Calculating ovulation using the Calendar/ Rhythm Method

The Rhythm/Calendar Method calculates ovulation using the length of previous cycles. It can be accurate, however, for women who do not have exactly regular menstrual cycles, predictions can be inaccurate. Even women with regular cycles experience irregular cycles from time to time

This article outlines 3 very common mistakes women make when trying to detect ovulation in their menstrual cycle with the rhythm method. It explains the rhythm method in detail and also summarizes a more comprehensive approach to charting the menstrual cycle. 

You can watch the video below or continue reading the article

Oh ovulation happens on the 14th day in the cycle, It’s 14 days before your next period. You know this and so you pick up a calendar counting back and forth like this and then over time, you notice this thing isn't working for me either because you haven’t achieved pregnancy which is your main goal or worse case scenario, you get pregnant while trying to avoid a pregnancy-not good lol. And guess what, most cycle tracking apps use this method.

The calendar/rhythm method uses the calendar, obviously to predict safe and fertile days in a woman’s cycle based on the length of previous cycles. 

Is it effective? It can be up to 80-87% effective in some ladies. 

So why isn’t it working for you? 

My name is Stephanie Nyong, the Health Teacher and MD here at Health Platforms, a centre providing comprehensive health education and quality preventive health services for women and I’ll be exposing you to the reasons why the calendar/rhythm method isn’t working for you or better still why it may not be the best option for you.

Mind you, this will be a very practical video as I will be teaching using pen and paper charting. You already know how much I like this kind of charting and I’ll explain why certain cycle tracking apps may not be the best option for you at this point. 

And now we start...

The first reason is:

1.You are still using the 14th day ovulation myth 

Sadly, many ladies think they ovulate on the 14th day. This is a myth and the reason why it is so popular is because almost all examples in textbooks, on the internet and several other places paint the picture that ovulation happens on the 14th day. 

The fact is not all women will ovulate on the 14th day. Only women who have a regular cycle length of around 28 days will ovulate on the 14th day. 

And why is this so? It is because there is a formula to it. To calculate your ovulation using the calendar/rhythm method, you have to subtract 14 days from your menstrual cycle length if you have a regular cycle.

A menstrual cycle length is the time from the beginning of one period to the beginning of another period i.e the number of days from the first day you bleed to the first day of the next time you bleed. A normal menstrual cycle length ranges from 21 to 35 days

Important to note is the word “bleed.” For some ladies, they experience spotting during ovulation, in between periods or a few days before a period. That shouldn’t be counted as the start of a menstrual cycle. It is the first day you bleed to the first day of the next time you bleed. 

It simply means that if you have a very regular 28 day menstrual cycle length, your ovulation day will be 28-14 days which will be Day 14 of your cycle and if you have very regular 30 day cycle length, your ovulation will be 30-14days and that will be the 16th day of your cycle. Same way if you have very regular 32 day menstrual cycle lengths, it will be 32-14 days and what day will that be?  The 18th day of your cycle yea you got it…. 

So, this is the reason why not all women will ovulate on the 14th day. You have to calculate yours using your menstrual cycle length. 

So, allow me show you a practical example using a menstrual cycle chart.


Juliet starts her period on the 3rd of July and the next period starts on the 1st of August. She has a menstrual cycle length of 30 days. The number of days from one period to the other, right here and here represented in red color. 

Using the same formula we used for Funke, let's calculate Juliet’s ovulation date. 

ML-14 = Ovulation day

30-14 = Day 16

Now let’s count. It brings us to the 18th of July.

So when will her fertile and safe days be?

The fertile window lasts from 5 days leading to ovulation and 1 to 2 days after. This formula is used because it is believed that the sperm can last for up to 5 days in a woman’s body and once ovulation occurs that is the release of a mature egg from the ovary, it could be fertilised.

However, the egg only lasts for 12 to 24 hours waiting for fertilisation after which it disintegrates if fertilisation does not occur.

Her’s will be from the 13th to the 20th of July. 5 days before the 16th, possible ovulation date and 1 to 2 days after. Days before and after the fertile window are considered safe.

If you have a regular cycle, I’ll want you to use this formula to calculate your possible  ovulation date, safe and fertile period and drop them in the comments.

The calendar/rhythm method will not work for everyone? You notice how I keep saying very regular menstrual cycle length, that is because, for the rhythm method to work perfectly, you need to have an extremely regular cycle length. Meaning your period, ovulation etc. has to happen around the same time every month. In other words, your menstrual cycle length is the same figure every single month. Not 28 from January to February, 32 from February to March  and 26 from March to April. It has to be the same!

Now, you’ve learnt this you are wondering; My cycle length isn’t the same every other month, I must have been doing this wrong. Yes you have! So, what should you do differently? 

And that brings us to the 2nd mistake women make. 


  1. You use the rhythm method the wrong way


Important for you to note is If your menstrual cycle is shorter than 26 days and longer than 32 days, this isn’t the method for you. And before you get excited thinking I use apps, so this doesn’t concern me, ask yourself; Are you inputting any other information asides the dates of previous periods, I mean other vital signs and symptoms. If you aren’t I'm sorry to burst your bubble, because many cycle tracking apps use the algorithm that predicts your safe and fertile periods using the length of the previous cycle.

So if your cycle length is shorter than 26 and longer than 32 days whether or not you use a calendar or these kinds of cycle tracking apps, it is not the best option for you and I’ll be telling you what you can do instead.

