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Ovulation Calculator

Estimate your fertile window, ovulation day and predicted next period from your last period date and cycle length.

✓ Last reviewed: Sources: ACOG Committee Opinion No. 651 (2015) — Menstruation as a Vital Sign (zakres cyklu 21-35)

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Number of days from the first day of one period to the first day of the next. Typical range is 21-35 days; we default to 28.

Advanced options

Number of days from ovulation to your next period. Typical range is 10-17 days, most commonly 14. Leave the default if you don't know yours.

Also show predicted ovulation and next period for upcoming cycles (2-3), counted from the current one.

Your result

Fill in the fields above to see your result.

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What the ovulation calculator tells you

This ovulation calculator answers the question most people ask when they’re trying to conceive: when do I ovulate, and which days are fertile? Enter the first day of your last period and your cycle length, and you get your predicted ovulation day, your whole fertile window (a range of dates, not a single day), your predicted next period, and a suggested day to start ovulation tests.

Most tools online just show the ovulation date. We treat it as a full ovulation calendar instead:

  • your ovulation day — the most fertile day of your cycle,
  • your fertile window — the six days when sex can lead to pregnancy,
  • your next period — when to expect your period if you don’t conceive,
  • your LH test start — when it’s worth beginning ovulation tests,
  • optionally an irregular-cycle mode (a min–max range) and a forecast for the next 2–3 cycles.

Everything the ovulation calculator shows is an estimate based on a calendar model — a planning aid, not a diagnosis. We explain why, and when to trust it, below.

How to use the ovulation calculator

  1. Enter the first day of your last period. That’s day 1 of your cycle — the first day of bleeding, not the day it ended.
  2. Enter your cycle length. This is the number of days from the first day of one period to the first day of the next. The default is 28 days; the typical range is 21–35 days.
  3. Read your results. Your ovulation day, fertile window and predicted period appear instantly.
  4. Open the advanced options if you need them. Have irregular periods? Switch the mode and enter your shortest and longest cycle — we’ll show a wider window of possible fertility. Know your luteal phase? Set it instead of the default 14 days. Want to plan ahead? Turn on the multi-cycle forecast.

How we calculate ovulation and the fertile window

The ovulation formula

Ovulation doesn’t sit rigidly “in the middle” of your cycle — it falls a fixed number of days before your next period. The phase after ovulation (the luteal phase) is relatively stable within one woman, and is classically taken as 14 days. That gives the formula behind institutional calculators like the NHS and the US Office on Women’s Health:

Ovulation day = first day of your last period + (cycle length − luteal phase)

For a typical 28-day cycle with a 14-day luteal phase, ovulation lands on day 14 of the cycle — the “about 14 days before your period” rule of thumb. But on a 21-day cycle ovulation comes as early as day 7, and on a 35-day cycle not until day 21. That’s why cycle length matters so much, and why we don’t assume it for you.

You can set the luteal phase in the advanced options. 10–17 days is considered normal (Cleveland Clinic), and a phase shorter than 10 days occurs naturally in many healthy women — the calculator will still work it out and simply flag it, never block the result.

The fertile window is six days

Ovulation is a single day, but there are more fertile days than that. The classic Wilcox study (NEJM, 1995) found that conception almost only happened when intercourse fell within a six-day window ending on the day of ovulation:

Fertile window = [ovulation day − 5 days, ovulation day]

Why five days before ovulation? Sperm can survive in the female reproductive tract for up to about 5 days, so sex a few days before ovulation can still lead to pregnancy — the sperm “wait” for the egg. That’s why the fertile window opens well before ovulation itself.

Ovulation day is not the same as the fertile window

It’s worth separating two things. The fertile window lasts 6 days, because that’s roughly how long sperm live. But the egg itself can only be fertilized for 12–24 hours after ovulation. So the day of ovulation and the day just before it offer the best odds of conceiving, and after ovulation the chance drops sharply. In short: the fertile window is when it’s worth having sex; ovulation is the peak of fertility.

When to start ovulation tests

LH tests detect the hormone surge that precedes ovulation by roughly a day. To avoid missing it, it’s worth starting to test a few days before your expected ovulation — the calculator suggests starting about 3 days earlier (per MedlinePlus/NIH).

Irregular cycles

If your cycle varies from month to month, a single ovulation day won’t capture the truth. In irregular-cycle mode you enter your shortest and longest cycle, and the calculator works out two extreme ovulation days and stretches a wide window of possible fertility between them. With irregular cycles, though, the calendar alone is less reliable — real-time signs are more telling: LH tests, cervical mucus and basal body temperature (per ASRM).

A worked example, step by step

Say your last period started on 1 June 2026, with a typical 28-day cycle.

  1. Ovulation day = 1 June + (28 − 14) = 15 June 2026.
  2. Fertile window = from 15 June − 5 days to 15 June = 10 to 15 June 2026 (six days).
  3. Next period = 1 June + 28 days = 29 June 2026.
  4. LH test start ≈ 15 June − 3 days = 12 June 2026.
  5. If conception happened in that window, the estimated due date would be 15 June + 266 days = 8 March 2027 — exactly the result our due date calculator gives from a conception date.

