LilaLila

Free tool · With perimenopause irregularity check

Menstrual Cycle Calculator

Predict your next 3 periods, fertile window, and ovulation day. If your cycle length is shifting, we'll flag it as a perimenopause signal.

From the first day of one period to the first day of the next.

How much your cycle length swings month to month.

Next period

Wed, Jun 10
Ovulation dayWed, May 27
Fertile windowMay 22 – May 28
Following periodWed, Jul 8
Period after thatWed, Aug 5

Cycle pattern

Regular. Variation under 7 days is typical for premenopausal cycles.

Calendar prediction. Not contraception, not a diagnosis.

Reading your cycle in your 40s

In your 20s and 30s, your cycle is mostly a calendar exercise. In your 40s, it becomes one of the clearest windows into where you are in the menopause transition. The numbers below are how clinicians stage perimenopause — well before any blood test.

STRAW staging at a glance

The STRAW+10 system is the gold standard for staging the menopause transition:

  • Late reproductive: cycles still regular but flow may change.
  • Early perimenopause: persistent 7+ day variation in cycle length.
  • Late perimenopause: 60+ day gaps between periods.
  • Postmenopause: 12 consecutive months without a period.

When predictions stop working

Calendar calculators assume cycle length is stable. In perimenopause, anovulatory cycles, shorter follicular phases, and skipped periods make calendar math unreliable. Track symptoms and basal body temperature alongside dates for a fuller picture.

Yes, you can still get pregnant

Until you've gone 12 months without a period, perimenopause does not equal infertility. Use proper contraception if pregnancy isn't the goal.

Track cycle, symptoms, and food in one place.

Lila connects the dots so you can see what's hormonal, what's lifestyle, and what actually moves your symptoms.

Get the app