Estimate an ovulation date and calendar fertile window from the last period date and average cycle length; timing can vary.
Estimated Ovulation Date
How the Ovulation Window Is Estimated
The calculator uses the entered last-period date and cycle length. It estimates ovulation by assuming a luteal phase of about 14 days, then displays a fertile window from five days before the estimated ovulation date through the following day. The method is a calendar projection based on the cycle length supplied.
Calendar predictions cannot confirm ovulation
Ovulation timing can vary between people and from one cycle to another. Stress, illness, travel, postpartum changes, and irregular cycles can make calendar estimates less reliable. Ovulation predictor tests and clinical assessment may provide additional information.
Health note: This calculator cannot confirm fertility, diagnose an ovulation disorder, or prevent pregnancy. Do not use its dates as a contraceptive method.
Example and limitations
Ovulation does not always occur on the same cycle day, and a calendar window cannot confirm ovulation or prevent pregnancy. Consider a qualified healthcare professional for contraception or fertility concerns.
Source and important context
The NHS notes that ovulation timing varies between people and cycles. A calendar estimate cannot confirm ovulation or reliably identify days when pregnancy cannot occur. NHS: Fertility in the menstrual cycle. This page provides general educational estimates, not a diagnosis or individualized care advice. Page information updated: 4 November 2026.