Menstrual Cycle Calculator
Next period and cycle phase calendar from last period and cycle length.
next_period = LMP + cycle_length ovulation = LMP + cycle_length − 14 fertile_start = ovulation − 5 days
Next period
in 21 days
- Ovulation
- Jun 6
- Fertile window
- Jun 1 – Jun 7
- Current phase · day 8/28
- Follicular
- Menstruation · day 1–5
- May 23 – May 27
- Follicular · day 6–13
- May 28 – Jun 4
- Ovulatory · day 14–15
- Jun 6 – Jun 7
- Luteal · day 16–28
- Jun 8 – Jun 19
The Four Phases of the Menstrual Cycle
The menstrual cycle is driven by a coordinated interplay of four hormones: follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone. Phase one — menstruation — begins on day one with the shedding of the endometrium as progesterone withdraws. FSH simultaneously begins rising to initiate the follicular phase, stimulating several ovarian follicles to compete for dominance.
The dominant follicle produces rising estrogen, which thickens the uterine lining and, at sufficient concentration, triggers the LH surge that causes ovulation. The ruptured follicle becomes the corpus luteum, secreting progesterone throughout the luteal phase. If fertilization does not occur, the corpus luteum degenerates, progesterone falls, and the cycle resets.
The Luteal Phase and Cycle Prediction
While the follicular phase varies considerably in length (explaining most cycle-to-cycle variation), the luteal phase is relatively fixed at 12–16 days. This asymmetry is why the "subtract 14 days from the next period" rule for estimating ovulation is reasonably accurate for most people, even when total cycle length varies.
Luteal phase defects — where progesterone production is insufficient or the phase is shorter than 10 days — can impair implantation and are associated with early pregnancy loss. A consistently short luteal phase detected by tracking is worth discussing with a reproductive endocrinologist.
Factors That Shift the Cycle
Stress activates the HPA axis, suppressing GnRH and delaying or preventing the LH surge — a mechanism thought to have evolved to prevent pregnancy during famine or threat. Intense exercise, particularly in athletes with low energy availability, can cause functional hypothalamic amenorrhea (absent periods). PCOS, the most common hormonal disorder in people with ovaries, disrupts the follicular phase and causes irregular or absent ovulation.
Travel across time zones, illness, significant weight change, and starting or stopping hormonal contraception can all shift cycle timing by several days to weeks. Building a 3–6 month tracking history gives a more reliable personal baseline than population averages.
Tracking for Fertility Awareness
Calendar-based methods like this calculator have limited contraceptive efficacy (typical-use failure rate around 24% per year) because they cannot detect real-time cycle shifts. Combining cycle tracking with basal body temperature (BBT) monitoring and cervical mucus observation — the sympto-thermal method — improves accuracy. LH urine tests ("ovulation tests") detect the LH surge 24–36 hours before ovulation and are the most direct at-home confirmation of ovulation timing.
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