About the Ovulation calculator
If you're trying to conceive, knowing your most fertile days can help with timing. This ovulation calculator estimates when you are likely to ovulate and your fertile window, the days when sex is most likely to lead to conception.
Enter the first day of your last period, your usual cycle length and, if you know it, your luteal phase. You get your next ovulation date and fertile window, plus a table of predicted period, fertile and ovulation dates for the next six cycles.
Important: this is a calendar estimate. It is not a reliable form of birth control.
How to use it
- Pick the First day of last period (day 1 is the first day of full bleeding).
- Enter your Cycle length: the days from the start of one period to the start of the next.
- Leave Luteal phase at 14 days unless you know yours from tracking.
- Read your next ovulation date and fertile window, then check the six-cycle table.
The formula
Ovulation happens a fairly steady number of days before the next period. That gap is the luteal phase, usually around 14 days.
Ovulation = period start + (cycle length − luteal phase) daysFertile window = 5 days before ovulation through 1 day after
- With a 28-day cycle and a 14-day luteal phase, ovulation is estimated 14 days after your period starts, about 14 days before the next one.
- Later cycles are projected by adding your cycle length to each period start.
- The window spans 7 days because sperm can survive up to about five days, and the egg only about a day.
Worked example
Say your last period started on March 1, your cycle is 28 days and your luteal phase is 14 days. Ovulation is estimated at 28 − 14 = 14 days after the start, on March 15. The fertile window runs from March 10 to March 16, and the next period is expected around March 29.
If your cycle is 32 days with the same luteal phase, ovulation shifts to 18 days after the start, on March 19. Longer cycles mostly lengthen the time before ovulation, not after it.
Making the estimate more useful
Cycles vary, even in people who think of themselves as regular. Stress, illness, travel and changes in weight can shift ovulation by several days. A calendar method can't see those changes, so its predictions are best treated as a guide to when to pay closer attention.
You can narrow things down with body signs and tests. Urine ovulation tests detect the LH surge that usually comes a day or two before ovulation. Clear, stretchy cervical mucus is a sign of fertile days. A small, sustained rise in basal body temperature confirms that ovulation has already happened. Tracking a few cycles also tells you your own luteal phase length.
Many couples take several months to conceive, and that is normal. It is common advice to see a doctor if you have been trying for a year without success, or after six months if you are 35 or older. See a doctor sooner if your cycles are very irregular or absent. Once you have a positive test, the due date calculator estimates when your baby may arrive.
Frequently asked questions
Can I use this ovulation calculator to avoid getting pregnant?
No. Calendar predictions can be off by several days and ovulation can shift without warning. Talk to a doctor or clinic about reliable contraception.
When is the best time to have sex to conceive?
The two or three days before ovulation and the day of ovulation are usually the most fertile. Having sex every one to two days through the fertile window covers it well.
What is the luteal phase?
It's the part of the cycle after ovulation, ending when your next period starts. It is often around 14 days and tends to vary less than the time before ovulation.
What if my cycles are irregular?
Predictions become much less reliable. Ovulation tests, cervical mucus and temperature tracking help more, and a doctor can check for causes of irregular cycles.
How long does ovulation last?
The egg is released in a single event and survives for roughly 12 to 24 hours. The fertile window is longer because sperm can survive for several days.
Last updated October 24, 2026. Results are estimates for education and planning — see our disclaimer.