Ovulation Calculator
Ovulation Calculator
Estimate your ovulation day, fertile window, most fertile days and next period from the first day of your last period and your cycle length, with the pregnancy test date, the due date if you conceive and the next six cycles.
Your cycle day by day
Next period = last period + cycle length. Ovulation = next period − luteal phase, 14 days unless you know yours. Fertile window = the five days before ovulation plus the day itself (Wilcox 1995). Due date if you conceive = ovulation + 266 days, matching the Due Date Calculator for a 14 day luteal phase. Calendar method = shortest cycle − 18 to longest cycle − 11, in cycle days. Calendar estimates for planning, not a diagnosis and not contraception. Exports contain only the values you entered.
How to use this tool
Enter the first day of your last period and your usual cycle length. Leave the luteal phase, the days from ovulation to the next period, at 14 unless ovulation tests or temperature charting have shown otherwise. The next period date doubles as the first day a pregnancy test is likely to be reliable.
The input people most often get wrong is the date. It is the first day of real bleeding, not the last day and not spotting before it; counting from the end of the period pushes every result about five days late. If your cycles change from month to month, choose “vary from month to month” and enter the shortest and longest of your last six or more cycles to add the wider calendar method window.
What the calculation is doing
The count runs backwards from the next period, because the luteal phase is the steadier part of the cycle. Next period = last period + cycle length. Ovulation = next period − luteal phase, so with the default 14 days it falls cycle length − 14 days after day 1: cycle day 15 of a 28 day cycle, cycle day 22 of a 35 day cycle. The NHS and the Mayo Clinic both place ovulation about 14 days before the next period.
The fertile window is the five days before ovulation plus the day itself. Wilcox, Weinberg and Baird followed 221 women through 625 cycles (New England Journal of Medicine, 1995) and found conception only when intercourse fell in the six days ending on the day of ovulation. The last three are shown as the most fertile. The due date if you conceive is ovulation + 266 days, which with a 14 day luteal phase equals the site’s Due Date Calculator: last period + 280 days, adjusted for cycle length.
For cycles that vary, the calendar method sets the first fertile day at shortest cycle − 18 and the last at longest cycle − 11, in cycle days, a wider span that allows for ovulation anywhere in that range.
Why the window is six days and why it still moves
Sperm can fertilise an egg for several days, while the egg lasts only about 12 to 24 hours after release. Intercourse before ovulation can lead to conception; a day after it, almost never. So the window is lopsided, and in the Wilcox data the chance from a single act rose from about 10% five days out to about 33% on the day of ovulation.
The window moves because the phase before ovulation is not fixed. In more than 600,000 cycles logged in an app and analysed by Bull and colleagues in 2019, the follicular phase averaged 16.9 days with most between 10 and 30, while the luteal phase averaged 12.4 days with most between 7 and 17. When Wilcox, Dunson and Baird pinned down ovulation with daily hormone measurements in 696 cycles (BMJ, 2000), only about 30% of women had their fertile window entirely within days 10 to 17, 17% were already fertile by day 7, and women with regular cycles still had a 1 to 6% chance of being fertile on the day their period was due. Read the dates as the centre of a spread wider than six days.
Textbooks often put ovulation on day 14 of a 28 day cycle. Counting 14 days back from the next period puts it on day 15, the convention this tool and the Due Date Calculator share; the one day difference is small next to the real spread.
Ovulation day and fertile window by cycle length
For a 14 day luteal phase, with the first day of the period as day 1. The calendar method column is what shortest − 18 to longest − 11 gives when every cycle has that length: it opens with the six day window and runs two days past ovulation as a margin.
| Cycle length | Ovulation, cycle day | Fertile window, cycle days | Calendar method, cycle days | Ovulation after a March 1, 2026 period |
|---|---|---|---|---|
| 21 days | day 8 | days 3 to 8 | days 3 to 10 | March 8 |
| 22 days | day 9 | days 4 to 9 | days 4 to 11 | March 9 |
| 23 days | day 10 | days 5 to 10 | days 5 to 12 | March 10 |
| 24 days | day 11 | days 6 to 11 | days 6 to 13 | March 11 |
| 25 days | day 12 | days 7 to 12 | days 7 to 14 | March 12 |
| 26 days | day 13 | days 8 to 13 | days 8 to 15 | March 13 |
| 27 days | day 14 | days 9 to 14 | days 9 to 16 | March 14 |
| 28 days | day 15 | days 10 to 15 | days 10 to 17 | March 15 |
| 29 days | day 16 | days 11 to 16 | days 11 to 18 | March 16 |
| 30 days | day 17 | days 12 to 17 | days 12 to 19 | March 17 |
| 31 days | day 18 | days 13 to 18 | days 13 to 20 | March 18 |
| 32 days | day 19 | days 14 to 19 | days 14 to 21 | March 19 |
| 33 days | day 20 | days 15 to 20 | days 15 to 22 | March 20 |
| 34 days | day 21 | days 16 to 21 | days 16 to 23 | March 21 |
| 35 days | day 22 | days 17 to 22 | days 17 to 24 | March 22 |
| 36 days | day 23 | days 18 to 23 | days 18 to 25 | March 23 |
| 38 days | day 25 | days 20 to 25 | days 20 to 27 | March 25 |
| 40 days | day 27 | days 22 to 27 | days 22 to 29 | March 27 |
| 42 days | day 29 | days 24 to 29 | days 24 to 31 | March 29 |
| 45 days | day 32 | days 27 to 32 | days 27 to 34 | April 1 |
A worked example you can check on paper
Take the defaults: a period that began on March 1, 2026, a 28 day cycle and a 14 day luteal phase. Next period: March 1 + 28 days = March 29. Ovulation: March 29 − 14 = March 15, cycle day 15. Fertile window: March 15 − 5 = March 10, so March 10 to 15, with March 13 to 15 the most fertile. Pregnancy test: from March 29. Due date: March 15 + 266 days. The 16 days left in March plus 30, 31, 30, 31, 31, 30, 31 and 30 for April to November make 260, and 6 more gives December 6, 2026, the same as March 1 + 280 days.
