Ovulation Calculator

How it works

The phase after ovulation (the luteal phase) is relatively consistent at about 14 days in many people, while the phase before it varies. So ovulation is estimated as 14 days before the next expected period:

Ovulation ≈ LMP + cycle length − 14 days
Fertile window ≈ 5 days before ovulation through ovulation day

The six-day fertile window comes from Wilcox and colleagues (1995), who found that conception occurred only from intercourse in the five days before ovulation and on the day of ovulation itself, because sperm can survive several days and the egg about a day.

Improving the estimate

Real cycles vary. More reliable ways to identify your fertile days include ovulation predictor kits that detect the luteinizing hormone (LH) surge, tracking basal body temperature (which rises after ovulation), and noticing changes in cervical mucus, which becomes clear and stretchy near ovulation. ACOG notes that fertility awareness-based methods require training and consistent daily tracking.

Irregular cycles

If your cycles vary by more than about a week, or are shorter than 21 or longer than 35 days, calendar predictions are much less reliable. Irregular cycles can have many causes, such as polycystic ovary syndrome or thyroid problems, and are worth discussing with your provider.

Trying to conceive

Intercourse every one to two days during the fertile window gives the best chance. Most couples conceive within a year; ACOG suggests talking to a provider if you haven't after 12 months of trying (or 6 months if you are 35 or older).

Signs of ovulation

• Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the days before ovulation.
• Basal body temperature rises by about 0.2–0.5 °C (0.4–1.0 °F) after ovulation and stays up until your period.
• LH surge: urine test kits detect the hormone surge that triggers ovulation about 24–36 hours later.
• Some people notice mild one-sided pelvic pain or breast tenderness.

Factors that shift ovulation

Stress, illness, travel across time zones, intense exercise, significant weight change, breastfeeding and recently stopping hormonal contraception can all delay or prevent ovulation in a given cycle. That's why a calendar estimate based on past cycles can miss.

When to call your provider

See a doctor or gynecologist if your cycles are regularly shorter than 21 days or longer than 35 days, your periods stop for three months or more (and you're not pregnant), you bleed between periods or after sex, your periods are very heavy or painful, or you have been trying to conceive for 12 months (6 months if you are 35 or older) without success. Earlier advice is wise if you have a known condition such as endometriosis or PCOS. See ACOG's Evaluating Infertility FAQ and the NHS page on infertility.

Tracking cycles

Record the first day of each period for several months. Your average cycle length — and how much it varies — makes this estimate more useful and is valuable information to share with a healthcare provider.

After stopping birth control

Fertility usually returns quickly after stopping the pill, patch, ring or a copper or hormonal IUD, though the first few cycles can be irregular. After the contraceptive injection it can take several months. Calendar estimates are least reliable during this time.

Worked examples

Example 1: LMP January 1, 2026, 28-day cycle

Ovulation ≈ January 1 + 28 − 14 = January 15. Fertile window ≈ January 10–15.

Example 2: LMP January 1, 2026, 32-day cycle

Ovulation ≈ January 1 + 18 days = January 19; fertile window ≈ January 14–19.

Limitations

• Assumes a 14-day luteal phase; actual length varies (about 10–16 days).
• Ovulation can shift from cycle to cycle due to stress, illness, travel or breastfeeding.
• Not reliable for irregular cycles or soon after stopping hormonal contraception.

Who should not rely on this calculator

Do not use this calculator as contraception or to diagnose fertility problems. People with irregular cycles, PCOS, or who are breastfeeding should consult a provider.