Ovulation Calculator
Trying to get pregnant? Use this free ovulation calculator to find your most fertile days. Enter the first day of your last period and your usual cycle length to see your estimated ovulation date, fertile window and next period for the next three cycles.
What is ovulation?
Ovulation is when one of your ovaries releases a mature egg, usually once in each menstrual cycle. The egg travels into the fallopian tube, where it can be fertilised by sperm for about 12–24 hours. Sperm, however, can survive in the body for up to five days. This is why you can get pregnant from sex in the days before ovulation, not only on the day itself.
How is ovulation calculated?
The second half of the cycle, from ovulation to the next period, is called the luteal phase. It is fairly constant and lasts about 14 days for most people. The first half of the cycle is what varies. So the calculator estimates ovulation by counting back 14 days from your next expected period:
Ovulation day = first day of last period + (cycle length − 14)
Examples: With a 28-day cycle, ovulation is usually around day 14. With a 32-day cycle, it is around day 18, and with a 25-day cycle, around day 11. If your last period started on 1 October with a 28-day cycle, you would likely ovulate around 15 October, and your fertile window would be about 10–16 October.
What is the fertile window?
Your fertile window is the six days made up of the five days before ovulation and the day of ovulation. Some calculators, including this one, add one day after ovulation to allow for small timing differences. Your chances of pregnancy are highest if you have sex in the two to three days before ovulation and on the day of ovulation. Having sex every one to two days during the fertile window gives you a good chance without needing to pinpoint the exact day.
Signs of ovulation
Many people notice changes in their body around ovulation. Common signs include:
| Sign | What to look for |
|---|---|
| Cervical mucus | Becomes clear, slippery and stretchy, like raw egg white |
| Basal body temperature | Rises slightly (about 0.2–0.5 °C) after ovulation and stays higher until your period |
| LH surge | Ovulation test strips detect a rise in luteinising hormone 24–36 hours before ovulation |
| Mild pelvic pain | A twinge or ache on one side of the lower tummy (mittelschmerz) |
| Other changes | Breast tenderness, higher sex drive or light spotting in some people |
Ovulation with irregular periods
If your cycles vary by more than a week, or are often shorter than 21 days or longer than 35 days, it is harder to predict ovulation from dates alone. Irregular cycles can be caused by stress, weight changes, thyroid problems, breastfeeding or polycystic ovary syndrome (PCOS). In this case, ovulation test kits or tracking your temperature and cervical mucus can help, and it is a good idea to talk to your doctor.
Tips for getting pregnant
Have regular sex, every two to three days throughout your cycle, or every one to two days in your fertile window. Start taking a folic acid supplement (usually 400 micrograms a day) before you try to conceive, as advised by your doctor. Aim for a healthy weight (check yours with our BMI Calculator), avoid smoking and alcohol, and limit caffeine. See a doctor if you have been trying for 12 months without success, or after 6 months if you are 35 or older, or sooner if your periods are very irregular.
Already think you might be pregnant? Work out when it may have happened with our Conception Calculator, or find your baby's arrival date with the Due Date Calculator.
Frequently Asked Questions
When do I ovulate?
Most people ovulate about 14 days before their next period starts. With a 28-day cycle that is around day 14, counting the first day of your period as day 1. With longer or shorter cycles, ovulation happens later or earlier.
How many days are you fertile each month?
You are fertile for about six days each cycle: the five days before ovulation and the day of ovulation. The two or three days just before ovulation are the most fertile.
Can I get pregnant right after my period?
It is less likely but possible, especially if you have short cycles. Sperm can live for up to five days, so sex soon after your period can lead to pregnancy if you ovulate early.
How accurate is an ovulation calculator?
It gives a good estimate if your cycles are regular, but ovulation can vary from month to month. Combining the calculator with ovulation test strips or tracking body signs gives more accurate results.
Can I use this calculator to avoid pregnancy?
No. Because ovulation can change without warning, calendar-based estimates are not reliable for contraception. Speak with your doctor about effective birth control options.
When can I take a pregnancy test?
For the most reliable result, take a home pregnancy test from the first day of your missed period. Some sensitive tests can detect pregnancy a few days earlier, but results are more accurate if you wait.