Ovulation Calculator
Estimate your fertile days and likely ovulation day from your last period, with your next period and a calendar of the months ahead.
Your cycle runs from the first day of one period to the day before the next. 28 days is common, but anything from 21 to 35 is normal.
Separate them with commas, newest or oldest first. We use the average, and the shortest and longest to show the widest window.
The luteal phase is the time from ovulation to your next period. It's usually 12 to 14 days; leave it at 14 unless you track ovulation.
How your fertile days are worked out
Find your next period, count back to ovulation, then mark the days before it.
- 1
Find your next period
9 October + 28 d
= 6 November - 2
Count back to ovulation
− 14 days (luteal)
= 23 October
cycle day 15 - 3
Mark the fertile days
18 October – 23 October
best: 21 October – 23 October - 4
Test if it's late
test from
6 November
due ~16 July
Read more: the formula, a worked example, irregular cycles and when to see a doctor
Ovulation is when an ovary releases an egg. It usually happens about 12 to 14 days before your next period starts, whatever the length of your cycle, so the calculator finds your next period first and counts back from there. You can get pregnant from sex in the 5 days before ovulation and on the day itself, because sperm can live for up to 5 days and the egg for about 12 to 24 hours.
The formula
next period = first day of last period + cycle length
ovulation day = next period − luteal phase (14 days)
fertile window = the 5 days before ovulation + ovulation day
most fertile days = the 2 days before ovulation + ovulation day
If you enter several recent cycle lengths, the average (rounded to a whole day) is used as your cycle length. The shortest cycle gives the earliest your window could open and the longest gives the latest you might ovulate. A pregnancy test is most reliable from the day your next period is due. The due date shown is ovulation day + 266 days (38 weeks), which is the same as our due date calculator.
Worked example
Say your last period started on 1 January 2026 and your cycle is 28 days. Your next period is due on 1 January + 28 days = 29 January. Count back 14 days: you most likely ovulate on 15 January (cycle day 15, counting the first day of your period as day 1). Your fertile window is the 5 days before that plus the day itself, 10 to 15 January, and your most fertile days are 13 to 15 January. You could start ovulation tests on 12 January, take a pregnancy test from 29 January, and if you conceived this cycle your due date would be 15 January + 266 days = 8 October 2026.
With cycles of 26, 30, 28, 31, 29 and 26 days instead, the average is 170 ÷ 6 = 28.3, which rounds to 28, so the main dates stay the same. The shortest cycle (26 days) puts the earliest ovulation on 13 January and the window opening on 8 January; the longest (31 days) puts ovulation as late as 18 January. So the widest range is 8 to 18 January.
Cycle length and ovulation day
Counting the first day of your period as day 1, with a 14-day luteal phase:
| Cycle length | Likely ovulation | Fertile days |
|---|---|---|
| 21 days | day 8 | days 3–8 |
| 24 days | day 11 | days 6–11 |
| 26 days | day 13 | days 8–13 |
| 28 days | day 15 | days 10–15 |
| 30 days | day 17 | days 12–17 |
| 32 days | day 19 | days 14–19 |
| 35 days | day 22 | days 17–22 |
| 40 days | day 27 | days 22–27 |
Irregular cycles
If your cycle length changes by more than a few days from month to month, a single date is less likely to be right. Use the "Varies" option in Advanced and enter at least 6 recent cycles: the widest fertile range then runs from cycle day (shortest − 18) to cycle day (longest − 13). The older calendar "rhythm" rule uses shortest − 18 to longest − 11, adding two extra days at the end to be safe.
Ways to spot ovulation in the moment: ovulation (LH) test strips turn positive about a day before you ovulate; clear, stretchy, slippery vaginal discharge (like raw egg white) shows the most fertile days; and your resting temperature rises slightly after ovulation, so a chart shows it has already happened.
Please note
- This is not a form of contraception. Calendar dates alone are not a reliable way to avoid pregnancy.
- Stress, illness, travel, breastfeeding, recently stopping hormonal contraception and conditions such as PCOS can all move or stop ovulation.
- Common advice is to see a doctor if you haven't got pregnant after a year of trying, after 6 months if you're 35 or older, or sooner if your periods are very irregular or have stopped.
For information only. Please speak to your doctor, nurse or midwife about your fertility and health. Checked on 10 October 2026.
Common questions
How do I calculate my ovulation day?
Find the date your next period is due (the first day of your last period plus your cycle length), then count back 14 days. Ovulation usually happens about 12 to 14 days before your next period, whatever your cycle length. With a 28-day cycle that started on 1 January, that's 15 January.
What is the fertile window?
It's the 5 days before ovulation plus ovulation day itself, because sperm can live for up to 5 days and the egg for about 12 to 24 hours. The 2 days before ovulation and the day itself are usually the most fertile, and the calculator marks them.
Can I use an ovulation calculator with irregular periods?
Yes, but a single date is less likely to be right. Use the "Varies" option in Advanced and enter at least 6 recent cycle lengths. The widest fertile range then runs from cycle day (shortest cycle − 18) to cycle day (longest cycle − 13).
When should I take a pregnancy test?
A pregnancy test is most reliable from the day your next period is due, and the calculator shows that date for each cycle. If you conceived, the calculator also shows an estimated due date, which is ovulation day plus 266 days.
How can I tell when I'm ovulating?
Ovulation (LH) test strips turn positive about a day before you ovulate. Clear, stretchy, slippery discharge like raw egg white shows the most fertile days, and your resting temperature rises slightly after ovulation, so a temperature chart shows it has already happened.
Can I use an ovulation calculator as contraception?
No. Calendar dates alone are not a reliable way to avoid pregnancy. Stress, illness, travel, breastfeeding, recently stopping hormonal contraception and conditions such as PCOS can all move or stop ovulation. Please speak to your doctor, nurse or midwife about your fertility and health.
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Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.