Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated ovulation
2026-06-03
Fertile window start
2026-05-29
Fertile window end
2026-06-04
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter first day of last period.
- 2Enter cycle length (days).
- 3Read your estimated ovulation on the right — it updates as you type.
- 4Hit Share to keep the scenario or send it to someone.
About this calculator
Estimates the next ovulation date and fertile window from the last period and cycle length.