Ovulation & fertile days calculator
Estimate your next ovulation and fertile window in seconds from your last period date and average cycle length. We use the calendar method: ovulation happens about 14 days BEFORE the next period, and the fertile window is the 5 prior days plus ovulation day. An educational estimate for regular cycles — not a contraceptive method.
Enter your last period date (within 12 months) and a cycle of 21-40 days.
Calendar method for regular cycles: an educational ESTIMATE. NOT a contraceptive method and not a substitute for medical advice. With irregular cycles, prediction reliability drops.
How are ovulation and fertile days calculated?
The key is the luteal phase: the stretch between ovulation and the next period lasts about 14 days quite stably across women, while the first half of the cycle (follicular phase) is what varies. So ovulation is estimated by subtracting 14 days from your next period date: in a 28-day cycle it falls around day 14, in a 35-day cycle around day 21. The fertile window spans the 5 days before ovulation plus ovulation day, because sperm survive up to 5 days and the egg about 24 hours. Note: with irregular cycles the calendar method loses reliability; cervical mucus, basal temperature or LH tests are far more precise. Apps like Renzy learn from your real logs to improve the prediction cycle after cycle.
FAQ
When do I ovulate on a 28-day cycle?
Around day 14 (counting from the first day of your period). But it is an average — real ovulation can shift by days even in regular cycles. Longer cycles ovulate later (day 21 on a 35-day cycle).
Which days have the highest pregnancy probability?
The 2-3 days right before ovulation are the peak; the full window spans the 5 prior days plus ovulation day. Outside it the probability is low but NOT zero — which is why this is not a contraception method.
Can I use this as contraception?
NO. The calendar method has a high failure rate as contraception because ovulation shifts between cycles (stress, travel, illness). Talk to your doctor about effective methods.
What if my cycles are irregular?
With irregular cycles (variations over 7-9 days) the calendar loses reliability. LH tests, basal temperature and cervical mucus are more dependable — and persistent irregularity is worth discussing with your gynecologist.