Cycle & Fertility Calculator
Enter your last period and cycle length. We'll show your ovulation day, fertile window, and next period on a beautiful calendar.
Ovulation is when your ovary releases a mature egg. It typically happens around day 14 of a 28-day cycle — but can vary by several days depending on your individual cycle length.
The egg survives for 12–24 hours after release. Sperm can survive in the reproductive tract for up to 5 days, which is why the fertile window spans several days around ovulation.
Your fertile window is typically 6 days long — the 5 days before ovulation and the day of ovulation itself. The two days before ovulation and the day of are your peak fertile days.
This calculator uses the standard formula: ovulation = cycle length − 14. If your cycles are irregular, the fertile window may shift — tracking basal body temperature helps pinpoint it.
Common signs include a change in cervical mucus (becomes clear and stretchy, like egg whites), a slight rise in basal body temperature (0.2–0.5°C), and sometimes mild one-sided pelvic pain (mittelschmerz).
Some people also notice increased libido, light spotting, or breast tenderness around ovulation.
A typical cycle is 21–35 days, counted from the first day of one period to the first day of the next. A 28-day cycle is the average but not universal — shorter cycles mean earlier ovulation, longer cycles mean later.
If your cycles vary by more than 7–9 days, they're considered irregular. This calculator works best for regular cycles — consult a doctor for irregular cycles.
How accurate is this calculator?
This calculator uses the standard textbook formula (Ogino-Knaus method) and is a good starting estimate for women with regular cycles. It assumes ovulation occurs 14 days before your next period. For irregular cycles, actual ovulation may differ — temperature tracking or ovulation predictor kits (OPKs) give more precise results.
When am I most likely to conceive?
The highest probability is on the day before ovulation and the day of ovulation itself (marked as peak fertile days). Having intercourse during the 5-day fertile window, especially the final 2–3 days, maximises the chance of conception.
Can I use this to avoid pregnancy?
This calculator should not be used as contraception. Calendar-based methods have high failure rates, especially with irregular cycles. Please consult a healthcare provider for reliable contraception options.
What if my cycle is irregular?
Irregular cycles make calendar prediction unreliable. Instead, use daily basal body temperature (BBT) tracking, cervical mucus monitoring, or over-the-counter ovulation predictor kits. Apps that combine multiple signals can also help. A gynaecologist can help identify the cause of irregular cycles.
What does luteal phase mean?
The luteal phase is the second half of your cycle — from ovulation to the start of your next period. It's typically 12–16 days (average 14). During this time, progesterone rises to prepare the uterus for a possible pregnancy. If fertilisation doesn't occur, progesterone drops and menstruation begins.