So, if your menstrual cycle length is between 26 and 32 days but varies across the months looking something like this in the past 4 months; 28 days, 32 days, 26 days and 31 days, what you should do is first, keep track of your longest and shortest menstrual cycle length (at least 6 to 12 cycles)

Once you are able to deduce that, to get the first fertile day of your cycle, you will need to subtract 18 from the number of days in your shortest cycle. And to get the last fertile day, you will need to subtract 11 from your menstrual cycle length. 

Now let’s look at  Aisha’s charts to understand better. Aisha doesn't exactly have a regular cycle.


Aisha’s first day of her menstrual cycle is the 7th of May and we can see from the top of the chart that her shortest menstrual cycle length was 28 days and the longest 34 days. 

Aisha will need to subtract 18 days from her shortest menstrual cycle length and 11 from her longest menstrual cycle length.

Using the chart we see the 10th day of her cycle is the 16th of May and the 23rd day to be the 29th of May which makes the 16th to 29th of May her fertile window. It’s a pretty long one right. Yea and that’s because she doesn't have a regular cycle. 

So if Aisha is looking to prevent pregnancy she should abstain from sex totally or use protection from the 16th to 27th of may while is she is looking to achieve pregnancy, she should do otherwise if you understand what I mean. This would mean abstaining or going at it for 12 consecutive days right. 

I have another exercise for you. Use this formula to calculate your safe and fertile times and then drop it in the comments. I like to know that you are following this lesson lol

This method will work for some people, no doubt but it can be even more effective if combined with other methods of fertility awareness especially because this can still be regarded as a prediction. Aisha still doesn’t know when exactly ovulation will occur or if ovulation occurs at all. However, if she can totally get at it or abstain for that long, then she could be lucky.  

Using the rhythm method alone can be up to 80 to 87% effective in preventing or achieving pregnancy but when combined with other methods of fertility awareness, and  done correctly, it can be up to 98% effective in both preventing and achieving pregnancy. And that right there is the 3rd mistake women make, You rely on the rhythm method alone.

First, you should understand that there are several reasons why a woman may have slightly irregular or irregular cycles, most of which happen as a result of stress, weight gain or loss, hormonal imbalance etc. and this of course can cause a shift with how the different phases of the menstrual cycle happen. Links to learn more about irregular cycles-it’s causes and how to chart it are just below this video, you can check it out after watching this video. 

The menstrual cycle isn’t just about periods , there’s a process. You know first you have the :

  • menstrual phase, the beginning of a cycle when you bleed
  • The follicular phase where the egg cells begin to grow until they mature
  • the ovulation phase when the egg is released from the ovary and taken to the fallopian tube for possible fertilisation and lastly
  • the luteal phase which lasts from after ovulation until the start of the next period, if a pregnancy does not occur. 

So, with irregular cycles, there's a possibility that as a result of the imbalance, you may not be ovulating or your ovulation sometimes may be delayed. Usually, your period starts when the lining of the uterus sheds if fertilisation did not occur and that is about 12 to 14 days after ovulation.

However, if you do not ovulate, the lining of your uterus still sheds and causes bleeding when it can no longer sustain itself and when that happens, you are not having a period, it is just bleeding.

So you may be there marking calendars every month predicting an ovulation that doesn’t occur or marking the wrong day not knowing your ovulation has been delayed . 

So what are these other  methods? These methods in addition to the rhythm method, use the body’s vital signs; basal body temperature or waking temperature, cervical fluid/mucus, cervical position and hormonal tests to pinpoint and confirm ovulation. 

I’ll show you an example quickly of what a delayed ovulation looks like in a menstrual cycle chart

Sometimes when our bodies are stressed, or when we start a very rigorous exercise routine or fall sick, or even travel your body may decide to hold on ovulation and when this happens you might see that your cycle becomes longer in your chart. 


These are Adeola’s charts for the month of July and August. Adeola has a menstrual cycle length of between 31 to 33 days which means that her ovulation should happen around the 19th day if we calculate using the calendar/rhythm method.

But here you see her ovulation happening on the 25th day. 



If you look closely into her chart,  you will see she started an exercise routine which she indicated to be an intense one on the 11th day of her cycle.

As a result of the stress on her body, her body decided to hold on ovulation until the 25th day. She  can confirm ovulation happened following her rise in temperature and cervical mucus.

She’s not predicting because she has more than one sign telling her. And as you can see from the chart, she maximized her fertile window as she is trying to conceive

Using the calendar alone will never give you this much information. You may just be predicting wrongly month after month and guess what, most cycle tracking apps do the same thing. They use the length of previous cycles to predict future ones. 

Once you start keeping track of your cycle using this method, it gives you an assurance and your mind can be at ease.  If there’s a problem as well, maybe you are not ovulating at all, you will see it too and you will agree with me, it’s easier to find a solution when you know what the problem is.

There are links to videos below this video or in the description discussing how to chart your menstrual cycle using these methods. You can click the links to watch them.

If you have any questions, drop them in the comments or send me an email on  I answer every single email myself. I’ll be glad to be of help. 

Start your journey to mastering your menstrual cycle







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Stephanie Nyong
About Stephanie Nyong 70 Articles
Stephanie O. Nyong is a seasoned health educator and content strategist at Health Platforms International where she empowers individuals especially women and girls with knowledge and skills to prevent and manage a number of health conditions. She is the founder of BE-ME Africa, a Breast Examination and Menstrual Education project for young girls. Stephanie also organises pop-up clinics regularly to provide affordable quality healthcare services to a target population. She is admired in the industry and has the capacity to deliver on the current demand for health literacy for improved health outcomes in Nigeria. You can connect with her across all social media handles @stephanie_nyong

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