A familiar situation
View a calendar-based ovulation estimate and fertile window while accounting for cycle uncertainty.
How to use this calculator
- Enter first day of last period, usual cycle length.
- Select Calculate to see the answer and its breakdown.
- Keep a scenario, change an input and compare the results.
Use consistent units and check the assumptions below before applying the result.
How ovulation is calculated
A common estimate places ovulation about 14 days before the next period; the fertile window is approximated as five days before through one day after ovulation. For a 28-day cycle starting on March 1, the estimate places ovulation near day 14 and the fertile window around March 9–15.
Ovulation≈next expected period−14 days; fertile estimate=−5 to +1 days.For a 28-day cycle starting on March 1, the estimate places ovulation near day 14 and the fertile window around March 9–15.
Assumptions & useful context
Cycle-based estimate; irregular cycles can make calendar predictions unreliable.
Health calculations are estimates for education. They cannot diagnose a condition or replace a clinician’s assessment. Individual needs vary, especially during pregnancy, illness and growth.
Frequently asked questions
Can this confirm ovulation?
No. It estimates from cycle timing only.
Does every cycle ovulate on day 14?
No. Cycle length and ovulation timing vary.