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Fertile window
Ovulation Calculator
Ovulation is when an ovary releases an egg, and it happens once in each menstrual cycle. This ovulation calculator estimates your ovulation date and your fertile window — the days when having sex is most likely to lead to pregnancy — from the first day of your last period, your average cycle length and, optionally, your luteal phase length. It also shows your next period, the due date you would have if you conceived this cycle, and the next few cycles. It is a calendar-based estimate, not a medical test.
Please enter the first day of your last period.
Estimated ovulation date
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Most fertile days
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Next period
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Due date if conceived
| Cycle starts | Fertile window | Ovulation |
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How to Use the Ovulation Calculator
- Enter the first day of your last period — the first day of full flow, not spotting.
- Enter your average cycle length, counted from the first day of one period to the first day of the next. The default is 28 days.
- Leave the luteal phase at 14 days unless you know yours is consistently different.
- Select Calculate to see your ovulation date, fertile window, next period and the next six cycles.
Ovulation Calculator — How It Works
Ovulation day = cycle start + (cycle length − luteal phase)
The luteal phase — the time from ovulation to your next period — is fairly steady from cycle to cycle for most people, and is often around 14 days, though anywhere from about 10 to 16 days can be normal. It is the first half of the cycle that varies most. So the calculator counts backwards from your expected next period: in a 28-day cycle with a 14-day luteal phase, ovulation falls on day 14, and in a 32-day cycle it falls on day 18.
The fertile window is the day of ovulation and the five days before it. Sperm can survive for up to about five days, while an egg lasts only around 12 to 24 hours, so the most fertile days are the two days before ovulation and the day of ovulation itself. If you conceived on your ovulation day, the due date is 266 days (38 weeks) later, which matches the standard 280-days-from-last-period rule for a 28-day cycle.
Ovulation Calculator Example
Say your last period started on 1 October, your cycles are 28 days long, and you use the default 14-day luteal phase. Ovulation is estimated on 15 October, and your fertile window runs from 10 October to 15 October, with 13 October to 15 October the most fertile days. Your next period would be expected around 29 October. If you conceived that cycle, the estimated due date would fall in early July.
When to use this tool
Use this when you are trying to conceive and want to time intercourse, or when you simply want to understand where you are in your cycle. Many people find it most useful for planning ahead, using the table of upcoming cycles, and then confirming with ovulation test strips.
Key Features & Benefits of This Ovulation Calculator
- Fertile window and peak days — see the full window and the two days before ovulation when conception is most likely.
- Adjustable luteal phase — fine-tune the estimate if your luteal phase is shorter or longer than 14 days.
- Six cycles at a glance — plan ahead with projected ovulation and fertile windows.
- Instant, free and private — runs entirely in your browser, no sign-up, nothing stored.
Good to know
This tool gives a calendar-based estimate and is not medical advice. It should not be used as birth control, because ovulation can vary from cycle to cycle. It is least reliable with irregular cycles, after stopping hormonal contraception, while breastfeeding, or with conditions such as PCOS. If you have been trying to conceive for 12 months without success (6 months if you are 35 or older), or your cycles are very irregular, speak with a doctor or midwife.
Ovulation Calculator — FAQs
How is the ovulation date calculated?
Ovulation is estimated as the first day of your last period plus your cycle length minus your luteal phase. With a 28-day cycle and the typical 14-day luteal phase, that puts ovulation on day 14 of your cycle.
How long is the fertile window?
About six days: the five days before ovulation plus the day of ovulation. Sperm can survive for up to about five days, while an egg survives only around 12 to 24 hours after it is released, so the two days before ovulation and the day itself are usually the most fertile.
Can I use this calculator as birth control?
No. This is a calendar-based estimate, and ovulation can shift from cycle to cycle even when periods seem regular. It should not be relied on to prevent pregnancy.
What if my cycles are irregular?
The estimate becomes less reliable when cycle length varies by more than a few days. In that case, methods that detect ovulation directly, such as ovulation (LH) test strips or basal body temperature tracking, are more dependable than a calendar estimate.
What is the luteal phase and should I change it?
The luteal phase is the time between ovulation and your next period. It is often around 14 days, and the default of 14 suits most people. Change it only if you know your own luteal phase is consistently shorter or longer, for example from ovulation tracking.