Ovulation & Fertile Window Calculator

What the Ovulation and Fertile Window Calculator calculates

Predicts ovulation, the six-day fertile window, the peak fertility days and the implantation window from the last period and cycle length, and projects them across the next several cycles.

How to use this Ovulation and Fertile Window Calculator

Enter First day of your last period, Average cycle length, Luteal phase length and Cycles to project. Estimated ovulation updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.

How the result is worked out

Open “How this is worked out” beside the result for the step-by-step calculation behind Estimated ovulation, Ovulation falls on and Fertile window opens.

The method follows Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle, Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation, Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy and ACOG, Fertility Awareness-Based Methods of Family Planning.

Textbook 28-day cycle from a 1 January 2026 period example

With First day of your last period: 2026-01-01, Average cycle length: 28, the calculator returns Estimated ovulation: 2026-01-15, Fertile window opens: 2026-01-10, Fertile window closes: 2026-01-15. This example is checked against an independent source in the test suite.

What you can compare

Change one input at a time to see how it affects Estimated ovulation, Ovulation falls on and Fertile window opens. The inputs you can vary are First day of your last period, Average cycle length, Luteal phase length, Cycles to project and Today.

Understanding Estimated ovulation, Ovulation falls on and Fertile window opens

An estimate from calendar arithmetic, not a clinical assessment. Only about 4% of births occur on the estimated due date; ultrasound dating by a clinician supersedes menstrual dating when they disagree.