How to use the Ovulation Calculator
- Pick the first day of your last period.
- Enter your usual cycle length in days, counted from the first day of one period to the day before the next.
- Read the estimated ovulation day, the fertile window and the date of your next period.
- If your cycle is irregular, treat the dates as a rough guide and consider ovulation tests.
Formula used
Ovulation day ≈ first day of last period + cycle length − 14. Fertile window ≈ ovulation day − 5 to ovulation day + 1. Next period = first day of last period + cycle length.
Worked example
Here is the Ovulation Calculator worked through with the example values shown above, so you can check each step by hand.
| Inputs | |
|---|---|
| First day of your last period | 2027-01-05 |
| Average cycle length | 28 days |
| Result | |
| Estimated ovulation day | 19 Jan 2027 |
| Fertile window | 14 Jan 2027 to 20 Jan 2027 |
| Next period expected | 2 Feb 2027 |
| Pregnancy test from about | 2 Feb 2027 |
How ovulation is estimated
Ovulation usually happens about 14 days before the next period starts, not 14 days after the last one began. With a 28-day cycle that is day 14. In the default example, a period starting on 5 January and a 28-day cycle give ovulation on 19 Jan 2027, a fertile window of 14 Jan 2027 to 20 Jan 2027 and the next period on 2 Feb 2027.
A longer or shorter cycle shifts the date. With a 32-day cycle, ovulation is estimated for 23 Jan 2027, because the part of the cycle before ovulation varies more than the 14 days after it. With a 24-day cycle it is 15 Jan 2027.
The fertile window
Sperm can survive in the body for up to about five days, and the egg survives for roughly a day after ovulation. So the fertile window is about six days: the five days before ovulation and the day of ovulation or the day after. The days just before ovulation, including the day before, are generally the most fertile.
The calculator shows this window from five days before to one day after the estimated ovulation date.
Why the estimate may be off
The calculation assumes you have a regular cycle and that the second half of your cycle is about 14 days long. Stress, illness, travel, breastfeeding, hormonal conditions and age can all change when you ovulate, and it can differ from month to month, even in regular cycles.
For a more precise picture, many people use ovulation predictor kits that detect a hormone surge, track their basal body temperature or watch for changes in cervical mucus. These methods can be combined with this calculator.
Important notes
This calculator is not a method of contraception and must not be used to avoid pregnancy, because the fertile window can shift unpredictably. It gives general information and is not medical advice.
If you have been trying to conceive for a year without success (or for six months if you are over 35), or if your cycles are very irregular or absent, speak to a doctor. A pregnancy test is most reliable from the day your period is due. The due date calculator on this site can estimate a due date once you are pregnant.
Frequently asked questions
Is this reliable as contraception?
No. It is an estimate and must not be used to prevent pregnancy. Ovulation can vary from cycle to cycle.
What if my cycle is irregular?
The estimate is less reliable. Ovulation tests, temperature tracking or advice from a doctor can help.
How is cycle length counted?
From the first day of one period to the day before the first day of the next period.
When is the best time to try to conceive?
The fertile window, especially the two or three days before and including ovulation.
When can I take a pregnancy test?
Most tests are reliable from the day your period is due. Follow the instructions on the test.
Why is ovulation not on day 14 for everyone?
Day 14 applies only to a 28-day cycle. Ovulation is about 14 days before the next period, so it moves with cycle length.
This calculator gives estimates for information only. Actual figures from a bank, employer, tax authority or doctor may differ. It is not financial, tax, legal or medical advice.