Ovulation Calculator: Find Your Fertile Window

Our ovulation calculator estimates when you are most likely to ovulate and which days make up your fertile window, based on the first day of your last period and your usual cycle length. It is a quick planning tool for couples who are trying to conceive, not a contraceptive and not a diagnosis.

How the ovulation calculator works

A menstrual cycle runs from the first day of one period to the day before the next. It has two halves: the time before ovulation, which varies from woman to woman, and the luteal phase after ovulation, which is more consistent and lasts about 14 days for most women.

So the calculator works backwards from your next expected period:

• Next period = first day of your last period + your average cycle length.
• Estimated ovulation day = next period minus about 14 days. In a 28-day cycle, that is around day 14; in a 32-day cycle, around day 18.
• Fertile window = the five days before ovulation plus ovulation day itself, six days in total.

The window starts before ovulation because sperm can survive in the body for up to about five days, while an egg lives for only around 12 to 24 hours after release. Having sex every one to two days during the fertile window, or regularly throughout the month, gives you the best chance. The two or three days just before ovulation are usually the most fertile.

Limits of any ovulation calculator

A calculator can only predict from averages. It will be less reliable if:

• Your cycles are irregular, shorter than about 21 days or longer than about 35 days.
• You have polycystic ovary syndrome (PCOS), thyroid problems or another condition that affects ovulation.
• You have recently stopped hormonal contraception, had a baby, or are breastfeeding.
• You are going through a period of high stress, illness, shift work or big changes in weight or exercise.

Ovulation can also shift from month to month even in women with regular cycles. Treat the dates as a guide, not a guarantee.

Signs of ovulation to look for

Your body gives clues that ovulation is near. Common signs include:

• Cervical mucus becoming clear, slippery and stretchy, like raw egg white.
• A small, sustained rise in basal body temperature after ovulation, which confirms it has happened.
• Mild one-sided pelvic twinges, light spotting, breast tenderness or a higher sex drive.

Our guide to ovulation signs explains how to track each one.

Ovulation predictor kits (OPKs)

OPKs are urine tests that detect the surge in luteinising hormone (LH) that comes before ovulation. Ovulation usually follows about 24 to 36 hours after a positive result, so a positive test means your most fertile days are now. Start testing a few days before the calculator’s estimated ovulation date. OPKs can be harder to read if you have PCOS, as LH levels may be high throughout the cycle. Compare options in our round-up of the best ovulation tests in Singapore.

When to see a doctor in Singapore

Most couples conceive within a year of regular, unprotected sex. Consider seeing a GP, gynae or fertility clinic:

• After 12 months of trying, if you are under 35.
• After 6 months, if you are 35 or older.
• Sooner, if you are 40 or older, have very irregular or absent periods, known PCOS or endometriosis, a history of pelvic infection, previous miscarriages, or if your partner has a known fertility issue.

Fertility checks usually look at both partners, including blood tests, a pelvic ultrasound and a semen analysis. The public hospitals KKH, NUH and SGH have assisted reproduction centres, and eligible Singaporean couples can receive government co-funding for assisted reproduction treatment there. The Ministry of Health’s page on marriage and parenthood schemes outlines current support.

See a doctor promptly, whatever stage you are at, if you have severe pelvic pain, very heavy or painful periods, or bleeding between periods or after sex.

Next steps

For a fuller picture of preparing your body, timing and what to expect, visit our trying to conceive hub. If your period is late, our guide to the early signs of pregnancy explains when to take a test.