Ovulation & Fertile Window Calculator
Estimate your likely ovulation date, fertile window and next menstrual period from your last period and usual cycle length. These dates are approximate and cannot confirm ovulation.
Enter menstrual cycle information
Ovulation & Fertile Window Calculation Results
Calculation methodology
Estimated next period = last period start + usual cycle length. Estimated ovulation = predicted next period - 14 days. Estimated fertile window = five days before estimated ovulation through the estimated ovulation day (six calendar days).
The luteal phase is not always 14 days. Fertile dates may shift from cycle to cycle. This calendar method is not reliable contraception and cannot diagnose or confirm ovulation.
Frequently asked questions
What is a fertile window?
The fertile window is an estimated six-day interval ending on the predicted ovulation day, when conception may be possible.
How is ovulation estimated?
This tool estimates ovulation about 14 days before the next period. Actual ovulation varies.
Does a 28-day cycle mean ovulation on day 14?
Day 14 is only a common estimate. Individual cycles can vary.
Can this calculator confirm ovulation?
No. Ovulation prediction kits and clinical advice may provide additional information.
Can I use it with irregular cycles?
Cycle-based estimates are less reliable when periods are irregular.
Can this be used as birth control?
No. Do not rely on a calendar estimate alone to prevent pregnancy.
What if I am trying to conceive?
Tracking cycles may help identify a possible fertile window, but timing is uncertain. Seek medical advice if you have concerns.
Important health information
This calculator is for general education. Do not use these estimates alone to avoid pregnancy. If your periods are irregular, you have concerns about fertility or need contraception advice, speak with a qualified healthcare professional.