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Menstrual Cycle Phase Calculator

Find which phase of your cycle you are in today — menstrual, follicular, ovulatory or luteal — from your cycle length and the day of your cycle.

Average cycle length
Period length
Days since your period started
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How it works

Cycle day = (days since period start mod cycle length) + 1 · Ovulation day ≈ cycle length − 14 · Phases: menstrual (day 1 to period length), follicular, ovulatory (ovulation ±1), luteal

A cycle is counted from the first day of bleeding (day 1). The luteal phase — from ovulation to the next period — is fairly constant at about 14 days, so ovulation is estimated by subtracting 14 from the cycle length rather than from a fixed mid-point. That is why longer cycles ovulate later, not just menstruate later. The four phases reflect the hormonal rhythm: menstruation, the estrogen-driven follicular build-up, the ovulatory surge, and the progesterone-dominant luteal phase.

Worked example

On a 28-day cycle, 10 days after your period started you are on day 11. Ovulation is estimated at day 28 − 14 = day 14, so day 11 sits in the follicular phase, with the fertile window running roughly day 9 to day 15 and the next period due in 18 days.

Frequently asked questions

Is the ovulation day exact?

No — it is an estimate assuming a 14-day luteal phase. Actual ovulation shifts with stress, illness and cycle variability. Ovulation predictor kits, basal body temperature or cervical-mucus tracking give a more personal signal.

Why does a longer cycle ovulate later?

The luteal phase (ovulation to next period) is relatively fixed at about 14 days, so the extra days in a longer cycle fall in the follicular phase before ovulation. A 32-day cycle ovulates around day 18, not day 16.

Can I use this to avoid or plan pregnancy?

It is educational only. Cycle-based timing has a meaningful failure rate because ovulation moves. Use a clinically supported fertility-awareness method or contraception if pregnancy timing matters.

Related calculators

Menstrual Cycle Calculator
Map your menstrual cycle — period, follicular phase, fertile window, ovulation, luteal phase and likely PMS days — with today’s cycle day and phase, a luteal-phase setting and dates for your next six cycles.
Fertility Window by Cycle Length
See which days of your cycle are fertile from your average cycle length alone — as cycle-day numbers, no dates needed.
PMS Symptom Window Calculator
Estimate the days each cycle when PMS symptoms are most likely — the late-luteal window before your next period — from your last period date and cycle length.
Cycle-Synced Fitness Plan Calculator
Match your training to your menstrual cycle — see which phase you are in today and the strength, HIIT or recovery focus that suits your hormones.

Educational estimate only, not medical advice. Cycles, nutrient needs and risk vary between people — discuss decisions with a qualified clinician.