Ovulation Calculator

Enter the first day of your last period and your usual cycle length to estimate your ovulation day, fertile window and next period, plus the next three cycles.

Day 1 is the first day of real bleeding, not spotting.
days
From one period’s first day to the next (21–45 days).
days
Ovulation to next period. Leave at 14 unless you have measured it (10–16).

Private: your dates stay on this device. They are not saved, added to the page address or sent anywhere.

Example Enter the first day of your last period above.

Estimated ovulation

Friday, October 2, 2026

Your fertile window starts in 2 days, on Sun, Sep 27.

Fertile window
Sep 27 – Oct 2, 2026
6 days: 5 before ovulation + ovulation day
Most fertile days
Sep 30 – Oct 2, 2026
2 days before ovulation + ovulation day
Next period due
Fri, Oct 16
28-day cycle
Due date if you conceive
Fri, Jun 25, 2027
Ovulation + 266 days

This cycle at a glance

Next 3 cycles (every 28 days)
Period startsFertile windowOvulation
Fri, Oct 16Oct 25 – Oct 30Fri, Oct 30
Fri, Nov 13Nov 22 – Nov 27Fri, Nov 27
Fri, Dec 11Dec 20 – Dec 25Fri, Dec 25

Working

  1. First day of your last period (cycle day 1): Friday, September 18, 2026.
  2. Ovulation is estimated at 14 days (the luteal phase) before your next period. With a 28-day cycle that is 28 − 14 = 14 days after your period started: September 18, 2026 + 14 days = Friday, October 2, 2026.
  3. Fertile window: the 5 days before ovulation plus ovulation day, September 27, 2026 to October 2, 2026 (6 days). The chance of conceiving is highest on the last three: Sep 30 – Oct 2, 2026.
  4. Next period: September 18, 2026 + 28 days = Friday, October 16, 2026.
  5. Later cycles repeat every 28 days, so the next 3 periods are due October 16, 2026, November 13, 2026, December 11, 2026; each ovulation is 14 days after its period starts.
  6. Today is September 25, 2026: the fertile window starts in 2 days, and ovulation is 7 days away.
  7. If you conceive this cycle, the estimated due date is ovulation + 266 days = Friday, June 25, 2027.

An estimate for information only, not medical advice, and not a method of birth control. Ovulation can shift from cycle to cycle. Planning a pregnancy? See the due date calculator once you have a positive test.

How to use the ovulation calculator

Enter the first day of your most recent period and your average cycle length. The estimated ovulation day, your fertile window, the most fertile days and the date your next period is due appear straight away, with a calendar of the whole cycle, the next three cycles and the working underneath. Until you change the date, the example uses a period that began a week ago. If you have tracked your own luteal phase, enter it too; otherwise leave it at 14 days.

Nothing you enter leaves your device. Unlike most of our calculators, this one deliberately does not put your numbers in the page address, so a shared or bookmarked link never reveals your dates.

How the dates are calculated

Calendar methods count backward from the next period: ovulation usually happens about two weeks before a period starts. Everything else follows from that:

Ovulation = first day of last period + (cycle length − luteal phase) Fertile window = ovulation − 5 days … ovulation day (6 days) Next period = first day of last period + cycle length Due date if you conceive = ovulation + 266 days

With a 28-day cycle and a 14-day luteal phase, ovulation lands 14 days after your period started, which is the same assumption ACOG’s standard due date rule makes. Longer cycles move ovulation later: in a 35-day cycle it comes about 21 days after the period starts. The luteal phase stays roughly the same while the first part of the cycle stretches or shrinks.

The six-day fertile window comes from a landmark 1995 study by Allen Wilcox and colleagues at the NIH’s National Institute of Environmental Health Sciences. They followed 221 women trying to conceive, pinpointed ovulation from daily urine hormone tests, and recorded 192 pregnancies over 625 cycles. Conception happened only when intercourse took place in the six days ending on ovulation day. The chance was about 10% for intercourse five days before ovulation and rose to about 33% on ovulation day. Sperm can survive in the body for several days, but the egg can be fertilized only for a short time after it is released, which is why the window ends at ovulation.

