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How to Calculate your Ovulation Day & Safe Period using just a Calendar

July 20, 2022

Calculating Ovulation Day with a Calendar - image of a calendar with a red pin on the 20th day

This post helps you determine your safe period by identifying your most likely ovulation date.

Knowing how to work out your ovulation date will show you the times of the month when you are most likely to fall pregnant after sex without protection.

You will only need two items for this calculation:

  • a calendar
  • and an idea of the dates of your last two menstrual cycles.

At this stage, it is essential to explain that this method will work more accurately for a woman with a regular menstrual cycle. 

Woman in white briefs holding a red flower in front of her private area

Regular Menstrual Cycle

This means your flow happens at roughly the same interval from month to month.

So, for example, when you count from the 1st day of bleeding in one month to the 1st day of bleeding the next month, the duration is about the same between other months of the same.

These ladies with regular cycles get their periods about the same time of the month like clockwork.

They also regularly bleed for an average of 5-7 days each month. 

If your periods are not regular – and many of us fall into this category, this method of predicting your safe period may not work.

Essential Facts for Calculation

Ok, let’s begin. The only thing you need to remember is the number 14.


14 is the estimated number of days from the 1st day of your menstrual flow to when your ovary releases a new mature egg, i.e. ovulation.

With this information, you can quickly work out when you should ovulate – given that you have the 1st day of your last menstrual period.

Once released, your egg will likely survive for 12 – 24 hours before it naturally dies unless fertilised.

Sperm survival

Given that a man’s sperm can remain alive in the woman’s womb for an average of 5 days after sex, we can also work out your fertile window.

Fertile Window

The fertile window is the time in your menstrual cycle when you are most likely to fall pregnant.

We usually estimate it is six days long, five days before ovulation when the sperm is alive in your womb, plus the day when the egg is released.

(Remember, we are trying to work out when you are safest, i.e. least likely to get pregnant).

So you should know that even if you had sex four days before ovulation, the man’s sperm is still alive and can fertilise your egg.

You have the same risk of falling pregnant four days before ovulation as on ovulation day – because the sperm can survive and wait for the egg to be released four days later!!

Pretty black woman holding a pregnancy test with an expression of surprised pleasure - did she get her fertile window right?

Get The Calendar

So let’s make this practical – get your calendar!

 Say your last period was one week ago, Friday the 24th of June.

By this, I mean the day you started to bleed was the 24th of June – the first day of your last menstrual period.

Using our magic number 14, the estimated ovulation date will fall around the 8th of July.

Please remember that this date is an estimate. Ovulation could happen on the 8th or the days before or after.

But let’s stick with the 8th. That means your fertile window is from the 3rd of July to the 8th of July.

So these are the days of your cycle when you are most likely to get pregnant.

How Safe is the Safe Period?

But, when are you safe?

Well, first, the safe time of your period is a relative term.

That means any other time is safer when compared to the fertile window. 

Scientists estimate that women are 30% more likely to fall pregnant in their fertile window.

According to the Faculty of Sexual and Reproductive Health FSRH, UK, a woman is least likely to fall pregnant during the first three days of her menstrual cycle.

These are the first, second and third days of bleeding.

Any other day, pregnancy is theoretically possible, with the highest chance in the fertile window.

Your safe periods are then the days before and after your fertile window.

Does that mean you can confidently go ahead to have sex without protection or birth control on those “so-called” days? 


A Final Note

Remember, we’ve talked about estimates, so these calculations may be on or off by a day or two.

Even a woman with a regular cycle may not ovulate at the same time every month.

So now you know how to work out your ovulation day, when you are most fertile and when your safe period is.

But I can take it a step further. What happens with all these dates when you use the emergency birth control pill?

Well, check out this video, where we work out the reasons for the delay in periods after taking the birth control pill, Postinor looking at dates only.

Please use our email information service if you want specific answers to your health care questions.

Editing by AskAwayHealth Team


All AskAwayHealth articles are written by practising Medical Practitioners on a wide range of health care conditions to provide evidence-based guidance and to help promote quality health care. The advice in our material is not meant to replace the management of your specific condition by a qualified health care practitioner.
To discuss your condition, please contact a health practitioner 

Image Credits: Unsplash

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