
Safe Days Calendar
Do you want to know how to prevent pregnancy and to know when to conceive? Use our Safe day Calculator to find out this important piece of information
How to Calculate your safe days
Written by Mindsmaking Medical Writer
Fact Checked by Mindsmaking Professionals
24th February, 2025
Safe days are the days outside of your fertile window, typically days before ovulation begins and after it ends. On these days, the chances of conception are lowest. This method is based on the fertility awareness methods, which involve observing fertility signs to identify fertile days of the menstrual cycle [1].
The safe days method can be used if you are looking for a natural, hormone-free method to avoid pregnancy. However, it is most accurate if you know your menstrual cycle length and regularity, as irregular cycles make it harder to predict ovulation and, consequently, your safe days.
In this article, you will learn how to use our safe days calculator to calculate your safe days and the effectiveness of the safe days method in preventing pregnancy.
Key Facts
Track your menstrual cycle for 6–12 months to identify patterns, which makes your safe days calculation more accurate and reduces the chance of error.
Calculate your safe days by subtracting 18 from your shortest cycle and 11 from your longest cycle—this reveals your fertile window, and the days outside it are your safe days.
With perfect use, the safe days method is 95% effective; however, with typical use, 12 in 100 women may still get pregnant. Therefore, combining it with other tracking methods can improve reliability.
How Our Safe Days Calculator Works
To accurately calculate your safe days, it is essential to understand your menstrual cycle properly, as this forms the foundation for accurate estimations. Our safe days calculator is a tool designed to help you identify your less fertile days, which aids in natural family planning. The safe days method, also known as the standard days method (SDM), is based on the physiology of the menstrual cycle and the functional lifespan of sperm and eggs [2].
To get started, you only need to enter two key pieces of information about you: the first day of your last menstrual period (LMP) and your menstrual cycle length. The calculator then utilizes this menstrual cycle data to estimate your fertile window and, consequently, your safe days. It applies the established calculations of the calendar method. By pinpointing when you are least likely to be fertile, the calculator helps you identify those days when the chances of conception are lowest.
How Do I Calculate My Safe Days?
The safe days method, also known as the standard days, calendar, or rhythm method, helps you identify your fertile window and safe days by tracking your menstrual cycle. It relies on your unique cycle to pinpoint when you are most likely to conceive. Here's a step-by-step guide to calculating your safe days [3].
- Keep a detailed record of your menstrual cycles for at least 6 to 12 months, noting the first day of each period to calculate the total length of every cycle.
- From your recorded cycle lengths, identify both the shortest and longest cycle you have experienced.
- Calculate your first fertile day by subtracting 18 from your shortest cycle length, and your last fertile day by subtracting 11 from your longest cycle length. The days between these two calculated numbers, inclusive, are your fertile window.
- Your safe days are the days before your first fertile day and the days after your last fertile day, during which you can have unprotected intercourse with a lower, though not zero, risk of pregnancy.
For instance, if you tracked your cycle and found out that your shortest cycle is 26 days and your longest cycle is 31 days, your first fertile day will be on day 8 (that is, 26 - 18), and your last fertile day will be day 20 (that is, 31 − 11). Between days 8 and 20, you have your fertile window. Your safe days are therefore between day 1 and day 7 (before ovulation), and day 21 to day 31 (after ovulation).
While the calendar method is a basic approach, other tracking methods provide more real-time indicators of your safe days and can offer greater accuracy, especially when combined.
- Tracking your basal body temperature (BBT) helps you monitor your fertile window, as a slight rise in temperature can indicate that ovulation has occurred [5], and you are approaching your safe period.
- Around ovulation, your vaginal discharge becomes thin, watery, slippery, and similar to an egg white. After ovulation, the amount of discharge usually decreases. [11]. By tracking these changes, you can identify both the start and end of your fertile window.
- The symptothermal method combines basal body temperature (BBT) and cervical mucus tracking [4]. By comparing these signs, you gain a more precise confirmation of when your fertile window ends, thus identifying your safe days more accurately.
