrual cycle has two main phases separated by ovulation. The follicular phase runs from the first day of your period to ovulation and can vary more from person to person. The luteal phase runs from ovulation to the next period and is usually more consistent (often 12–16 days). Because luteal length is steadier, ovulation can be estimated by subtracting it from the next expected period date.
3) Estimating ovulation & fertile days
Fertility tends to peak in the days leading up to ovulation because sperm can survive for several days in the reproductive tract. That’s why the fertile window is defined as ovulation day plus the previous five days. Keep in mind: stress, illness, travel, and other factors can shift dates.
4) Safe days and limitations
“Safe days” here simply means outside the fertile window and outside predicted period days. They are not a guarantee against pregnancy. If you need contraception, use medically approved methods. If you are trying to conceive, combine this estimate with ovulation tests or basal body temperature tracking for better accuracy.
5) Irregular cycles & when to see a professional
If your cycle length varies by more than a few days month to month, predictions become less accurate. Consider keeping detailed logs and consult a healthcare professional if you have concerns such as very heavy bleeding, severe pain, or cycles consistently outside 21–35 days.
6) How to use this page
- Enter last period date and your average cycle/period lengths.
- Optionally set luteal length if you know it; otherwise the tool assumes ~14 days.
- Press Calculate. You’ll see projected dates and a color calendar for the next three months.
- Use Copy Share Link to save your parameters, or Print to create a PDF.