Ovulation Calculator (Fertile Window and Next Period)
Ovulation day and the six-day fertile window from your last period and cycle length, with the next period, the test date and the next three cycles.
Inputs
| Period starts | Fertile window | Ovulation |
|---|---|---|
| Aug 1 | Aug 10 – Aug 15 | Aug 15 |
| Aug 29 | Sep 7 – Sep 12 | Sep 12 |
| Sep 26 | Oct 5 – Oct 10 | Oct 10 |
- Fertile window (6 days)
- Monday, August 10, 2026 – Saturday, August 15, 2026
- Most fertile days
- Aug 13 – Aug 15 (the two days before ovulation and the day itself)
- Cycle day of ovulation
- Day 15, 14 days before the next period (the first day of bleeding is day 1)
- Next period expected
- Saturday, August 29, 2026
- Earliest reliable pregnancy test
- Saturday, August 29, 2026 (the day the period is due)
- Due date if conceived this cycle
- Saturday, May 8, 2027
Use this calculator on your own site
Free embed
- Two lines of HTML, unlimited views
- Shows a small CalculateEveryday bar, with a credit link under the calculator
What each Ovulation input means
- Last period start
- The first day of bleeding in your most recent period.
- Cycle length
- Days from the first day of one period to the first day of the next. Average a few recent cycles; if they vary by more than a few days, the estimate is less reliable.
- Luteal phase (if known)
- Leave at 14 unless you know yours from temperature charting or ovulation tests — it is the gap from ovulation to the next period, and it usually stays within a day or two of 14.
Ovulation formula
Ovulation ≈ first day of last period + (cycle length − luteal phase, usually 14) · fertile window = ovulation − 5 days through ovulation day
The calendar method estimates ovulation by counting back from the next period rather than forward from the last one. The second half of the cycle — the luteal phase, from ovulation to the next period — is fairly constant at about 14 days, while the first half varies, so a 28-day cycle ovulates around 14 days after the period starts and a 32-day cycle around 18 days. The fertile window comes from Allen Wilcox's 1995 study in the New England Journal of Medicine, which followed 221 women trying to conceive with daily hormone measurements: every pregnancy came from intercourse in the six days ending on ovulation day, with the chance per act rising from about 10% five days before to about 33% on the day itself. Sperm can survive several days and the egg only about a day, so the window closes when ovulation passes. The page lists this cycle and the next two, the date the next period is due — the earliest day a home test is reliable — and the due date if you conceive this cycle.
Ovulation worked example
Last period started 1 August 2026 and cycles run 28 days. The next period is due 29 August, and 14 days before that is Saturday, August 15, 2026 — cycle day 15 counting the first day of bleeding as day 1. The fertile window is the six days from 10 to 15 August, with 13 to 15 August the most fertile. A pregnancy conceived then would be due around 8 May 2027.
More Ovulation examples
A 32-day cycle starting 20 September 2026: the next period is due 22 October, so ovulation is about 14 days earlier, Thursday, October 8, 2026, on cycle day 19. The fertile window runs 3 to 8 October.
Where the Ovulation calculator can give the wrong answer
- Cycles move. Stress, illness, travel and breastfeeding can shift ovulation by days in a single cycle even when the average stays the same; the calendar method can only use the average.
- Irregular cycles, polycystic ovary syndrome and the months after stopping hormonal contraception make a calendar estimate unreliable. LH test strips, basal temperature or cervical mucus track the cycle you are actually in.
- Cycles outside 21–45 days are refused. Persistently short, long or absent cycles are worth discussing with a doctor.
- This is not contraception. The calendar method alone has a high failure rate in typical use, because the window cannot be predicted precisely enough.
- Some apps add the day after ovulation to the window because the egg can live up to about a day. Wilcox's data found the window ended on ovulation day, which is what the page uses.
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Ovulation reference table
| Cycle length | Ovulation on cycle day | Fertile window, cycle days |
|---|---|---|
| 24 days | 11 | 6–11 |
| 26 days | 13 | 8–13 |
| 28 days | 15 | 10–15 |
| 30 days | 17 | 12–17 |
| 32 days | 19 | 14–19 |
| 35 days | 22 | 17–22 |
Ovulation FAQ
- Usually about 14 days before your next period. With a 28-day cycle that is around cycle day 14–15; with a 32-day cycle, around day 18–19. Enter the first day of your last period and your cycle length and the page gives the date.
- The two days before ovulation and ovulation day itself. In Wilcox's study the chance of conception was highest from intercourse on those days and fell to about 10% five days before; outside the six-day window it was essentially zero.
- Six days, ending on ovulation day. Sperm can survive in the reproductive tract for up to about five days, while the egg lives for roughly a day after release, so the window opens five days before ovulation and closes soon after it.
- From the day your next period is due. Home tests detect hCG, which rises after implantation; testing earlier risks a false negative. The page gives that date as 'earliest reliable pregnancy test'.
- Because the luteal phase — ovulation to the next period — is the steady part of the cycle at about 14 days, while the follicular phase before ovulation varies. A 26-day cycle ovulates around day 12–13 and a 35-day cycle around day 21–22.
- With short cycles, yes. In a 24-day cycle ovulation falls around day 10–11, so the fertile window starts around day 6, which can overlap the end of a period. The table under the result shows the window for the next three cycles.