Find your most fertile days by calculating ovulation date and fertile window from your last period and cycle length.
Ovulation usually happens about 14 days before your next period starts. In a regular 28-day cycle, that is around day 14. Your fertile window is roughly the five days before ovulation plus ovulation day itself, because sperm can survive several days. Timing intercourse in this window gives the best chance of conceiving.
| Event | Date |
|---|
Ovulation is estimated by counting back from your next expected period, because the second half of the cycle (the luteal phase) is fairly consistent at about 14 days.
The fertile window is the span of days when intercourse can lead to pregnancy. It reflects how long sperm and the egg survive.
| Day (relative to ovulation) | Fertility | Why |
|---|---|---|
| 5 days before | Low–moderate | Sperm can survive up to ~5 days |
| 2–3 days before | High | Sperm in place ready for the egg |
| 1 day before | Very high | Peak fertility |
| Ovulation day | Very high | Egg released |
| 1 day after | Low | Egg survives ~12–24 hours |
Alongside the calculator, your body gives physical signals that ovulation is near. Tracking these improves timing.
| Sign | What to Look For |
|---|---|
| Cervical mucus | Clear, stretchy, egg-white texture |
| Basal body temperature | Small rise (~0.3°C) just after ovulation |
| LH surge | Positive ovulation predictor kit (OPK) |
| Mild cramping | One-sided twinge (mittelschmerz) |
| Other | Slight breast tenderness, increased libido |
Calendar-based estimates work best with regular cycles. If your cycles vary a lot, tracking cervical mucus and using ovulation predictor kits is more reliable than the calendar alone.
After a miscarriage, ovulation can return surprisingly quickly — sometimes about two weeks later, even before your first period. If you are tracking ovulation after a pregnancy loss, your cycle timing may be different from before, so physical signs and OPKs can help. It is a good idea to discuss timing and any concerns with your doctor.
Ovulation typically occurs 14 days before the next expected period (not 14 days after the last period). The fertile window is approximately 5 days before ovulation plus the day of ovulation (6 days total), as sperm can survive 3–5 days in the reproductive tract.
Ovulation occurs when a mature egg is released from the ovary. For a 28-day cycle, ovulation typically occurs around day 14 — but this is calculated as 14 days BEFORE the next expected period (the luteal phase), not 14 days after the last period began. The luteal phase (ovulation to period) is relatively consistent at 12-16 days; it is the follicular phase (period to ovulation) that varies in length between cycles and between women.
Sperm survive in the female reproductive tract for up to 5 days, while the egg survives for only 12-24 hours after ovulation. This creates a fertile window of approximately 6 days: the 5 days before ovulation plus the day of ovulation itself. Having sex during this window maximises conception probability.
| Day Relative to Ovulation | Pregnancy Probability per Cycle |
|---|---|
| 5 days before ovulation | ~10% |
| 3-4 days before ovulation | ~15-20% |
| 1-2 days before ovulation | ~25-30% |
| Ovulation day | ~20-25% |
| 1 day after ovulation | ~8-10% |
When do I ovulate in my cycle?
Ovulation occurs approximately 14 days before your next period — not necessarily 14 days after your last period. For a 28-day cycle: ovulation on day 14. For a 32-day cycle: ovulation around day 18. For a 24-day cycle: around day 10. The luteal phase (ovulation to period) is relatively constant at 12-16 days; it is the follicular phase that varies between women and cycles.
How long is the fertile window?
The fertile window spans approximately 6 days: the 5 days before ovulation (when sperm can survive awaiting the egg) plus the ovulation day itself (when the egg survives 12-24 hours). Pregnancy probability is highest in the 1-2 days immediately before ovulation and on ovulation day itself — approximately 25-30% per cycle for healthy couples under 35.
Are ovulation calculator predictions accurate?
Calendar-based ovulation prediction is accurate for women with very regular cycles. For cycles varying by more than 2-3 days between months, or cycles shorter than 25 days or longer than 35 days, calendar prediction is less reliable. LH surge testing (ovulation predictor kits), cervical mucus monitoring, and basal body temperature tracking are more accurate methods for women with irregular cycles.
What if my period is irregular?
For women with irregular cycles (varying by more than 3-5 days month-to-month), calendar-based ovulation prediction has limited accuracy. Recommended approaches: basal body temperature charting, cervical mucus monitoring (Billings method), LH surge testing with ovulation predictor kits, or consultation with a GP or fertility specialist who can order hormone testing and ultrasound follicle tracking.
How do I calculate my ovulation day?
Subtract 14 days from the expected start of your next period, or add (cycle length − 14) to the first day of your last period. For a 30-day cycle, ovulation is around day 16. Enter your last period date and cycle length in the calculator to see your ovulation day and fertile window.
What are the most fertile days to conceive?
The most fertile days are the day before and the day of ovulation. Because sperm can survive up to about five days, the full fertile window spans roughly five days before ovulation plus ovulation day. Having intercourse during this window gives the best chance of conception.
How do I calculate ovulation after a miscarriage?
Ovulation can return about two weeks after a miscarriage, sometimes before your first period, so timing may differ from your usual cycle. Because dating from a last period is less reliable then, ovulation predictor kits and cervical mucus tracking are especially helpful. Discuss timing with your doctor.
How accurate is an ovulation calculator?
Calendar-based ovulation estimates are a helpful guide but assume a regular cycle with ovulation about 14 days before your period. Actual ovulation can shift from cycle to cycle. For more precision, combine the calculator with ovulation predictor kits, cervical mucus changes and basal body temperature.
Can I use an ovulation calculator to avoid pregnancy?
It is not reliable for contraception. Fertility awareness methods require careful daily tracking and training to be effective, and a simple calculator does not account for cycle variation. If you want to avoid pregnancy, use a proven contraceptive method and speak with your doctor.
What are the signs of ovulation?
Common signs include clear, stretchy egg-white cervical mucus, a small rise in basal body temperature after ovulation, a positive ovulation predictor kit (LH surge), mild one-sided cramping, and sometimes increased libido. Tracking these alongside the calculator improves timing accuracy.
How long does ovulation last?
The egg itself survives only about 12 to 24 hours after being released. However, the fertile window is longer — around six days — because sperm can survive in the body for up to five days, ready to fertilise the egg once it is released.