Ovulation Calculator
Your likely ovulation day and the six days you are most likely to conceive, for this cycle and the next ones, worked out on this device from your period and cycle length.
From the first day of one period to the first day of the next. 28 if you’re not sure.
Your dates stay on this device: nothing is sent, saved or added to the address bar.
More optionsLuteal phase 14 days
14 by convention. If ovulation tests or temperature charts have shown you yours, use it: in a study of 612,613 cycles the average was 12.4 days.
ExampleA period that started 5 days ago, 28-day cycle
Oct 17–22
Ovulation most likely around Thursday, October 22, 2026; the two or three days up to it give the best chance. This is an estimate, not contraception.
PeriodFertile daysToday
- Ovulation, about
- Thu, Oct 22, 2026
- Next period
- Thu, Nov 5, 2026
- Due date if you conceive
- Thu, Jul 15, 2027
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. This is not contraception: a calendar estimate can’t tell you which days are safe from pregnancy. Terms
The next six cyclesFertile windows from Oct 17 to Mar 11
| Period starts | Fertile window | Ovulation |
|---|---|---|
| Thu, Oct 8, 2026 | Oct 17–22 | Oct 22 |
| Thu, Nov 5, 2026 | Nov 14–19 | Nov 19 |
| Thu, Dec 3, 2026 | Dec 12–17 | Dec 17 |
| Thu, Dec 31, 2026 | Jan 9–14 | Jan 14 |
| Thu, Jan 28, 2027 | Feb 6–11 | Feb 11 |
| Thu, Feb 25, 2027 | Mar 6–11 | Mar 11 |
Each cycle the same length as the one you entered. Real cycles vary, so the further ahead, the rougher.
How this works: Method, 8 sources, Checked against 2 worked examples,
How this works
Method
Ovulation is counted back from the next period, not forward from the last one. The time between ovulation and the next period (the luteal phase) is the steadier half of the cycle, so the calculator takes the date your next period is due and goes back 14 days. The six days that end on that day are your fertile window: sperm can live for several days, and an egg for about a day after it is released. Full method
Sources
- Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (N Engl J Med, 1995)
- Wilcox, Dunson and Baird, The timing of the “fertile window” in the menstrual cycle (BMJ, 2000)
- Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digit Med, 2019)
- Committee Opinion No. 700: Methods for Estimating the Due Date (ACOG, reaffirmed 2025, 2017)
- Committee Opinion No. 579: Definition of Term Pregnancy (ACOG, 2013)
- Predicting date of birth and examining the best time to date a pregnancy (Khambalia et al.) (Int J Gynaecol Obstet, 2013)
- Length of human pregnancy and contributors to its natural variation (Jukic et al.) (Hum Reprod, 2013)
- Naegele’s rule: a reappraisal (Baskett and Nagele) (BJOG, 2000)
How it’s tested
2 worked examples for this page are checked by automated tests before every release: given the inputs, the tool must show the expected answer.
Limits
General information from published research, not a diagnosis or medical advice. Talk to a doctor before changing medication or treatment. This is not contraception: a calendar estimate can’t tell you which days are safe from pregnancy.
Changes
- First published: a due date from your last period, conception, IVF or a scan, with ovulation, conception, period and weeks-pregnant pages; nothing you enter leaves your device.
How the fertile window is worked out
Ovulation is counted back from the next period, not forward from the last one. The time between ovulation and the next period (the luteal phase) is the steadier half of the cycle, so the calculator takes the date your next period is due and goes back 14 days. The six days that end on that day are your fertile window: sperm can live for several days, and an egg for about a day after it is released.
For example, a period that started on Wednesday, March 3, 2027 with a 31-day cycle puts the next one on Saturday, April 3, 2027. Fourteen days before that is Saturday, March 20, 2027, so the fertile window is Mar 15–20, 2027.
The window is an estimate on a calendar. Ovulation tests (which detect the hormone surge a day or so before ovulation) or a basal body temperature chart show what your body is actually doing, and are worth adding if you are trying to conceive.
ovulation ≈ next period − 14 days; fertile window = the 5 days before ovulation + ovulation day
- In 221 women trying to conceive, every pregnancy came from sex in the six days ending on ovulation day; the chance from a single day rose from about 10% five days before ovulation to 33% on the day itself. Source: Wilcox, Weinberg and Baird, Timing of sexual intercourse in relation to ovulation (N Engl J Med, 1995).
- Only about 30% of women had their whole fertile window between days 10 and 17 of the cycle; for every day from day 6 to day 21 there was at least a 10% chance of being fertile. Source: Wilcox, Dunson and Baird, The timing of the “fertile window” in the menstrual cycle (BMJ, 2000).
- Across 612,613 tracked cycles the average luteal phase was 12.4 days (95% of cycles 7 to 17), so a real ovulation day can sit a few days either side of the estimate. Source: Bull et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (npj Digit Med, 2019).
Ovulation day and fertile window by cycle length
Cycle days counted from the first day of the period (day 1), with a 14-day luteal phase.
| Cycle length | Likely ovulation | Fertile window | Next period |
|---|---|---|---|
| 21 days | day 8 | days 3–8 | day 22 |
| 22 days | day 9 | days 4–9 | day 23 |
| 23 days | day 10 | days 5–10 | day 24 |
| 24 days | day 11 | days 6–11 | day 25 |
| 25 days | day 12 | days 7–12 | day 26 |
| 26 days | day 13 | days 8–13 | day 27 |
| 27 days | day 14 | days 9–14 | day 28 |
| 28 days | day 15 | days 10–15 | day 29 |
| 29 days | day 16 | days 11–16 | day 30 |
| 30 days | day 17 | days 12–17 | day 31 |
Show all 18 rowsShow fewer
| 31 days | day 18 | days 13–18 | day 32 |
| 32 days | day 19 | days 14–19 | day 33 |
| 33 days | day 20 | days 15–20 | day 34 |
| 34 days | day 21 | days 16–21 | day 35 |
| 35 days | day 22 | days 17–22 | day 36 |
| 36 days | day 23 | days 18–23 | day 37 |
| 38 days | day 25 | days 20–25 | day 39 |
| 40 days | day 27 | days 22–27 | day 41 |
Frequently Asked Questions
Can I use this to avoid getting pregnant?
No. A calendar estimate can be several days out, ovulation can come early or late in any cycle, and sperm can survive for days, so there is no day this page can call safe. If you want to avoid pregnancy, a clinician or a family-planning service can help you choose a method that works.
What are the best days to try to conceive?
The two or three days up to and including ovulation day have the highest chance in the research, and the whole six-day window counts. Because the estimate can be off by a few days, sex every one to two days through the window, or tracking with ovulation tests, covers more of the possibilities than aiming at one day.
My cycles are irregular. Will this work for me?
It will be rougher. The window moves with every cycle, and with irregular cycles the next period, and so ovulation, is hard to predict. Use your shortest and longest cycles to see the earliest and latest windows, and consider ovulation tests. If your cycles are often shorter than 24 days or longer than 38, that is worth mentioning to a clinician.
Do you keep the dates I enter?
No. The calculation runs on your device and nothing you type is stored, sent or put in the address bar. If you want a record of your cycles, the period calculator can keep one on this device only, and only if you turn it on.