Period and ovulation estimator
See estimated dates for your next 3 periods, the day you're 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 the steadiest part, at about 14 days. The first half is the part that varies.[1][2] 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 5 days before ovulation, ovulation day and the day after, the days ACOG gives for when sex can lead to pregnancy.[1]
When the dates are less reliable
Calendar estimates are rough when cycles vary by more than a few days, when periods have just started, after stopping hormonal birth control, after having a baby, and as you approach menopause.[1] Ovulation tests or tracking body temperature add information.[1] If you're trying to conceive, ACOG suggests a fertility check after 12 months of trying, or after 6 months if you're over 35.[3]
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're 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, ACOG says sperm can survive about 3 days and sometimes up to 5, 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.
Sources
- ACOG: Fertility awareness-based methods of family planning Checked October 30, 2026
- NHS: Periods and fertility in the menstrual cycle Checked October 30, 2026
- ACOG: Evaluating infertility Checked October 30, 2026
Spotted an error? Write to [email protected]. How we check health information.