About the Ovulation Calculator
This ovulation calculator predicts the day you are most likely to ovulate and the fertile window around it (the five days before ovulation, ovulation day and the day after), based on the first day of your last period and the usual length of your cycle. It also shows when to expect your next period, when a pregnancy test is likely to be reliable, and the due date if you conceive this cycle — then repeats the forecast for up to 12 upcoming cycles so you can plan ahead.
It is designed for people trying to conceive who want to time intercourse, and for anyone learning how their cycle works. The method is the calendar approach: ovulation usually happens one luteal phase — about 14 days — before the next period starts, so a 28-day cycle ovulates around day 14 and a 32-day cycle around day 18. If you know your luteal phase from tracking, enter it for a closer estimate.
Calendar predictions work best with regular cycles. Ovulation can shift with stress, illness or travel, so pairing this calculator with ovulation (LH) test strips or basal body temperature charting gives the most reliable picture. It is not a method of contraception.
How to use the ovulation calculator
- 1Enter the first day of your most recent period.
- 2Enter your average cycle length — track 3–6 cycles for the best estimate.
- 3Adjust the luteal phase if you know it from temperature or LH tracking.
- 4Read your ovulation day and fertile window, then use the calendar for the next cycles.
Formula and method
The luteal phase — the time between ovulation and the next period — is fairly constant for each person, usually 12–14 days, while the phase before ovulation varies. So the calculator finds ovulation by counting back one luteal phase from the next expected period (LMP + cycle length).
Sperm can survive up to five days and an egg about 12–24 hours, so the fertile window is the five days before ovulation, ovulation day, and the day after. The due date if you conceive is ovulation + 266 days, and a home pregnancy test is typically reliable from the day your period is due.
- LMP
- First day of the last menstrual period
- cycle length
- Days from one period start to the next
- luteal phase
- Days from ovulation to the next period (default 14)
Worked examples
28-day cycle starting September 15, 2026
With a 28-day cycle and a 14-day luteal phase, ovulation falls 14 days after the period starts: September 29, 2026. The fertile window runs September 24–30, the next period is due October 13, and conceiving this cycle would give a due date of June 22, 2027.
Long 34-day cycle
A 34-day cycle means ovulation around day 21, which is October 21, 2026. The most fertile days start October 16, and the next period is expected on November 4.
26-day cycle with a 12-day luteal phase
Counting back 12 days from the next period (November 28) puts ovulation on November 16, cycle day 15. A home pregnancy test is most reliable from November 28 onward.
Frequently asked questions
How many days after my period do I ovulate?+
In a 28-day cycle ovulation usually happens about 14 days after the period starts (cycle day 14–15, counting the first day of bleeding as day 1). With longer cycles it comes later — roughly cycle length minus 14 days after the period starts.
What is the fertile window?+
It is the five days before ovulation plus ovulation day — six days in all — with a small extra chance the day after, because the egg survives 12–24 hours. This calculator shows all seven days. The two days before ovulation and ovulation day itself have the highest chance of conception.
How accurate are ovulation calculators?+
They are estimates based on averages and work best for regular cycles. Studies show ovulation day varies even in regular cycles, so confirm with LH test strips, cervical mucus changes or basal body temperature.
Can I use an ovulation calculator as birth control?+
No. Calendar predictions alone are not reliable contraception because ovulation can shift unpredictably. Fertility awareness methods require daily tracking of several signs and proper training to be effective.
What if my cycles are irregular?+
Use your shortest and longest recent cycles to get an earliest and latest ovulation estimate, and rely more on ovulation test strips. Cycles regularly shorter than 21 or longer than 35 days are worth discussing with a doctor.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.