Ovulation Calculator: Fertile Window, Next Periods, Due Date

Ovulation by luteal phase, pregnancy test date and due date

The tool places ovulation "cycle length minus luteal phase" days after the first day of your last period: 28 − 14 = 14 days for a standard cycle. From this it derives a 7-day fertile window, an early pregnancy test date (ovulation + 11 days) and an estimated due date if conception happens during this cycle.

This page lets you set the luteal phase between 10 and 16 days, which helps if you know it from a temperature chart or LH tests. It also shows the peak fertility days and a forecast over three cycles, the current one included.

Calculations

• Ovulation = last period + (cycle − luteal phase), cycle from 21 to 40 days.
• Fertile window = ovulation − 5 days to ovulation + 1 day.
• Peak fertility = ovulation − 2 days to ovulation day.
• Next period = last period + cycle.
• Early test = ovulation + 11 days.
• Due date = last period + 280 days + (cycle − 28), Naegele's rule adjusted to cycle length.

Example

Last period on 1 September 2026, 32-day cycle, 12-day luteal phase: ovulation on 21 September, fertile window from 16 to 22 September, peak fertility from 19 to 21 September, next period on 3 October, early test on 2 October, estimated due date 12 June 2027.

Three-cycle forecast

The tool repeats the calculation over three cycles of the same length, starting with the current one: each cycle begins on the expected period date of the previous one, and ovulation always falls "cycle − luteal phase" days after that start. With the example inputs (last period on 1 September 2026, 32-day cycle, 12-day luteal phase), cycle 2 starts on 3 October, with ovulation on 23 October and a fertile window from 18 to 24 October; cycle 3 starts on 4 November, with ovulation on 24 November, a fertile window from 19 to 25 November and the following period expected on 6 December. Errors add up: if every cycle lasts 2 days longer than entered, the third cycle starts 4 days later than shown.

Limits

The due date only accounts for cycle length, not for the luteal phase you set: in the example, estimated ovulation plus 266 days would give 14 June 2027. A test taken before the expected period can be negative even though a pregnancy is starting; if it is and your period does not come, repeat it a few days later. In France the due date that counts is the one set by the dating scan, done between 11 and 13 weeks + 6 days after the last period. This calendar is not contraception: French health insurance gives natural methods a high failure rate. With no pregnancy after a year of regular sex (6 months after age 35), it advises seeing a doctor.

Sources and dates: NHS, Periods and fertility in the menstrual cycle (reviewed 5 January 2023); French national health insurance (ameli), Natural methods of contraception (updated 1 October 2025); ameli, Medical assessment of infertility (updated 8 December 2025); ameli, Pregnancy: follow-up programme and first consultation (updated 14 July 2026); Naegele's rule (280 days from the last period), cited in text (checked on 14/10/2026)

Frequently Asked Questions

How do I find out how long my luteal phase is?

It is the number of days between ovulation and the first day of your next period. You can spot it with a temperature chart or an LH test, whose surge comes 24 to 48 hours before ovulation according to French health insurance, then count the days until your period. The tool accepts 10 to 16 days and uses 14 by default.

When should I take a pregnancy test after ovulation?

The tool suggests ovulation + 11 days: for ovulation estimated on 21 September, the early test falls on 2 October. A test taken before the expected period can be negative even though a pregnancy is starting. If it is negative and your period does not come, repeat it a few days later.

How is the due date calculated if I conceive this cycle?

The tool adds 280 days to the first day of your last period, plus the difference between your cycle and 28 days: 284 days for a 32-day cycle. The luteal phase you set is not used in this step. In France the final due date is set by the dating scan, between 11 and 13 weeks + 6 days after the last period.

Does this calendar work with irregular cycles?

It assumes the next cycle will have the length you entered. If your cycles vary from 26 to 32 days, the estimated ovulation also varies by 6 days: run the calculation with your shortest and longest cycle to bracket the window. It is not a method of contraception.

How does this ovulation calculator forecast my fertile days over three cycles?

It starts from the first day of your last period and adds the cycle length to get the start of each following cycle, then places ovulation "cycle − luteal phase" days after that start. With a period on 1 September 2026, a 32-day cycle and a 12-day luteal phase, the fertile windows run from 16 to 22 September, 18 to 24 October and 19 to 25 November, and periods are expected on 3 October, 4 November and 6 December. The forecast assumes regular cycles of the same length.

Which day do you ovulate with a 28-day cycle?

With the default settings (28-day cycle, 14-day luteal phase), the tool places ovulation 14 days after the first day of your period: day 15 of the cycle. With a 32-day cycle it moves to day 19; with a 12-day luteal phase instead of 14, it comes 2 days later. The NHS notes that in most women ovulation happens 10 to 16 days before the next period, hence a margin of a few days.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.