Skip to content

Ovulation Calculator

HealthPregnancy and Fertility

Count from the first day of one period to the day before the next.

Days from ovulation to the next period. Keep 14 unless you have tracked your own.

Estimated ovulation day —
Fertile window
—
Ovulation on cycle day
—
Next period expected
—
Due date if conceived this cycle
—

Ovulation usually happens about 14 days before the next period starts, and the fertile window is the five days before it plus the day itself and the day after. With a 28-day cycle that began on 1 January 2026, the next period is due on 29 January, so ovulation is estimated for 15 January.

About this tool

People trying to conceive, and people tracking their cycle for other reasons, often want to know which days carry the highest chance of pregnancy. Enter the first day of your last period and your usual cycle length, and this calculator estimates the ovulation date, the fertile window, the next period and the due date if conception happens this cycle. A table projects the same dates for six cycles. The luteal phase can be set from 10 to 16 days if you have tracked it with temperature charts or test strips. The estimate is calendar arithmetic only: it assumes every cycle is the same length, so irregular cycles, recent hormonal contraception or breastfeeding can move ovulation by several days.

How to use it

  1. Pick the start date

    Choose the first day of full bleeding in your most recent period.

  2. Set your cycle length

    Use the − and + buttons to enter your average cycle length in days, from 21 to 45.

  3. Adjust the luteal phase if known

    Leave the slider at 14 days unless your own tracking shows a different length.

  4. Read the dates and table

    The ovulation day, fertile window and the next six cycles update as you change any input.

Examples

28-day cycle starting 1 January

First day of your last period
2026-01-01
Average cycle length
28
Luteal phase
14

Result Estimated ovulation day: Thursday, 15 January 2026
Fertile window: 10 Jan 2026 – 16 Jan 2026
Ovulation on cycle day: 15
Next period expected: Thursday, 29 January 2026
Due date if conceived this cycle: Thursday, 8 October 2026

  1. Ovulation = 1 Jan 2026 + (28 − 14) days = 15 Jan 2026 (cycle day 15)
  2. Fertile window = ovulation − 5 days to ovulation + 1 day = 10 Jan 2026 to 16 Jan 2026
  3. Next period = 1 Jan 2026 + 28 days = 29 Jan 2026
  4. Due date if conceived = 1 Jan 2026 + 280 + (28 − 28) days = 8 Oct 2026

The next period is due on 29 January, so ovulation falls 14 days earlier, on 15 January, with the fertile days running from 10 to 16 January.

32-day cycle starting 10 March

First day of your last period
2026-03-10
Average cycle length
32
Luteal phase
14

Result Estimated ovulation day: Saturday, 28 March 2026
Fertile window: 23 Mar 2026 – 29 Mar 2026
Ovulation on cycle day: 19
Next period expected: Saturday, 11 April 2026
Due date if conceived this cycle: Saturday, 19 December 2026

  1. Ovulation = 10 Mar 2026 + (32 − 14) days = 28 Mar 2026 (cycle day 19)
  2. Fertile window = ovulation − 5 days to ovulation + 1 day = 23 Mar 2026 to 29 Mar 2026
  3. Next period = 10 Mar 2026 + 32 days = 11 Apr 2026
  4. Due date if conceived = 10 Mar 2026 + 280 + (32 − 28) days = 19 Dec 2026

A longer cycle moves ovulation later: day 19 rather than day 15, because the luteal phase stays about 14 days.

How it is calculated

Ovulation = LMP + (C − L) days; fertile window = ovulation − 5 days to ovulation + 1 day; due date = LMP + 280 + (C − 28) days

LMP
first day of the last menstrual period
C
average cycle length in days
L
luteal phase, the days from ovulation to the next period, typically 14

The luteal phase is fairly constant between people, so the calculator counts back from the expected next period rather than forward from the last one. Sperm can survive up to five days in the reproductive tract and an egg about a day, which sets the six-to-seven-day fertile window. The due date applies Naegele's rule, 280 days from the last period, shifted by the difference between your cycle and 28 days.

Sources

When not to use it

  • As contraception: calendar estimates alone miss shifted ovulation often enough to cause unplanned pregnancy.
  • When cycles vary by more than a week from month to month, since one average length cannot place ovulation.
  • In the months after stopping hormonal contraception, giving birth or while breastfeeding, when cycles are often irregular.

Common mistakes

  • Entering the last day of the period instead of the first day of full bleeding.
  • Counting cycle length to the first day of the next period inclusive, which adds one day.
  • Treating the ovulation date as exact rather than as the centre of a range of likely days.

Frequently asked questions

Why does a longer cycle move ovulation later?

The phase after ovulation, the luteal phase, usually lasts close to 14 days in most people. Extra length in a cycle sits in the phase before ovulation, so a 35-day cycle ovulates around day 22 rather than day 15.

How long is the fertile window?

About six days: the five days before ovulation and ovulation day, because sperm can live up to five days. This calculator also includes the day after ovulation, as the egg survives roughly 12 to 24 hours and the exact moment of ovulation is uncertain.

Which days give the highest chance of pregnancy?

The two to three days before ovulation and ovulation day itself. Sex every two to three days through the cycle covers the window without needing to know the exact date, which is the approach the NHS describes.

How can I confirm ovulation instead of estimating it?

Urine ovulation test strips detect the LH surge one to two days before ovulation. A rise in basal body temperature of about 0.2 to 0.5 °C confirms ovulation afterwards, and cervical mucus becomes clear and stretchy near the fertile days.

What luteal phase length should I enter?

Fourteen days is the textbook value. If you chart basal temperature, count the days from the temperature rise to the day before your period and enter that, between 10 and 16 days.

Is the due date shown here the same as a dating scan?

No. It is Naegele's rule adjusted for cycle length. An early ultrasound measures the embryo and is the reference clinicians use when the two dates disagree.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 19). Ovulation Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/health/pregnancy-and-fertility/ovulation-calculator/

MLA

Tools Directory Editorial Team. “Ovulation Calculator.” Tools Directory, 19 Oct. 2026, https://tools.milunir.com/health/pregnancy-and-fertility/ovulation-calculator/. Accessed 28 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Ovulation Calculator.” Tools Directory. Last modified October 19, 2026. Accessed October 28, 2026. https://tools.milunir.com/health/pregnancy-and-fertility/ovulation-calculator/.

Link<a href="https://tools.milunir.com/health/pregnancy-and-fertility/ovulation-calculator/">Ovulation Calculator – Tools Directory</a>