Menstrual Cycle Period Calculator
Predict your next period, ovulation, and fertile window based on your cycle length and last period start date.
Published by:
isCalculator Editorial Team
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How to use this tool?
Follow these steps to use the Menstrual Cycle Period Calculator:
- Enter Last Period Start Date – Select the first day of your most recent menstrual period using the date picker.
- Set Cycle Length – Input your average cycle length in days (typical range: 21–35 days, allowed: 20–45).
- Set Period Length – Enter how many days your period usually lasts (1–10 days).
- Set Luteal Phase Length – Enter your luteal phase length (typically 14 days, range 10–16).
- Click "Calculate" – The calculator will display:
- Next Period Start – Predicted start date of your next period.
- Ovulation Day – Estimated day of ovulation.
- Fertile Window – The 6-day window (5 days before ovulation to 1 day after) when conception is most likely.
Note: All inputs are in days (no metric/imperial conversions needed). The calculator assumes a regular cycle. If you see an error, check that your cycle length is longer than your luteal phase length.
Previous Results
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Frequently Asked Questions
A menstrual cycle is the time from the first day of one period to the first day of the next period. The average cycle is 28 days, but it can range from 21 to 35 days.
Ovulation typically occurs 14 days before the next period starts. By subtracting the luteal phase length from the predicted next period date, we estimate the ovulation day.
The fertile window is the days when pregnancy is most likely. It includes the 5 days before ovulation and the day of ovulation itself, due to sperm lifespan.
