Period calculator to estimate your next period, period days, ovulation date, fertile window, and upcoming cycles from your last period date, period length, and cycle length.
Use this period calculator to estimate your next period date, period days, ovulation date, and fertile window from the first day of your last period, your usual bleeding length, and your average cycle length.
It also lays out several upcoming cycles in table and calendar form, which makes it useful for general cycle tracking, planning ahead, and understanding when your next period or ovulation is most likely. The results are estimates only and can vary from month to month.
First day of your last menstrual period
How long did it last?
days
Number of days in your cycle
days
Result
Current cycle estimate
- Estimated period days:
-
- Estimated ovulation date:
-
- Most fertile days:
-
- Fertile window:
-
- Estimated next period date:
-
Upcoming cycle estimates
| Cycle |
Period |
Ovulation |
Fertile window |
Next period |
Estimated period calendar
Period days
Fertile window
Ovulation
This calculator gives cycle estimates only and is not a medical tool. Period timing, bleeding length, and ovulation can vary from cycle to cycle.
Period Calculator FAQ
What does a period calculator estimate?
A period calculator estimates your next period date, expected period days, ovulation date, and fertile window from your last period date, period length, and average cycle length.
How is the next period date calculated?
It is estimated as the first day of your last period + average cycle length. If your cycle is fairly regular, this gives a practical estimate of when the next period may begin.
How is ovulation estimated in this calculator?
Ovulation is estimated at about 14 days before the next period. That is a common cycle-tracking rule of thumb, but real ovulation can happen earlier or later.
What is the fertile window?
The fertile window is usually the 5 days before ovulation plus the day of ovulation. This is the part of the cycle when pregnancy is most likely.
Can I use this as a medical or birth control tool?
No. This calculator provides estimates only and should not replace medical advice, cycle-specific fertility testing, or contraception guidance. Stress, illness, hormone changes, travel, and irregular cycles can all shift timing.
How many future cycles can this help me plan?
This version shows your current cycle estimate, a table of upcoming cycles, and a compact calendar view so you can see likely period days and ovulation timing at a glance.
