Ovulation Calculator
Estimate ovulation and the six-day fertile window from your last period and cycle length, with the next three cycles mapped and an honest note on accuracy.
Ovulation Calculator: with the default inputs, estimated ovulation date is September 24, 2026.
From the first day of one period to the first day of the next. Average your last 3–6 cycles.
- Fertile window
- September 19, 2026 to September 24, 2026The five days before ovulation plus ovulation day. Conception odds peak in the two days before.
- Next period expected
- October 8, 2026
- Earliest reliable pregnancy test
- October 8, 2026The day your period is due. Earlier tests miss many pregnancies.
- Due date if you conceive this cycle
- June 17, 2027
Assumptions
- Ovulation is placed at cycle length minus luteal phase (default 14 days) after the first day of the last period.
- Future cycles assume the same length as the average you entered.
- The fertile window is the six days ending on ovulation day (Wilcox 1995). This is an estimate for planning, not a contraceptive method.
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|---|---|---|---|
| 1 | September 10, 2026 | Sep 19 – Sep 24 | September 24, 2026 | October 8, 2026 |
| 2 | October 8, 2026 | Oct 17 – Oct 22 | October 22, 2026 | November 5, 2026 |
| 3 | November 5, 2026 | Nov 14 – Nov 19 | November 19, 2026 | December 3, 2026 |
| 4 | December 3, 2026 | Dec 12 – Dec 17 | December 17, 2026 | December 31, 2026 |
Cycle 1 is your current cycle. Each later cycle assumes the same length, which is the biggest source of error.
How this is worked out
The formula
Ovulation ≈ first day of last period + (cycle length − luteal phase length) Fertile window = ovulation − 5 days through ovulation day Next period = first day of last period + cycle length Due date if conceived = ovulation + 266 days
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- First day of last period
- A calendar date.defaults to today
- Average cycle length
- From the first day of one period to the first day of the next. Average your last 3–6 cycles.from 20 to 45 · whole numbers only · defaults to 28
- Luteal phase length(under More options)
- Days from ovulation to the next period. 12–14 is typical and fairly constant for each person.from 9 to 18 · whole numbers only · defaults to 14
What you get back
- Estimated ovulation datemain answer
- Fertile window
- The five days before ovulation plus ovulation day. Conception odds peak in the two days before.
- Next period expected
- Earliest reliable pregnancy test
- The day your period is due. Earlier tests miss many pregnancies.
- Due date if you conceive this cycle
What this assumes
- Ovulation is placed at cycle length minus luteal phase (default 14 days) after the first day of the last period.
- Future cycles assume the same length as the average you entered.
- The fertile window is the six days ending on ovulation day (Wilcox 1995). This is an estimate for planning, not a contraceptive method.
About this calculator
Ovulation is when an ovary releases an egg, and the egg lives only about 12–24 hours. Sperm survive up to five days in fertile cervical mucus, which is why the fertile window is six days long and sits mostly before ovulation. Getting that window right — and knowing how fuzzy any calendar estimate is — matters more than pinning down a single day.
How the estimate works
The calendar method counts backwards from the next period, not forwards from the last one. The luteal phase — ovulation to the next period — is fairly constant at 12–14 days for a given person, while the first half of the cycle is what varies. So: next period = last period + cycle length; ovulation ≈ next period − luteal phase. For a 28-day cycle with a 14-day luteal phase, that's day 14. For a 32-day cycle it's day 18, not day 16 — a common mistake.
The fertile window runs from five days before ovulation through ovulation day. Wilcox and colleagues' landmark 1995 study found essentially all conceptions came from intercourse in that six-day span, with the highest odds in the two days before ovulation.
How accurate is this?
Not very, for any single cycle — and it's important to say so. In a study of over 200 women with "regular" cycles, the fertile window fell entirely within days 10–17 in only about 30% of them. Stress, illness, travel and age shift ovulation by days. Use the calendar to know roughly when to start paying attention, then confirm with:
- Ovulation predictor kits, which detect the LH surge 24–36 hours before ovulation.
- Basal body temperature, which rises about 0.3 °C after ovulation — useful for confirming your luteal phase length for future cycles.
- Cervical mucus, which becomes clear and stretchy in the fertile days.
Reading the results
The table maps your next three cycles so you can plan ahead. The pregnancy test date is the day your period is due — testing earlier is tempting but produces false negatives, because hCG often isn't detectable until implantation completes 8–10 days after ovulation. The "due date if you conceive" line is simply ovulation plus 266 days.
When it doesn't apply
If your cycles vary by more than 7–9 days, or are shorter than 21 or longer than 35 days, you may not ovulate every month and calendar prediction breaks down. That pattern is common with PCOS, thyroid problems, and around perimenopause, and it's worth a conversation with a clinician — especially if you've been trying to conceive for a year (six months if over 35). This calculator is not a contraceptive method.
Frequently asked questions
▸When am I most fertile?
The two days before ovulation and ovulation day itself. Intercourse every 1–2 days across the six-day fertile window covers the uncertainty in the estimate.
▸Do I ovulate on day 14?
Only if your cycle is about 28 days with a 14-day luteal phase. Ovulation is roughly cycle length minus 14: day 18 for a 32-day cycle, day 12 for a 26-day cycle — and it varies month to month.
▸How accurate is an ovulation calculator?
It gives a useful starting point but is wrong by several days in most cycles. Ovulation predictor kits and basal body temperature are far more reliable if timing matters.
▸Can I use this to avoid pregnancy?
No. Calendar-based estimates are too imprecise for contraception; with typical use, calendar methods have failure rates well above 10% per year.
▸When should I take a pregnancy test?
From the day your period is due. Implantation happens 8–10 days after ovulation and hCG needs a few more days to rise, so earlier tests often miss real pregnancies.
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