08/04/2025 12:35 p.m.

Track Your Ovulation Days and Identify Your Fertile Window

Dr. Niels van de Roemer
Dr. Niels van de Roemer Medical Adviser

The ovulation date calculator Daysy is quick, simple and accurate!

Do you want to calculate the ovulation days in your menstrual cycle but dread the workload? We understand that, and it‘s why we developed Daysy! Daysy is a particularly convenient and easy to use ovulation date calculator that determines your fertile and infertile phases based on your basal temperature . Several decades of research and studies have gone into the development of its unique algorithm. With Daysy, knowing whether you are currently fertile is a matter of minutes in your morning routine. 

And this is how it works: Every morning before you get up, you remove the protective cap, activate Daysy via the push of a button and then place Daysy's highly sensitive test prod underneath your tongue. After about 60 seconds, a beeping noise lets you know that the measurement was a success and Daysy immediately displays your current fertility status to you. If the red light is on, it means that you are currently fertile / possibly fertile. If the green light is on, it means that you are not fertile. Calculating your fertile days based on your menstrual cycle really is that simple when you have Daysy!

Calculate ovulation days with the menstrual cycle

So how does the calculation based on basal temperature work? The basis for Daysy to be able to calculate your fertile days based on your menstrual cycle is the natural female cycle and the influence that hormones have on your basal temperature . In the course of the cycle, the basal temperature changes in a certain way for every woman. Even when the strength of the temperature changes and the precise timing vary among every individual, this applies to all women and all menstrual cycles: Around the time of ovulation , the basal temperature rises by 0.3 to 0.5 degrees Celsius (32.5 – 41 degrees Fahrenheit) and stays slightly elevated until the onset of menstruation. Precisely this change in your basal temperature is what the ovulation date calculator Daysy can measure and evaluate with the help of its algorithm. Through this evaluation, Daysy can not only determine your ovulation days, but also the beginning and end of your fertile phases.

Knowing your ovulation days

Daysy helps you to find the perfect time to have sex in order to get pregnant. You are only fertile and able to conceive a child on the day of your ovulation, as well as a maximum of five days prior to that. This is what makes the right timing very important when you want to have a baby. With the right knowledge about your menstrual cycle and your ovulation date, you can help along your dream of becoming a mother.

Daysy and the free app "DaysyDay" offer you the option to get to know your body and your menstrual cycle - 100% natural and without any side effects.

Discover all the advantages of Daysy now and calculate your ovulation date based on your menstrual cycle!

What is the best way to measure?

For optimal use of Daysy, we recommend that you should have slept sufficiently (at least one hour) before the measurement. It is important that you take your temperature immediately after waking up, before getting up and becoming active. Measure below your tongue, with your mouth closed, and keep the device steady while it is measuring. Video Tutorial

Do I have to measure at the same time every day?

No. You should have at least one hour of good quality sleep before the measurement. It is important that you take your temperature immediately after waking up, before getting up and becoming active. For optimal use of Daysy, try to get at least 3 or more hours of sleep before measuring. Less sleep can result in more temperature fluctuations causing Daysy to give more fertile or potentially fertile days.

How do I enter or edit menstruation?

You can confirm or remove menstruation at any time during the current day. Simply press the button to turn on your Daysy, and then press and hold the activation button until the violet light lights up without flashing and you hear a "beep." Confirm you are menstruating for each day of active bleeding and for a minimum of three consecutive days. You can also remove menstruation data for the same day by turning Daysy on and pressing and holding the activation button until the purple/blue light is no longer solidly lit and begins flashing again.

Video Tutorial

How long until Daysy knows my personal cycle?

The first 2-4 cycles of use are considered Daysy's learning phase. Over the course of her learning phase, Daysy will come to understand your cycle and begin narrowing down your fertile window. Daysy remains quite cautious and conservative during this time, which is why Daysy is accurate starting the very first day.

When can I start to rely on Daysy's results?

You can rely on Daysy's results starting the very first day of use. In the beginning, you will see more yellow and red days because Daysy is still learning your unique cycle. You will see more green days as Daysy comes to understand your cycle and can narrow down your fertile window. The more regularly you measure, the better.

My cycle is irregular. Can I still use the devices?

Yes. Irregular cycles are not a problem for Daysy/Lady-Comp. Even with irregular cycles, all Valley devices distinguish between non-fertile and fertile days with a very high accuracy. However, in case of extreme fluctuations between cycles it will give you more red and yellow lights and less green lights. Fluctuations of several days are usually not a problem. If you have questions about your unique cycle, please reach out to our team of experts.

Are there any costs associated with the DaysyDay app?

No! DaysyDay is free! There are no in-app purchases or upgrade fees.

Does illness reduce the reliability of the device?

In general, you should skip measurements during illness and while you are taking medication that can have an influence on your body temperature or cycle. If you have doubts, contact your doctor or our customer service team.

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Get more information

If you’re ready to start a family, read on to learn helpful tips to enhance your fertility and prepare for pregnancy.

The fertility tracker method is a modern, digital evolution of natural family planning. It eliminates common sources of error – ensuring ovulation is accurately detected and recorded.

Below, we’ll show you how traditional NFP (Natural Family Planning) rules differ from the features of modern fertility monitors – and how the two compare.