Period and ovulation estimator
See estimated dates for your next three periods, the day you are likely to ovulate and your fertile window, worked out from your last period and usual cycle length.
Pick the first day of your last period and the estimate appears here.
How the estimate works
The second half of the cycle, from ovulation to the next period, is fairly steady at about two weeks for most people. The first half is the part that varies. So the estimator takes your cycle length, subtracts 14, and treats that day of the cycle as the likely ovulation day.
| Cycle length | Likely ovulation | Fertile window |
|---|---|---|
| 24 days | Day 10 | Days 5 to 11 |
| 28 days | Day 14 | Days 9 to 15 |
| 32 days | Day 18 | Days 13 to 19 |
Day 1 is the first day of bleeding. The fertile window shown here is the five days before ovulation, ovulation day and the day after.
When the dates are less reliable
Calendar estimates are rough when cycles vary by more than a few days, in the first years after periods start, after stopping hormonal contraception, after giving birth or while breastfeeding, and in perimenopause. Ovulation tests or tracking body temperature add information, and a doctor can help if you are trying to conceive and it is taking longer than you hoped.
Your history does the work
One date and an average only go so far. In Her Good Days you log each period, and the app shows your real cycle lengths as a range. If you are trying to conceive, you can add ovulation test results too.
Questions
How does an ovulation calculator work?
It assumes ovulation happens about 14 days before your next period starts, which is day 14 of a 28-day cycle. From your last period and your usual cycle length it works out the next period and the likely ovulation day, and marks the five days before it and the day after as the fertile window.
Can I use this as birth control?
No. Ovulation moves from cycle to cycle, sperm can survive for several days, and a calendar estimate can be off by days. Talk to a doctor, nurse or pharmacist about contraception.
What if my cycles are irregular?
Then any calendar estimate is less reliable, and the range of possible days is wider. Logging a few months of period dates shows how much your cycles vary, which is useful to bring to a doctor.
Do you store what I enter?
No. The estimate is worked out in your browser and nothing you type is sent to us.
Start with today
Her Good Days is free, with no third-party ads and nothing to subscribe to. Free on iPhone and Android.