Ovulation Calculator
Estimate your ovulation day and most fertile window from the first day of your last period and your usual cycle length, using the standard luteal-phase method.
Ovulation calculator
Calendar method (luteal phase ≈ 14 days).
An estimate only — not medical or contraceptive advice. See the NHS fertility guide.
How ovulation is estimated
Ovulation — when an egg is released — usually happens about 14 days before your next period starts. That is because the luteal phase (the second half of the cycle) is fairly constant at around 14 days, while the first half varies. So the calculator finds your next expected period from your cycle length, then counts back 14 days to estimate ovulation.
On a 28-day cycle, ovulation falls around day 14. Your most fertile days are the five days before ovulation plus ovulation day, because sperm can survive up to five days inside the body.
Your fertile window
| Phase | Roughly (28-day cycle) |
|---|---|
| Period | Days 1–5 |
| Fertile window opens | Day 9 |
| Most fertile | Days 13–14 |
| Ovulation | Day 14 |
| Next period | Day 28 |
Source: NHS — Fertility in the menstrual cycle.
When the calendar method is less reliable
If your cycle length varies from month to month, calendar-based predictions become rougher. Ovulation predictor kits, tracking your basal body temperature, and watching changes in cervical mucus give more personal signals. The calendar method is also not a reliable form of contraception. If you are planning a pregnancy, our due date calculator projects forward once you conceive.
Frequently asked questions
How is ovulation day calculated?
Ovulation usually happens about 14 days before your next period starts, because the luteal phase is fairly constant. So the calculator subtracts 14 days from your next expected period to estimate ovulation day.
When is my most fertile window?
Your fertile window is roughly the five days before ovulation plus ovulation day itself, because sperm can survive up to five days. The day before and the day of ovulation are the most fertile.
How accurate is an ovulation calculator?
It is an estimate based on an average cycle. Real ovulation can vary, especially with irregular cycles. Ovulation predictor kits, basal body temperature and cervical-mucus tracking give more personal signals.
What if my cycle is irregular?
If your cycle length varies a lot, the calendar method is less reliable. The NHS suggests tracking several cycles or using ovulation test kits, and speaking to your GP if you are trying to conceive and cycles are very irregular.
Is this medical advice?
No. This is an informational estimate, not medical or contraceptive advice. The calendar method is not a reliable form of contraception. Speak to your GP or the NHS for fertility and contraception guidance.