Period Calculator — Predict Next Period Date
Predict your next period date, fertile window, and ovulation day based on your cycle length. Shows the next 6 cycles. See also our Ovulation Calculator and Pregnancy Calculator.
How to Predict Your Next Period
- Enter the start date of your last menstrual period.
- Enter your average cycle length (typically 21-35 days, average 28).
- Enter your average period duration (typically 3-7 days).
- Click Calculate to see your next 6 predicted cycles.
- The fertile window is estimated as 5 days before ovulation to 1 day after.
Formula
Next Period = Last Period Start + Cycle Length
Ovulation Day = Next Period Start + Cycle Length - 14
Fertile Window = Ovulation Day - 5 to Ovulation Day + 1
Example
Last Period: January 5, 2025
Cycle Length: 28 days
Next Period: February 2, 2025
Ovulation: January 19, 2025 (day 14)
Fertile Window: January 14-20, 2025
Cycle Length Reference
| Cycle Length | Ovulation Day | Status |
|---|---|---|
| 21 days | Day 7 | Short cycle |
| 25 days | Day 11 | Below average |
| 28 days | Day 14 | Average |
| 30 days | Day 16 | Above average |
| 35 days | Day 21 | Long cycle |
Frequently Asked Questions
How accurate is this calculator?
This provides estimates based on average cycle patterns. Actual cycles can vary due to stress, illness, travel, and hormonal changes. Track multiple cycles for better accuracy.
What is a normal cycle length?
Normal cycles range from 21 to 35 days. The average is 28 days, but many women have slightly shorter or longer cycles.
When is the fertile window?
The fertile window is approximately 6 days: the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the reproductive tract.
Can I use this for birth control?
This calculator is for informational purposes only and should not be relied upon as a contraceptive method. Consult a healthcare provider for birth control advice.
Why is my period irregular?
Irregular periods can be caused by stress, weight changes, exercise, PCOS, thyroid issues, or perimenopause. Consult a doctor if cycles are consistently irregular.