How ovulation is estimated
A menstrual cycle runs from the first day of one period to the first day of the next. The part after ovulation (the luteal phase) is fairly steady at about 14 days for most people, while the part before ovulation varies more. So the calculator counts back 14 days from the expected start of the next period.
The fertile window is six days long because sperm can survive for up to about five days in the reproductive tract, while an egg lives for about a day after ovulation. Sex in the days just before ovulation, and on the day itself, is the most likely to lead to pregnancy.
Worked example
The last period started on 1 October 2026. Here is how the cycle length changes the dates:
| Cycle length | Fertile window | Ovulation | Next period |
|---|---|---|---|
| 21 days | 3–8 Oct | 8 Oct (day 8) | 22 Oct |
| 28 days | 10–15 Oct | 15 Oct (day 15) | 29 Oct |
| 35 days | 17–22 Oct | 22 Oct (day 22) | 5 Nov |
For the 28-day cycle, the next period is 1 October + 28 days = 29 October. Ovulation is 14 days earlier, on 15 October, and the fertile window runs from 10 to 15 October. The following cycles repeat every 28 days, with ovulation on 12 November and 10 December.
Using the calendar and table
- Next three cycles: the table starts with the cycle you are in today. If you enter a period from several months ago, the dates roll forward using your cycle length.
- Calendar: shows the current month. Use Prev and Next to see other months. Each day has a text label for screen readers, so the colours are not the only cue.
- Privacy: the dates stay in your browser on this device and are not sent anywhere.
Why your dates may differ
This is a calendar method, so it is only as good as the regularity of your cycle. Stress, illness, travel, breastfeeding, recent hormonal contraception and conditions such as polycystic ovary syndrome (PCOS) can all move ovulation, and the luteal phase is not exactly 14 days for everyone. If your cycle length changes by more than a few days from month to month, a calendar estimate becomes much less reliable.
More reliable ways to find ovulation:
- Ovulation predictor kits detect the rise in luteinising hormone (LH) in urine, which comes a day or so before ovulation.
- Basal body temperature rises slightly after ovulation. It confirms ovulation afterwards rather than predicting it.
- Cervical mucus often becomes clear, slippery and stretchy around the fertile days.
- A doctor can check ovulation with blood tests or ultrasound, which is worth doing if your cycles are irregular or you have been trying for a while.
Not a method of contraception
Do not rely on this calculator to avoid pregnancy. Ovulation can come earlier or later than expected, and sperm can survive for several days. If you want to prevent pregnancy, talk to a doctor, nurse or family planning service about reliable contraception.
Already pregnant? Use the pregnancy due date calculator to estimate your due date. To count days between dates, try the date calculator.
This calculator gives general information only. It is not medical advice.
Frequently asked questions
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