Ovulation Calculator

Created by Bogna Szyk and Łucja Zaborowska, MD, PhD candidate
Reviewed by Małgorzata Koperska, MD, Jack Bowater and Adena Benn
Based on research by
Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ (November 2000)See 1 more source
Bull JR, Rowland SP, Scherwitzl EB, Scherwitzl R, Danielsson KG, Harper J. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles npj Digital Medicine (August 2019)
Last updated: Sep 11, 2023

If you want to conceive, you will be asking yourself, "when are my most fertile days?". Luckily, this free ovulation calculator is here to help you! In just a few clicks, you can assess when you are ovulating over the next three cycles.

You can use this ovulation tracker not only to maximize your chances of conceiving but also to estimate your due date.

What is ovulation?

Ovulation is the release of an egg from one of your ovaries. It typically occurs around two weeks before the beginning of your period, in the middle of the menstrual cycle — see period calculator.

You are the most fertile around the time of your ovulation — to be more precise, during the so-called fertile window. It starts seven days before your ovulation day, as sperm is able to survive for up to seven days in your body. Since the egg has a much shorter lifespan, the fertile window ends 24 hours after ovulation. This period ends with the beginning of the luteal phase — see the luteal phase calculator to see how to calculate it.

Most birth control methods rely on blocking ovulation or altering the environment inside the uterus (see pearl index calculator).

Ovulation tracker principles

In order to use this free ovulation calculator, you need to input two pieces of information about your menstrual cycle:

  • Your cycle duration. A typical cycle lasts for around 28 days. However, it is perfectly normal to have a cycle that is a bit longer or a bit shorter, for example, lasting 27 or 30 days. If you're unsure what your cycle duration is (or you menstruate irregularly), stick to the default duration of 28 days.

  • Last period started on... The other piece of information this ovulation tracker needs is the date when your last period began.

When am I ovulating?

As mentioned before, you ovulate in the middle of your menstrual cycle, around two weeks before your next period. If you want to find a more precise date, you can always try charting your body temperature (or using our basal body temperature calculator) or checking your cervical mucus.

The fertile window starts seven days before ovulation and ends one day after. For example, let's assume your last period began on May 26th, and your cycle lasts for 30 days. If you're trying to get pregnant, your most fertile days in June will be the 4th to the 12th.

Our ovulation tracker displays your fertility windows for the next three cycles. It means that if you miss the fertility window in June, you can try to conceive between the 4th and 12th of July or between the 3rd and 11th of August.

Free ovulation calculator and due date

The last feature of this ovulation calculator is the estimation of the due date — that is, when your baby will be born if you conceive. We assume that the pregnancy lasts 280 days (40 weeks), but you should be aware that only five percent of all women deliver on their due date. Your child might be born a bit earlier or later.

For example, if you conceived during the fertile window of June 4th to 12th, then you can expect your baby around the middle of March. In the meantime, be sure to take a look at the pregnancy weight gain calculator!


How do I calculate when I am ovulating?

To calculate when you are ovulating, you can follow these steps:

  1. Note when was your last period.
  2. Add two weeks to your last period.
  3. That's all! You have now calculated your ovulation day — approximately in the middle of your menstrual cycle.

When is my ovulation day if my last period started on September 20th?

If your last period started on September 20th, assuming you have a 28-day cycle duration, your ovulation day will be around October 4th. You can visit the Omni Ovulation Calculator to learn more about ovulation, the fertility window, and the due date.

When is my fertility window if my last period started on September 20th?

Assuming you have a 28-day cycle duration, if your last period started on September 20th, the first day of your fertility window will be September 27th, and the last day of your fertility window will be October 5th.

Are there any physical changes associated with ovulation?

During ovulation, one might observe various physical changes, including:

  • Increase in basal body temperature;
  • Changes in cervical mucus consistency, often becoming more stretchy and clear;
  • Sensations of breast soreness or tenderness; and
  • Heightened senses, such as a more sensitive sense of smell.
Bogna Szyk and Łucja Zaborowska, MD, PhD candidate
Cycle duration
My last period started on...
First cycle
Ovulation day
Fertility window - first day
Fertility window - last day
Due date
Second cycle
Ovulation day
Fertility window - first day
Fertility window - last day
Due date
Third cycle
Ovulation day
Fertility window - first day
Fertility window - last day
Due date
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