Ovulation Calculator

How to use the ovulation calculator

1. Pick the first day of your last period: the first day of real bleeding, not spotting.
2. Enter your cycle length, counted from the first day of one period up to, but not including, the first day of the next. If it varies, use the average of your last few cycles. The calculator accepts 21 to 45 days.
3. Optional, under Advanced options: change the luteal phase, the days between ovulation and your next period. Leave it at 14 unless you know yours from tracking.
4. Read the estimated ovulation day, the fertile window, the next period and the due date, then check the table for the following cycles.

How the calculator estimates ovulation

• Ovulation = first day of last period + cycle length − luteal phase. With a 28-day cycle and a 14-day luteal phase, that is day 15 of the cycle, 14 days before the next period.
• Fertile window = 5 days before ovulation through 1 day after. ACOG explains that sperm can survive up to five days in the body, while an egg lives about 12 to 24 hours after release, so sex in the days before ovulation can lead to pregnancy.
• Next period = first day of last period + cycle length.
• Due date if you conceive = first day of last period + 280 days, adjusted by your cycle length minus 28. The luteal phase does not change this date.
• Six-cycle table: each new cycle starts one cycle length after the previous one. It assumes every cycle has the same length, which real cycles rarely do.

The luteal phase is the more stable part of the cycle. According to the Office on Women's Health, a period typically follows ovulation by 14 to 16 days, while the time before ovulation varies more. That is why the calculator counts back from the expected next period.

Ovulation calculator examples

28-day cycle, last period October 1, 2026

Estimated ovulation is October 15, 2026, with a fertile window from October 10 to October 16. The next period is due October 29, and the due date if you conceive this cycle is July 8, 2027. In the table, the second cycle runs from October 29, with a fertile window of November 7 to 13 and ovulation on November 12.

32-day cycle, last period September 20, 2026

A longer cycle moves everything later: ovulation October 8, 2026, fertile window October 3 to 9, next period October 22 and due date July 1, 2027.

26-day cycle with a 12-day luteal phase, last period December 28, 2026

Ovulation is estimated on January 11, 2027, with a fertile window from January 6 to 12. With the default 14-day luteal phase it would be January 9. Either way, the next period is due January 23 and the due date is October 2, 2027.

Limits: irregular cycles, tests and birth control

• It is an estimate. Ovulation can shift from cycle to cycle with illness, stress, poor sleep or travel. Normal cycles range from about 21 to 35 days. If your last period started October 1, 2026, a 21-day cycle puts ovulation on October 8 and a 35-day cycle on October 22. With irregular cycles, a single date says little.
• Body signs help. Urine ovulation tests detect the LH surge shortly before ovulation. Cervical mucus turns clear and slippery just before ovulation. Basal body temperature rises by about 0.4 to 0.8 °F after ovulation, which confirms it only afterward.
• Not birth control. A calendar estimate is not a reliable way to avoid pregnancy. Fertility awareness methods need daily tracking and training. Ask a doctor or nurse about contraception.
• When to get help: ACOG advises seeing a doctor after 12 months of trying if you are under 35, after 6 months from 35 to 39, and sooner from 40, or early with PCOS, endometriosis, prior pelvic surgery or very irregular periods.