Change the cycle length to 35 and ovulation moves to March 22, 2026, cycle day 22, with the window March 17 to 22. For cycles between 26 and 32 days the calendar method gives 26 − 18 = day 8, March 8, to 32 − 11 = day 21, March 21: 14 days that cover the six day window for every cycle length in that range.
Mistakes that catch people out
Counting the cycle one day long. Periods that start on the 1st and the 29th make a 28 day cycle, not 29. Count from day 1 to the day before the next period.
Treating the dates as exact. The estimate is right on average and often several days off in a given cycle. Ovulation test kits, a temperature rise or cervical mucus changes show what this cycle is doing.
Using a calendar as contraception. Ovulation can come early enough for the window to overlap the period; 17% of women in the BMJ study were fertile by day 7. A calendar alone does not prevent pregnancy.
Ignoring a known luteal phase. If charting shows your period arrives 12 days after ovulation, the default estimate is two days early. Enter 12 and every date moves with it.
Testing too early. A negative test days before the missed period is not reliable. Test from the day the period is late, and repeat a few days later if it still has not come.
Popular calculations
Each opens the calculator with those values filled in, ready to change.
28 day cycle, by the date the period began
Shorter and longer cycles
Luteal phase of 12 or 16 days
Cycles that vary in length
How this was checked
The date arithmetic was compared with JavaScript Date day setters and with Python’s datetime over 14 start dates (February 27 to 29 in leap and ordinary years, month and year ends, 2000 and 2100), every cycle length from 21 to 45 and luteal phases of 10, 12, 14 and 16 days: 1,400 cases each, all matching, weekdays included. The NHS rule was reproduced (March 1, 2026 and a 28 day cycle give ovulation on March 15, 14 days before the March 29 period), as was the Planned Parenthood calendar method example: a shortest cycle of 26 gives day 8, the 11th when day 1 is the 4th; a longest of 30 gives day 19, the 22nd.
The due date was run through the site’s Due Date Calculator engine. With a 14 day luteal phase, ovulation + 266 days matched both its last period route and its conception route in all 350 cases; other luteal lengths differ by exactly 14 minus the luteal length, as the working states. The six day window follows Wilcox, Weinberg and Baird, NEJM 1995, and the variability figures come from Wilcox, Dunson and Baird, BMJ 2000. Every address above was loaded and checked for a distinct title, the right canonical address and no missing values.
Common questions
How do you calculate ovulation from your last period?
Add your cycle length to the first day of your last period to get the next period, then count back the luteal phase, usually 14 days. A March 1 period and a 28 day cycle give March 29 and ovulation around March 15; a 35 day cycle gives April 5 and March 22.
How many days after my period do I ovulate?
That depends on the cycle length, not on when the bleeding stopped. Counting from the first day of the period, ovulation comes about 14 days later in a 28 day cycle, 21 in a 35 day cycle and 7 in a 21 day cycle, where the fertile window can open during the period.
When are you most fertile?
In the six days ending on the day of ovulation, and most of all the last three. Every conception in the 625 cycles studied by Wilcox, Weinberg and Baird came from intercourse in that window, with the chance about 33% on the day of ovulation.
Can I use this calculator to avoid pregnancy?
No. Ovulation moves from cycle to cycle even when periods are regular: in the BMJ study of 696 cycles only about 30% of women had their fertile window entirely within days 10 to 17. Even fertility awareness methods taught with temperature and mucus signs have typical use failure rates of 2 to 23% a year, according to the CDC. Use a reliable contraceptive if avoiding pregnancy matters to you.
When can I take a pregnancy test?
From the first day of the missed period, which is the next period date shown. Some sensitive tests claim earlier results, but a negative before the missed period is less reliable, so repeat it a few days later if the period has not arrived.
What is the luteal phase and why does it matter?
The time from ovulation to the next period, usually 12 to 16 days and steadier than the first half of the cycle, which is why ovulation is counted back from the expected period. If you have measured yours, enter it: a 12 day luteal phase moves every estimate two days later.
Related Calculators and Reference
- Due Date Calculator: the full 40 week timeline from the same period date once a test is positive.
- Date Duration Calculator: count the days between two period dates to find your real cycle lengths.
- Days From Today Calculator: how many days until the ovulation or test date.
- Add or Subtract Days Calculator: shift any of these dates by a set number of days.
- Sleep Calculator: steady bed and wake times, which help temperature charting give clean readings.
- Office on Women’s Health: Trying to conceive: the calendar, temperature and cervical mucus methods described by the US Department of Health and Human Services.