- Ovulation predictor kits (OPKs) detect the luteinizing hormone (LH) surge that precedes ovulation [12]. Once the surge passes, you know ovulation has occurred, helping to define the end of your fertile window for that cycle and the start of safe days.
- Fertility monitors track hormones or other signals to provide a daily assessment of fertility status. They help pinpoint your fertile window precisely, allowing you to identify your safe days.
To improve accuracy, track your menstrual cycles for at least 12 months to recognize consistent patterns and changes. For more reliable results, consider combining methods, like using the calendar method alongside cervical mucus observation or basal body temperature (BBT) tracking. You can simplify this process with our Mindsmaking Baby app, designed to support easy and accurate tracking of fertility and ovulation.
If your cycles are irregular, speak with your doctor, as this may indicate an underlying health issue. On days when fertility is uncertain, it’s best to use reliable contraceptives, such as condoms, especially if you’re trying to avoid pregnancy. Natural methods, such as tracking safe days, are less dependable and may not provide sufficient protection.
How Effective Is the Safe Days Calculator for Pregnancy Prevention?
While our safe days calculator can be a helpful guide, it is essential to understand its accuracy and limitations. It provides an estimation based on your input, but individual cycles vary due to many factors, making the results sometimes unreliable.
A study assessed the effectiveness of the Standard Days Method for preventing pregnancy. It found that with perfect use—when the method is followed exactly—the chance of pregnancy over one year is about 4.75%, meaning it’s around 95% effective [6]. However, with typical use, which accounts for occasional mistakes or inconsistent use, the chance of pregnancy rises to about 12%, making it roughly 88% effective.
Compared to other contraceptive methods, the safe days method is less reliable. With typical use, there is approximately a 11.96% probability of pregnancy [6], meaning that about 12 women may get pregnant within the first year. In contrast, implants have a typical-use failure rate of 0.6% per 100 episodes of use, intrauterine devices (IUDs) with a failure rate of 1.4%, oral contraceptive pills with a failure rate of 5.5%, and male condoms with a typical-use failure rate of 5.4% [7].
Accurate prediction of safe and fertile days can be more challenging at times because factors such as stress and diet can impact ovulation [8] [9]. Also, sperm can survive in your reproductive tract for up to four to five days [10], which means that having unprotected sex on a safe day could still result in pregnancy if you ovulate earlier than expected. For these reasons, the calculator should not be relied upon as your only method of contraception. We recommend using it alongside other forms of birth control, like condoms. For personalized family planning advice, consult your midwife, doctor, or other healthcare professional.
Frequently Asked Questions
What are safe days in a menstrual cycle?
Safe days are the days outside your fertile window, usually before and after ovulation, when the chances of conception are lowest.
When are the fertile and unsafe days?
Your fertile window, or unsafe days, are calculated by subtracting 18 from your shortest cycle length for the first fertile day, and 11 from your longest cycle length for the last fertile day.
Can I use this method if my periods are irregular?
Using this method is more challenging with irregular periods, as it relies on predictable cycle lengths for accurate estimation.
How accurate is a Safe Day Date Calculator?
The accuracy depends on consistent cycle tracking. With perfect use, there is a 4.75% probability of pregnancy, and with typical use, a 11.96% probability of pregnancy.
Can I rely on safe days to prevent pregnancy?
It is not entirely reliable, with typical use resulting in an 11.96% probability of pregnancy; therefore, it should be used with caution and ideally in conjunction with other methods.
Can I use this calculator to get pregnant?
Yes, as you can also identify your unsafe days, that is, your fertile window, and then time intercourse to achieve pregnancy.
Can I get pregnant on a safe day?
Yes, you can; sperm can survive for days, and factors like stress and diet can influence ovulation, making the calculation inaccurate and conception possible even on estimated safe days.
Are safe days the same for everyone?
No, safe days are specific to an individual's unique menstrual cycle lengths and patterns.
Are safe days a natural form of contraception?
Yes, safe days are based on fertility awareness methods, which are a natural, hormone-free approach to family planning.
What’s the difference between the calendar method and a Safe Day Calculator?
The calendar method is the underlying principle of the calculator, while a Safe Day Calculator is a tool that applies this method using your cycle data.