Ovulation Calculator
Enter the first day of your last period and your average cycle length to estimate your fertile window, peak days and ovulation date for the next three cycles.
Calendar estimates assume a regular cycle. If yours vary, confirm with BBT and ovulation kits.
How the ovulation calculator works
Ovulation usually happens a fixed number of days before your next period - your luteal phase, about 14 days. So we estimate:
Ovulation = first day of last period + (cycle length - luteal phase)
Because sperm can survive up to 5 days and the egg lives 12 to 24 hours, your fertile window is the 6 days ending on ovulation day, with the 3 peak days just before and including ovulation. We project the next three cycles so you can plan ahead.
Source: ACOG guidance on the fertile window and menstrual cycle.
Frequently asked questions
How accurate is the calendar (rhythm) method?
It is an estimate. Calendar predictions assume a regular cycle and a fixed luteal phase, so they can be off by several days if your cycles vary. Confirm ovulation with basal body temperature (BBT) and ovulation predictor kits (OPKs).
How long does ovulation last?
The egg survives about 12 to 24 hours after ovulation. Because sperm can live up to 5 days, the fertile window spans roughly 6 days, ending on ovulation day.
What are the best days to conceive?
The 2 to 3 days before and including ovulation are the highest-chance days. This calculator marks them as your peak days.
Can I ovulate right after my period?
If you have a short cycle (21 to 24 days) and a longer period, the fertile window can start soon after bleeding ends. This tool accounts for your cycle length.
My cycles are irregular. Is this still useful?
Use it as a starting estimate, but track BBT, OPKs and cervical mucus to pinpoint ovulation for your body. A TTC planner keeps all three in one place.
Medical disclaimer: This calculator is for general information and education only. It is an estimate, not medical advice, diagnosis or treatment. Always talk to your doctor or midwife about your health and any decisions.