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Free ovulation window calculator using your last period start and average cycle length. Estimates ovulation with a fixed 14-day luteal assumption and shows a 5-day-before through ovulation fertile band. Works with our period calculator and other medical & health tools.
Last updated: April 13, 2026
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Whole days between first days of successive periods (often 21–35). Default 28.
Fertile window (approximate)
2026-03-10 → 2026-03-15
5 days before ovulation through ovulation day
Estimated ovulation
2026-03-15
~Cycle day 15 (day 1 = LMP)
Next period (estimated start)
2026-03-29
LMP + 28 days
Model assumptions
Ovulation is placed 14 days before the next expected period (luteal phase estimate). Real cycles vary; LH tests, BBT, cervical mucus, and ultrasound give person-specific timing.
Placed using “next period minus ~14 days,” a common rule of thumb when you do not have LH or ultrasound data.
Includes the five days before estimated ovulation and ovulation day—often discussed in preconception education, with large individual variation.
Adds your cycle length to LMP so you can sanity-check against your typical calendar.
In a 28-day cycle, this model places ovulation 14 days before the next expected period start—often in the third week of the cycle depending on how you count “day 1.” Compare results to LH strips or BBT if you are trying to conceive.
The tool estimates the first day of your next menses as LMP plus your average cycle length in whole days. It then subtracts 14 days from that next-period date to locate estimated ovulation on the calendar. The fertile window begins 5 days before that ovulation date and ends on ovulation day. This is a simplified population model, not a hormonal simulation.
Next period ≈ LMP + cycle lengthOvulation ≈ Next period − 14 daysFertile start ≈ Ovulation − 5 daysFertile end = Ovulation dayAfter a positive test, try the gestational age calculator for LMP-based pregnancy dating.
Get a Custom Calculator for Your PlatformNext period estimated March 31. Ovulation is placed 14 days earlier: March 17. The fertile window begins March 12 and runs through March 17 in this model—wider spacing reflects the longer follicular phase assumption.
Share it with anyone planning pregnancy or learning cycle science
Suggested hashtags: #Fertility #WomensHealth #TTC #Education #Calculator