Luteal Phase Calculator: The Exact Formula to Use

by Jun 9, 2021

When it comes to getting pregnant, all of the hormone tracking and calculations can sometimes feel a bit like homework. Whether you’re optimistically estimating a due date, or planning the ideal time for intercourse, you can’t help but do the math.


Luckily some simple math is all it takes to calculate an important part of your cycle – the luteal phase.  Your luteal phase can be an important indicator of fertility and only takes a simple formula to figure out.  Read on for how to determine your luteal phase and why you need to know this information on your fertility journey.

Luteal phase calculator

Menstrual cycles vary from woman to woman and can last anywhere from 21 to 35 days.  Because menstruation happens in a fairly regular pattern, you can calculate your luteal phase by tracking your cycle and making note of important dates (like the first day of your last period and day of ovulation).  Plugging those numbers into a simple formula determines the length of your luteal phase.

What you need to know

To determine the length of your luteal phase you’ll need to know a couple of important dates before you can calculate anything:

  • The date of ovulation prior to your period
  • The date your menstrual period starts

Once you know these dates, you can plug them into the formula below to determine the length of your luteal phase.

How to calculate your luteal phase

The luteal phase is the number of days between when you ovulate and when you begin bleeding and can be calculated with a simple formula.

Date of menstrual period –  date of ovulation = number of days in luteal phase

To calculate the length of your luteal phase you simply subtract the date of your menstrual cycle  from the date of ovulation and you are left with the number of days in your luteal phase.  For example, if your date of ovulation was April 14th and your period began on April 28th the calculation would look like this: 28-14 = 14.  In this example, the luteal phase is 14 days long.

The trick with luteal phase calculations is knowing when you ovulate.  This can be done either through traditional methods like ovulation calendars or more precisely with a fertility tracker like the Mira digital fertility analyzer.

What’s a normal luteal phase?

A normal luteal phase can last 11-17 days and is an essential marker of fertility.  Anything outside of this range is considered abnormal and can negatively impact your fertility.  It is during this phase that implantation takes place, and a fertilized egg needs enough time for that to happen.

What if your luteal phase is short?

Luteal phases less than 10 days are considered short and may be a sign of a luteal phase deficiency.  Having a short luteal phase can affect your fertility since it doesn’t give the body a chance to develop enough to support a pregnancy.  This may be associated with a deficit in progesterone production.

Progesterone not only prepares the body for pregnancy, but ensures you are able to get and stay pregnant by thickening the lining of the uterus for a fertilized egg to implant.  A short luteal phase doesn’t give the uterine lining a chance to develop this thickness enough to receive and support an embryo, making it harder to maintain pregnancy.

What if your luteal phase is long?

As long as you are in the normal range, the length of your luteal phase shouldn’t be concerning.  By tracking your cycle, you will have a better idea of when things seem off or out of range.  Of course a long luteal phase could also mean you are pregnant and haven’t realized it yet.  If you are experiencing other worrisome symptoms like unusually heavy periods or pain during sex, it might be time to see a doctor though.

What is the luteal phase?

Your menstrual cycle is divided into two phases: follicular and luteal.  The part of the menstrual cycle that begins immediately after ovulation is known as the luteal phase.  Named after the structure that forms instead of the ruptured follicle from which the egg was released, or the corpus luteum, the luteal phase follows the follicular phase and ends when the next bleed begins.  During the luteal phase, progesterone is significantly higher than in the follicular phase of the cycle.

The luteal phase can be an important indicator of fertility so it’s useful to know what should be happening during this phase and how this plays out in your own cycle.  Tracking your menstrual cycle and monitoring the signs of fertility in your body will help you determine your luteal phase so you can use this information to your advantage on your fertility journey.

✔️ Medically Reviewed by Katerina Shkodzik, M.D., OB-GYN

Dr. Katerina Shkodzik is a certified OB-GYN with a special focus on reproductive endocrinology and infertility issues. She has been practising since 2015.

Dr. Shkodzik completed her residency program in the Department of OB/GYN at the Belarusian State Medical University and fellowship program in the Department of Gynecological Surgery at the Medical University of Bialystok, Poland.

Dr. Shkodzik is extensively involved in digital health projects providing her medical expertise and integrating of cutting edge technologies in medical science and clinical practice since 2018.

Dr. Shkodzik has participated in several studies focused on PCOS, endometriosis, menstrual cycle characteristics and their abnormalities based on big data of digital health in collaboration with leading universities.

She believes that paying special attention to women's health is a crucial step to improving the world we live in.

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