Ovulation Calculator

How to use it

1. Enter the first day of your last period – the day bleeding started.
2. Enter your usual cycle length. If it varies, use the average of the last three to six cycles.
3. Leave the luteal phase at 14 days unless charting or a clinic has shown yours is different (10–16 days is normal).
4. Read the estimated ovulation day, the fertile window and the expected next period; the cycle calendar colours every day and marks today.
5. Use the three-cycle table to plan ahead, and copy the dates or the link to keep them.

Example

Method, sources and limits

next period     = LMP + cycle length
ovulation       = next period − luteal phase length   (= LMP + cycle − luteal)
fertile window  = ovulation − 5 days … ovulation day
cycle n         = LMP + (n − 1) × cycle length

Counting back from the next period rather than forward from the last one reflects how cycles actually vary: the luteal phase is relatively constant (about 12–16 days), while the follicular phase stretches or shrinks. The six-day window comes from Wilcox, Weinberg and Baird (1995), who found that nearly all pregnancies resulted from intercourse in the five days before ovulation or on the day itself, with sperm surviving up to five days and the egg about 24 hours. The same group later showed how poorly the calendar predicts the window for an individual (Wilcox et al. 2000). The calculator cannot know when you ovulated this month; it only applies your averages. It is unsuitable for cycles outside 21–45 days, for the months after stopping hormonal contraception, during breastfeeding, and for conditions such as PCOS where ovulation is irregular or absent. Period length is drawn as a typical five days.

Sources: Wilcox, Weinberg & Baird 1995, NEJM – Timing of sexual intercourse in relation to ovulation, Wilcox, Dunson & Baird 2000, BMJ – The timing of the "fertile window" in the menstrual cycle, ACOG – Fertility awareness-based methods of family planning, NHS – Trying to get pregnant.

These dates are statistical estimates, not medical advice, and they are not a contraceptive method. If you have been trying to conceive for a year (six months if over 35) or your cycles are very irregular, talk to a doctor.

Signs that confirm ovulation

Urine LH tests turn positive 24–36 hours before ovulation; cervical mucus becomes clear and stretchy in the fertile days; basal body temperature rises by about 0.2–0.5 °C after ovulation and stays up until the next period. Combining one of these with the calendar estimate gives a much better picture than dates alone.

Questions

When do I ovulate with a 30-day cycle?

With a 14-day luteal phase, on cycle day 16 (30 − 14), i.e. 15 days after the first day of your period. The fertile window runs from day 11 to day 16.

Can I get pregnant outside the fertile window?

The window itself is an estimate, so yes: if ovulation comes earlier or later than calculated, the real fertile days move with it. That is why the calendar method alone is unreliable for avoiding pregnancy.

What if my cycles are irregular?

Enter the average length, but expect the estimate to be rough. Ovulation tests and cervical-mucus tracking work regardless of cycle length; a clinician can help if cycles are shorter than 21 or longer than 35 days.