Change the inputs and the window shifts with your cycle:

  • Short cycle (21 days): last period 1 March 2026 → ovulation 8 March, fertile window 3–8 March. Notice that on a short cycle the window opens just a few days after your period starts — which is why you can get pregnant soon after your period.
  • Irregular cycle (25–35 days): last period 1 May 2026 → ovulation possible between 12 and 22 May, with a wide fertile window stretching from 7 to 22 May.
  • Three-cycle forecast (28-day cycle from 1 June 2026): ovulation falls on 15 June, 13 July and 10 August 2026, with periods on 29 June, 27 July and 24 August.

How reliable is it, and what to keep in mind

The ovulation calculator assumes your cycle and luteal phase are fairly steady. In reality, ovulation can shift by several days from one cycle to the next — so treat the result as a starting point, not a fixed date. The more irregular your cycle, the wider the margin of error, and the more it pays to track the signs of ovulation directly: cervical mucus, basal body temperature and LH tests.

This calculator is not a method of contraception. Calendar methods have a meaningful failure rate in real-world use, because ovulation can be unpredictable — don’t rely on this tool to avoid pregnancy. If you want to plan or avoid pregnancy using fertility awareness methods, talk to a doctor or midwife.

If you’ve been trying to conceive for more than 12 months (or more than 6 months if you’re 35 or older), it’s worth discussing with a doctor — that’s the standard consultation threshold recommended by ACOG. And if you want to see when a due date would fall for a conception this cycle, head to our due date calculator and choose the “conception date” method.

Frequently asked questions

How many days after your period do you ovulate?

It depends on your cycle length. On a typical 28-day cycle you ovulate around day 14, roughly 9 days after a five-day period ends. On a short cycle (say 21 days) you ovulate as early as day 7, only a couple of days after your period. That’s why there’s no fixed “days after your period” answer — enter your own cycle length to get yours.

How long does the fertile window last?

The fertile window lasts about 6 days: the five days before ovulation plus the day of ovulation itself. That definition comes from the 1995 Wilcox study and is echoed by the NHS and ASRM. The six-day length reflects how long sperm survive — up to about 5 days in the reproductive tract, long enough to “wait” for the egg.

What’s the difference between ovulation day and the fertile window?

Ovulation day is a single day — when the egg is released and fertility peaks. The fertile window is the six-day span around it (ovulation plus the five days before), during which sex can lead to pregnancy. The difference exists because sperm live up to 5 days while the egg survives only 12–24 hours after ovulation.

What is the best time to have sex to get pregnant?

Your best odds come in the days just before ovulation and on ovulation day itself, when sperm are already in place waiting for the released egg. In practice it helps to have sex every 1–2 days throughout the fertile window, rather than waiting for one “perfect” day. LH tests and watching your cervical mucus help you time it more precisely.

How accurate is an ovulation calculator with irregular periods?

Less accurate than with regular cycles, because it relies on a predictable cycle length. Switch to irregular-cycle mode and enter your shortest and longest cycle — the calculator then shows a wide window of possible fertility rather than a single date. With irregular periods, LH tests, cervical mucus tracking and basal body temperature are more reliable than the calendar alone.

How long does an egg live after ovulation, and how long do sperm live?

The egg can only be fertilized for 12–24 hours after ovulation. Sperm are far hardier — they survive in the female reproductive tract for up to about 5 days. That difference is exactly why the fertile window opens several days before ovulation: sperm can wait for the egg, but not the other way around.

Can you get pregnant right after your period?

Yes, especially on shorter cycles. Since sperm live up to 5 days, and a 21-day cycle ovulates around day 7, sex right after your period ends can coincide with the opening fertile window. “Safe days” immediately after your period are a myth you shouldn’t rely on.

Can I use an ovulation calculator as a form of birth control?

No. The calculator assumes a regular cycle and an average ovulation day, but real ovulation can shift by several days — calendar-based methods have a meaningful failure rate in real-world use. Don’t rely on this tool as your only way to avoid pregnancy. If you want contraception based on fertility awareness, discuss it with a doctor or midwife.

What are the signs and symptoms of ovulation?

The most common signs are clear, stretchy cervical mucus (like raw egg white), a slight rise in basal body temperature after ovulation, sometimes one-sided ovulation pain low in the abdomen, and an increase in libido. LH tests detect the hormone surge about a day before ovulation. Signs vary between women, and combining a few methods gives the clearest picture.

How does cycle length change your ovulation date?

Because ovulation falls a fixed number of days before your next period, a longer cycle pushes ovulation later and a shorter one pulls it earlier. With a 14-day luteal phase, a 28-day cycle ovulates on day 14, a 35-day cycle on day 21, and a 21-day cycle on day 7. That’s why entering your real cycle length matters far more than assuming “day 14”.

Sources