Ovulation usually happens about 14 days before your next period starts — on day 15 of a 28-day cycle (often quoted as day 14), counting the first day of your period as day 1. You can get pregnant from 5 days before ovulation to 1 day after it, according to ACOG.
How ovulation is estimated
Ovulation is the day an ovary releases an egg. It usually happens about 14 days before your next period starts (NHS) — in most women, 10 to 16 days before it (NHS). So the calculator first predicts your next period from the first day of your last period and your cycle length, then counts back 14 days.
An egg lives for about a day, but sperm can survive in the body for up to five days (ACOG). That is why the fertile window is longer than one day. The calculator marks seven days — the five days before ovulation, ovulation day and the day after — the same window the Period Calc app uses, and highlights the two days before ovulation and ovulation day as your most fertile days. On very short cycles the window can begin right after a period ends (see the table).
The formula
Next period = first day of last period + cycle length
Ovulation ≈ next period − 14 days
Fertile window = ovulation − 5 days to ovulation + 1 day
A worked example
Your last period started on and your cycles are 28 days long.
- Next period
- ( + 28 days)
- Ovulation
- around ( − 14 days)
- Fertile window
- – June 2026
- In the Hijri calendar
- the fertile window runs from to , with ovulation around
With a 32-day cycle, the same start date moves ovulation to around : a longer cycle means a later ovulation, and a shorter cycle an earlier one.
Ovulation day for every cycle length
On cycles of 21 to 23 days the window starts on day 6: like the calculator, the table assumes a 5-day period and never counts period days as fertile.
Estimated ovulation day and fertile window for each cycle length, in days of the cycle (day 1 = the first day of your period)
| Cycle length | Ovulation day | Fertile window |
|---|---|---|
| 21 | 8 | 6–9 |
| 22 | 9 | 6–10 |
| 23 | 10 | 6–11 |
| 24 | 11 | 6–12 |
| 25 | 12 | 7–13 |
| 26 | 13 | 8–14 |
| 27 | 14 | 9–15 |
| 28 (average) | 15 | 10–16 |
| 29 | 16 | 11–17 |
| 30 | 17 | 12–18 |
| 31 | 18 | 13–19 |
| 32 | 19 | 14–20 |
| 33 | 20 | 15–21 |
| Cycle length | Ovulation day | Fertile window |
|---|---|---|
| 34 | 21 | 16–22 |
| 35 | 22 | 17–23 |
| 36 | 23 | 18–24 |
| 37 | 24 | 19–25 |
| 38 | 25 | 20–26 |
| 39 | 26 | 21–27 |
| 40 | 27 | 22–28 |
| 41 | 28 | 23–29 |
| 42 | 29 | 24–30 |
| 43 | 30 | 25–31 |
| 44 | 31 | 26–32 |
| 45 | 32 | 27–33 |
Signs that ovulation is near
Your body often gives clues. In the days before ovulation, cervical mucus becomes thin, stretchy and slippery, a bit like raw egg white. Around ovulation, your resting body temperature rises slightly and stays up until your next period. Ovulation tests from a pharmacy detect the rise in luteinising hormone (LH) that comes a day or two before ovulation (ACOG).
If your cycles are irregular
When your cycle length changes from month to month, ovulation moves too, and an estimate based on dates becomes less reliable. Use your average cycle length, and combine the calculator with the body signs above or with ovulation tests for a clearer picture.
When to see a doctor
See a doctor if you have been trying to get pregnant for a year without success — or for six months if you are over 35 (ACOG). If you are over 40, talk to a doctor now rather than waiting.
Want to see your next periods as well? The period calculator shows six cycles and a calendar. If you are already pregnant, the due date calculator estimates your due date, and the Period Calc FAQ answers questions about the app.
This calculator gives estimates, not medical advice. It cannot tell you which days are safe, and it is not a method of contraception.
Questions about ovulation
When am I most likely to get pregnant?
In the 2 to 3 days before your body temperature rises at ovulation, according to ACOG; the NHS says you are more likely to get pregnant around the time you ovulate. The calculator highlights the two days before ovulation and ovulation day as your most fertile days. Having sex every two to three days throughout your cycle also covers your fertile window without needing to know the exact day.
Can ovulation happen on a different day than predicted?
Yes. Ovulation is hard to pin down: in most women it happens 10 to 16 days before the next period, and it can move from one cycle to the next. Treat the result as a window, not an exact day.
Can I get pregnant during my period?
It is unlikely, but it can happen with short cycles: your fertile window can begin soon after your period ends, and sperm can live for several days.
Can the Period Calc app do this for me?
Yes. Choose “Trying to conceive” to see your fertile window, your estimated ovulation day and when to take a pregnancy test. Premium adds your best days, day by day, today’s chance of pregnancy and reminders before your fertile days.
Is what I type private?
Yes. The calculation runs in your browser, and nothing you enter is sent to us or anyone else.
How this page was made
Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the health organisations listed under Sources. They have not reviewed it, and neither has a doctor. The calculator runs the formula above in your browser, with the same rules as the Period Calc app. It works from averages, so it cannot know how your own body differs from them, and it cannot diagnose anything: if something worries you, see when to see a doctor.