Ovulation Calculator

Ovulation = last period + (cycle length − 14 days); fertile window = ovulation − 5 days to ovulation + 1 day

LMP

The first day of your last period, where the cycle is counted from. If that date is more than a cycle ago, the page moves the whole line forward by whole cycles until the window reaches today or later.

Cycle length

Days from the first day of one period to the first day of the next. It sets how far ovulation sits from your last period. (days)

14 days

The luteal phase, the stretch from ovulation to the next period. It is treated as the steady part of the cycle, which is why ovulation is placed by counting back from the next period rather than forward from the last one. (days)

5 days before

How long sperm can survive inside the body, described as about 3 days and sometimes up to 5. Taking the longer figure is the cautious direction: it can only widen the window. (days)

1 day after

How long the egg survives after ovulation, about 24 hours. Together with the sperm figure it gives a window of 7 days. (days)

Next period

Ovulation plus 14 days. It is the same date the period calculator reports, which is the point: the two pages describe one cycle from two directions.

Use it when you want to know which days could lead to pregnancy in this cycle. The period calculator covers the bleeding days themselves, and the conception calculator runs the other way, from a known due date back to the days that could have led to it.

1. Last period on 18 December 2025, 28-day cycle

   1. The cycle is 28 days, so ovulation falls 28 − 14 = 14 days after the last period: 2025-12-18 plus 14 days is 2026-01-01.
   2. That window ended on 2026-01-02, which is before today, so the whole line moves forward by one cycle of 28 days.
   3. Ovulation is now 2026-01-29, which is 14 days from today.
   4. The window runs from 5 days before it, 2026-01-24, to 1 day after it, 2026-01-30, and the next period is expected 14 days after ovulation, on 2026-02-12.

   The projection is what makes a date from last month useful today. A cycle that has already finished cannot tell you when to try, so the page advances by whole cycles only, never by part of one.

2. Last period on 28 December 2025, 30-day cycle

   1. The cycle is 30 days, so ovulation falls 30 − 14 = 16 days after the last period: 2026-01-13.
   2. That window has passed, so the line moves forward by one 30-day cycle, to 2026-02-12.
   3. The window is 2026-02-07 to 2026-02-13.
   4. The next period is 14 days after ovulation: 2026-02-26.

   A 30-day cycle does not move ovulation to day 30 minus 14 by accident: the 14 days after ovulation are the part of the cycle that stays put, so a longer cycle pushes ovulation later in the same amount.

3. Last period on 31 December 2025, 28-day cycle

   1. Ovulation falls 14 days after the last period: 2026-01-14.
   2. The window ends 1 day after ovulation, on 2026-01-15, which is today.
   3. Because the last day of the window is not behind today, the line is left where it is and nothing is projected.
   4. The count of days to ovulation is −1, since ovulation was yesterday, and the next period is 2026-01-28.

   This is the edge of the projection rule. Advancing here would hide a day that is still part of the current window, so the negative count is the useful answer rather than a mistake to correct.

Ovulation is estimated, not detected. The 14-day luteal phase is an average, and cycles that vary in length move the day with them, so a calendar cannot tell you when you ovulated in a given month. Conditions such as PCOS, recent hormonal contraception, and irregular cycles reduce the accuracy further. The window is deliberately wide, because sperm survival is given as about 3 days and sometimes up to 5, but a wide window is a cautious range rather than a schedule, and it is not a contraceptive method. If you are trying to conceive or to avoid pregnancy, treat these dates as a starting point for a conversation with your clinician.

Why is the fertile window wider before ovulation than after it?

Because sperm and eggs do not last the same length of time. Sperm can survive inside the body for about 3 days and sometimes up to 5, while an egg survives for about 24 hours after it is released. So the days that can lead to pregnancy start up to 5 days before ovulation and end 1 day after it, which makes a window of about 7 days that is not centred on ovulation day.

Does ovulation always happen 14 days after my period?

It happens about 14 days before your next period, which is not the same thing. The stretch after ovulation is the part of the cycle that stays closest to a fixed length, so the way to place ovulation is to count backwards from the next period. With a 28-day cycle that lands on day 14; with a 35-day cycle it lands on day 21, and the calculator shifts it for you.

The count of days says −1. Is that an error?

No. It means ovulation was yesterday and the fertile window is still open today, since the egg survives for about 24 hours after it is released. The page only moves the line forward when the whole window is behind you, so a window whose last day is today is left in place. A negative count is the honest answer in that case.

Why does the page show a date next month instead of the one I entered?

Because a cycle that ended last month cannot tell you when to try this month. When the window has entirely passed, the calculator advances your dates by whole cycles until the window reaches today or later. It moves by complete cycles only, so the day within the cycle stays the same and the rhythm you entered is preserved.

Can I use this instead of contraception?

No. These dates rest on an average cycle and on a sperm survival time that is given as a range, and ovulation itself is estimated rather than detected. Illness, travel, stress and irregular cycles all move it. ACOG treats fertility awareness as a method that has to be taught and used with rules of its own, not as a calendar you can read off a page, so talk to your clinician before relying on it in either direction.

Is the next period date the same as the one in the period calculator?

It should be. Here it is ovulation plus the 14-day luteal phase; in the period calculator it is your last period plus one cycle length. The two agree when your cycles are regular, and that agreement is the clearest way to see the rule underneath both pages: ovulation sits about two weeks before your next period begins.

• Fertility Awareness-Based Methods of Family Planning (patient FAQ) — American College of Obstetricians and Gynecologists, United States. Source of the survival times used here: sperm for about 3 days and sometimes up to 5, an egg for about 24 hours, and the resulting statement that pregnancy is possible from 5 days before ovulation until 1 day after it.
• Methods for Estimating the Due Date (Committee Opinion 700) — American College of Obstetricians and Gynecologists, United States, with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine (May 2017). Source of the convention that the estimated due date is 280 days after the first day of the last period, which places the cycle's second half where this page assumes it is.
• Fetal development (Medical Encyclopedia) — MedlinePlus, United States National Library of Medicine. States that an egg is released from an ovary at the end of the second week of the cycle and that this is when conception is most likely.