Calculating ovulation: how the Ovulation Calculator works
Ovulation takes place about 14 days before the next period. This time after ovulation is called the luteal phase, and in most women it is fairly stable, usually between 10 and 16 days. The time before it, until the egg matures, varies from cycle to cycle. That is why the Ovulation Calculator counts backward from the expected start of the next period:
- Ovulation: cycle length minus luteal phase. With a 28-day cycle and a 14-day luteal phase, that is cycle day 14; with a 32-day cycle, it is cycle day 18.
- Fertile days: the 5 days before ovulation, the day of ovulation, and, to be safe, the day after.
- Best chances: the 2 days before ovulation and the day of ovulation itself.
- Irregular cycle: The calculator takes the shortest and the longest cycle and shows the whole range in which ovulation can fall.
Cycle day 1 is always the first day of your period. The calculation happens exclusively in your browser, and Fertilio does not store any cycle data.
The calculator is both a cycle calculator and an ovulation calendar: It shows your whole cycle up to the next period and the fertile days of the coming months, and, if you want, transfers them to your phone calendar.
When are the fertile days?
Sperm can survive in a woman’s body for up to 5 days, while the egg survives only about 12 to 24 hours after ovulation. This results in a fertile window of around 6 days per cycle. In a well-known study by Wilcox and colleagues, pregnancies occurred only from sex in the 6 days ending with ovulation. The chance was about 10% five days before ovulation and rose to around 33% on the day of ovulation.
In practice, that means that if you have sex every 1 to 2 days during the fertile days, you do not need to know the exact day of ovulation.
How accurate is an ovulation calculator?
An ovulation calculator gives an estimate, not a measurement. Even with a regular cycle, ovulation can vary by a few days. In a prospective study, the fertile window fell entirely between cycle days 10 and 17 in only about 30% of women. An analysis of more than 600,000 cycles from a cycle tracking app found an average luteal phase of 12.4 days, with a wide range.
It gets more accurate when you combine the calendar estimate with signs from your body:
- Ovulation tests (LH tests): They show the LH surge, the rise of the hormone LH that comes about one to two days before ovulation. How to use and read them correctly is covered in the guide Ovulation test.
- Cervical mucus: When it becomes clear, fluid, and stretchy like raw egg white, ovulation is about to happen.
- Basal body temperature: It rises by about 0.4 to 0.9°F (0.2 to 0.5°C) after ovulation. So it only confirms ovulation after the fact. Instructions: How to measure basal body temperature.
Important: An ovulation or fertility calculator shows when you are likely to be fertile, not how fertile you are. Whether ovulation happens regularly and what your ovarian reserve looks like can only be shown by tests such as hormone testing and the AMH level. More on this in the guide Fertility test for women.
Ovulation tests: when to start?
With ovulation tests, you start a few days before the earliest possible ovulation, so about 17 days before the expected period. With a 28-day cycle, that is cycle day 11. With an irregular cycle, the start depends on the shortest cycle. The Ovulation Calculator shows you the day directly. Which devices measure temperature or hormones automatically, and what they cost in the first year, is shown in the fertility tracker comparison.
When does a pregnancy test make sense?
After fertilization, the embryo usually implants 8 to 10 days after ovulation, within an overall range of about 6 to 12 days. Only after that does the body produce the pregnancy hormone hCG. A urine test is therefore reliable from the day of the expected period. Early tests can turn positive a few days earlier, but they more often give a false negative.
When to get a fertility evaluation?
In a German study of couples who had sex specifically on their fertile days, 38% were pregnant after the first cycle, 81% after 6 and 92% after 12 cycles. If it has not worked for a longer time, an evaluation makes sense. Medical societies recommend:
- Under age 35: after 12 months without a pregnancy.
- From age 35: after 6 months.
- From age 40: without waiting long.
- Sooner with cycles shorter than 21 or longer than 35 days, absent periods, known endometriosis or PCOS, earlier fallopian tube inflammation, or known problems on the partner’s side.
An evaluation always includes both partners: for the woman, among other things, hormone testing, AMH, and the fallopian tubes; for the man, a semen analysis. You can find a suitable clinic near you with the Fertilio Clinic Finder.
Can I use the Ovulation Calculator for birth control?
No. A pure calendar estimate is too unreliable as a method of birth control, because ovulation can shift. If you want to use natural birth control, you need a symptothermal method with temperature measurement and mucus observation, and you should learn it thoroughly.
Frequently asked questions
How do I calculate my ovulation?
Subtract 14 days from your cycle length. The result is the likely cycle day of ovulation. With 28 days, that is day 14, with 30 days day 16, and with 35 days day 21. Cycle day 1 is the first day of your period.
How many fertile days does a woman have?
About 6 days per cycle: the 5 days before ovulation and the day of ovulation. The chance is highest in the 2 days before ovulation and on the day of ovulation itself.
What is the difference between an ovulation calculator, a cycle calculator, and an ovulation calendar?
All three work from your cycle using the same principle. An ovulation calculator mainly shows ovulation and fertile days, a cycle calculator also shows the next period, and an ovulation calendar shows the fertile days over several months. The Fertilio Ovulation Calculator does all three and, if you like, transfers the dates to your phone calendar.
Does the calculator work with an irregular cycle?
Yes. Choose “Irregular” and enter your shortest and longest cycle. The calculator then shows the whole range in which ovulation can fall. The more your cycle varies, the wider it becomes. Ovulation tests help you determine ovulation more accurately.
How does the reminder in the phone calendar work?
After the calculation, you can add the fertile days of the next 3, 6, or 12 cycles to your calendar. On iPhone, Mac, Outlook, and Samsung devices, this works with “Add to calendar,” and for Google Calendar with its own button. Your calendar reminds you the evening before the fertile days. If you like, discreet titles such as “Important days” appear there.
When can I take a pregnancy test?
Reliably from the day your period would be due. Early tests can turn positive a few days earlier, but they more often show a false negative result.
Does Fertilio store my cycle data?
No. The Ovulation Calculator calculates only in your browser. For the calendar file, the calculated dates are sent once to our server, converted into a file there, and not stored. Only if you sign up for the Fertility Companion do we store your email address and the date of the check reminder until you unsubscribe.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333:1517-1521. doi:10.1056/NEJM199512073332301
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321:1259-1262. doi:10.1136/bmj.321.7271.1259
- Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med. 1999;340:1796-1799. doi:10.1056/NEJM199906103402304
- Bull JR, Rowland SP, Scherwitzl EB et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digit Med. 2019;2:83. doi:10.1038/s41746-019-0152-7
- Gnoth C, Godehardt D, Godehardt E et al. Time to pregnancy: results of the German prospective study and impact on the management of infertility. Hum Reprod. 2003;18:1959-1966. doi:10.1093/humrep/deg366
- Practice Committee of the American Society for Reproductive Medicine. Fertility evaluation of infertile women: a committee opinion. Fertil Steril. 2021;116:1255-1265. doi:10.1016/j.fertnstert.2021.08.038