Worked example

Suppose your last period started on Friday, September 18, 2026, and your cycles are usually 30 days long.

  • Ovulation: 30 − 14 = 16 days after your period started, so September 18, 2026 + 16 days = Sunday, October 4, 2026.
  • Fertile window: the five days before plus ovulation day, Sep 29 – Oct 4, 2026; the most fertile days are Oct 2 – Oct 4, 2026.
  • Next period: September 18, 2026 + 30 days = Sunday, October 18, 2026. The following ones are due November 17, 2026 and December 17, 2026.
  • If you conceive in this cycle, the estimated due date is October 4, 2026 + 266 days = Sunday, June 27, 2027 — the same date the due date calculator gives from the last period with a 30-day cycle.

How accurate is a calendar estimate?

It is a reasonable starting point, not a prediction. Even with regular periods, the day of ovulation varies from one cycle to the next, and the luteal phase is not exactly 14 days for everyone. If your cycle length changes by more than a week from month to month, an average is a poor guide. The Office on Women’s Health notes that cycles typically range from 21 to 35 days in adults and from 21 to 45 days in young teens. To pin down your own ovulation:

  • Ovulation predictor kits detect the surge of luteinizing hormone (LH) in urine that comes shortly before ovulation. Start testing a few days before the fertile window shown here.
  • Basal body temperature (BBT) rises slightly after ovulation. It confirms ovulation afterward rather than predicting it, but a few months of charts show your pattern — and your real luteal phase.
  • Cervical mucus becomes clear, slippery and stretchy, like raw egg white, around the most fertile days.

This is not birth control. Fertility awareness-based methods of family planning combine several daily signs, need instruction and are less effective with typical use than many other methods. A calendar estimate on its own is not a safe way to avoid pregnancy. ACOG’s guide to these methods explains the options.

When to talk to a doctor

See a clinician if your periods are very irregular or stop for several months, or if you have been trying to conceive for 12 months (6 months if you are 35 or older) without a pregnancy. If you are planning a pregnancy, a preconception visit is a good time to start a daily folic acid supplement and review any medicines you take. Once you are pregnant, the due date calculator shows how far along you are, and the date difference calculator counts the days between any two dates.

Frequently asked questions

When am I most fertile?

In the six days that end on ovulation day: the five days before ovulation and the day itself. In the 1995 NIH study that defined this window, the chance of conceiving rose from about 10% for intercourse five days before ovulation to about 33% on ovulation day, so the last three days of the window are the most fertile.

Can I get pregnant outside the dates shown?

Yes. The calculator shows an average, and ovulation can come earlier or later than expected in any cycle, even when periods are regular. Stress, illness, travel and breastfeeding can all shift it. That is why these dates are not a reliable way to avoid pregnancy.

How do I work out my cycle length?

Count from the first day of one period (the first day of real bleeding) up to the day before the next period starts. For example, a period on March 1 followed by one on March 29 is a 28-day cycle. Track three to six cycles and use the average. If your cycles differ by more than a week from month to month, a calendar estimate will be rough.

What is the luteal phase and should I change it?

It is the time from ovulation to the start of your next period, usually about two weeks. Calendar methods assume 14 days. Leave it at 14 unless you have tracked ovulation yourself, for example with ovulation tests or a basal body temperature chart, and know your own luteal phase is shorter or longer.

Is my information saved or shared?

No. Everything is calculated in your browser. The dates are not stored, not added to the page address and not sent anywhere, so sharing the page link shares only the empty calculator.

How long should we try before seeing a doctor?

ACOG advises an infertility evaluation after 12 months of trying for women under 35, after 6 months for women 35 and older, and sooner for women over 40 or with known problems such as irregular or absent periods. The ACOG infertility FAQ explains what the tests involve.

Sources

Last reviewed · Built and checked by the Reeliy team · How we test our tools