Number Ovulation Calculator
Your fertile window, ovulation day and next period from two inputs — plus a forward calendar covering the next three cycles so you can plan ahead, not just this month.
Your next three cycles
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|
The fertile window runs from five days before ovulation through the day after — sperm can survive up to five days, the egg about 24 hours.
How it works
Ovulation is driven by the luteal phase — the stretch from ovulation to the next period — which stays near 14 days for most people even when total cycle length varies. So ovulation day ≈ period start + (cycle length − 14). The fertile window opens five days earlier (sperm survival) and closes a day after ovulation (egg lifespan). If a cycle leads to pregnancy, the due-date estimate is ovulation + 266 days.
Frequently asked questions
How is ovulation calculated from my period?
Ovulation happens about 14 days before the NEXT period, so with a 28-day cycle it lands on day 14, but with a 32-day cycle it lands on day 18. Enter your real average cycle length — the count matters more than the textbook 28.
What exactly is the fertile window?
The six days ending just after ovulation: sperm can live up to five days in the reproductive tract, and the egg survives roughly 24 hours. Conception is most likely in the two days before ovulation and on the day itself.
My cycles are irregular. Will this still work?
Only roughly. The calculator assumes every cycle matches your average length; irregular cycles can ovulate a week earlier or later than the estimate. For irregular cycles, treat the window as a guide and consider ovulation predictor kits.
If I conceive this cycle, when would the due date be?
About 266 days after ovulation (or 280 days after the period that started the cycle). The Pregnancy Due Date Calculator gives the full picture with trimester milestones.
🤰 Already pregnant?
The Due Date Calculator works from your last period, conception date or an ultrasound.