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About this tool
See your ovulation day, fertile window, and best days to conceive for the next six cycles on a clear calendar.
The Ovulation Calculator projects your ovulation day, six-day fertile window and best conception days across the next six cycles from two inputs: the date your last period started and your usual cycle length. It works from the calendar method, placing ovulation 14 days before the next period is due — cycle day 14 on a 28-day cycle — and marking the fertile window as the five days before ovulation through one day after. Results are laid out on a month calendar and in a per-cycle table you can copy. This is general information for planning, not a substitute for medical advice.
Open Ovulation Calculator on AltFTool — it loads instantly in your browser.
Enter First day of your last period and your Average cycle length (days), anywhere from 21 to 40.
Check the banner, which names today's fertility level and the cycle day you are on out of your cycle length.
Read the Next 6 cycles table for period start, fertile window, ovulation and best days to conceive, check the Calendar view, then press Copy schedule.
The table projects period start, fertile window, ovulation day and best days for six consecutive cycles, which is enough to plan around travel or leave.
The window runs from five days before ovulation to one day after, because sperm can survive up to five days while the egg lives only about 12–24 hours.
Ovulation is placed at cycle length minus 14 days for any length from 21 to 40, so a long or short cycle moves the whole window rather than breaking the estimate.
Around day 14, counting day 1 as the first day of your period. The calculation is cycle length minus 14, because the luteal phase after ovulation is the more consistent half of the cycle — so a 31-day cycle puts ovulation nearer day 17.
About six days: the five days before ovulation plus ovulation day itself, with one more day after usually included as marginally fertile. Sperm can survive up to five days in the reproductive tract, whereas the released egg is viable for only roughly 12 to 24 hours.
The two days before ovulation and ovulation day itself are generally the highest-probability days. This tool highlights that three-day stretch separately from the wider fertile window.
It is an estimate, and it is least reliable if your cycles vary or are outside the 21-to-40 day range. Ovulation test strips detecting the LH surge, cervical mucus changes and basal body temperature give a better real-time signal; see a doctor if you have been trying for 12 months, or 6 months if you are 35 or older.
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