Free
Period & Ovulation Calculator
Free online period and ovulation calculator. Enter your last period date and cycle length to find your next period, ovulation day, and most fertile days.
How to use this cycle calculator
- Enter your last period date
Select the first day your last period started. If unsure, use your best estimate.
- Set your cycle length
Enter your average cycle length (21-35 days is typical). If unsure, use 28 as a starting point.
- Tap Calculate
The calculator shows your next period date, estimated ovulation day, and fertile window.
- Track for better accuracy
For more precise predictions over time, install the Ayol app and log daily.
How does this period calculator work?
This calculator uses the calendar method to estimate your next period, ovulation day, and fertile window. It adds your cycle length to your last period start date to predict the next period. Ovulation is estimated at 14 days before your next period. The fertile window spans 5 days before ovulation through ovulation day itself.
When does ovulation happen?
Ovulation typically occurs 12-16 days before your next period, most commonly around 14 days before. If your cycle is 28 days, ovulation likely falls on day 14. If your cycle is 30 days, expect it around day 16. Tracking cervical mucus changes and body temperature can help confirm ovulation timing.
What is the fertile window?
The fertile window is the span of days when conception is most likely. It includes the 5 days before ovulation and ovulation day itself. Sperm can survive in the reproductive tract for 3-5 days, while the egg is viable for 12-24 hours after release. This creates a 6-day window where pregnancy is possible.
How accurate is the calendar method?
The calendar method provides 60-80% accuracy for regular cycles. It works best when your cycle length is consistent month to month. For irregular cycles, combining calendar tracking with symptom observation (cervical mucus, basal body temperature) significantly improves accuracy.
Why install the Ayol app instead?
This online calculator gives a one-time estimate. The Ayol app continuously learns from your daily logs, symptoms, and cycle history to provide increasingly accurate predictions. It also includes app lock protection, prayer time reminders, and works offline.
Frequently asked questions
Is this period calculator free?
Yes, completely free. The Ayol app is also free to download and use.
Can this calculator detect pregnancy?
No. This calculator only estimates cycle timing. For pregnancy confirmation, use a home pregnancy test.
What if my cycle is irregular?
Use the average of your last 3-6 cycles. The Ayol app calculates this automatically and improves over time.
Does this tool store my data?
No. All calculations happen in your browser. Nothing is sent to any server.
Get more accurate predictions with the Ayol app
The Ayol app learns from your daily logs and provides better predictions over time. Protected with app lock, works offline.
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