Ovulation & Fertile Window Calculator

The first day of bleeding.
First day of one period to the first day of the next.
Ovulation to the next period. About 14 days for most people, and far more stable than the follicular phase.
Used only for the countdown; leave blank to see the dates alone.
Estimated ovulation
2026-09-24
Ovulation and Fertile Window Calculator
Ovulation falls on
Thursday
Fertile window opens
2026-09-19
Fertile window closes
2026-09-24
Fertile window
2026-09-19 to 2026-09-24
Days in the fertile window
6
Peak fertility begins
2026-09-23
Peak fertility ends
2026-09-24
Next period expected
2026-10-08
Implantation window opens
2026-09-30
Implantation window closes
2026-10-06
Earliest reliable pregnancy test
2026-10-08
Due date if you conceive this cycle
2027-06-17
Cycle day today
5
Days until ovulation
10
Follicular phase length
14
Worked out from Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle and 3 other sources.
The period you entered is 9 whole cycle(s) in the past, so the dates shown are for the cycle beginning 2026-09-10.
Calendar prediction assumes a regular cycle. In the Wilcox 2000 cohort only 10% of women with 28-day cycles ovulated on day 14, so treat these dates as a planning aid rather than a guarantee.
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.

How this is worked out

5 steps
  1. The next period is expected 28 days after 2026-09-10, on 2026-10-08.
  2. Ovulation is one luteal phase earlier: 2026-10-08 - 14 days = 2026-09-24 (cycle day 15).
  3. The fertile window is the six days ending on ovulation: 2026-09-19 to 2026-09-24.
  4. Intercourse on the two days ending at ovulation carries the highest per-cycle chance, so 2026-09-23 and 2026-09-24 are the peak days.
  5. A pregnancy conceived this cycle would implant between 2026-09-30 and 2026-10-06 and be due on 2027-06-17.
Projected cyclestable
cycleperiodStartfertileWindowStartovulationDatenextPeriodStart
12026-09-102026-09-192026-09-242026-10-08
22026-10-082026-10-172026-10-222026-11-05
32026-11-052026-11-142026-11-192026-12-03
42026-12-032026-12-122026-12-172026-12-31
52026-12-312027-01-092027-01-142027-01-28
62027-01-282027-02-062027-02-112027-02-25

Results are general information, not financial, tax, legal, medical or engineering advice.