Menstrual Cycle Calculator - Next Period, Ovulation & Fertile Days

You track the dates but still get surprised

Most period tracking comes down to counting days on a calendar and hoping for the best. The problem is that ovulation does not land on the same calendar date each month - it depends on your personal cycle length. This calculator takes your last period date, your cycle length and your period duration, then maps out all four phases: menstruation, follicular, ovulation and luteal. It predicts your next 3 periods, pinpoints the 5-day fertile window and tells you which phase you are in right now.

How to use this calculator - step by step

First day of last period - pick the date when your most recent bleeding started (not spotting). This anchors all predictions.
Cycle length - enter the number of days from one period start to the next. Normal range is 21-35 days, average 28. Leave blank if unsure.
Period length - how many days your bleeding lasts, typically 3-7 days. Defaults to 5 if not entered.
Read results - the calculator shows your next period date, ovulation day, fertile window, current phase with symptoms and a 3-cycle forecast.

The four phases of a menstrual cycle

Each cycle has four distinct phases. Their length varies depending on your total cycle length, but the luteal phase is remarkably consistent at 14 days in most women.

Phase	Days (28-day cycle)	Hormones	Key signs
Menstruation	1-5	Estrogen and progesterone low	Bleeding, cramps, fatigue
Follicular	6-12	Estrogen rising	Energy up, clear skin, focus
Ovulation	13-15	LH surge, estrogen peaks	Temp rise +0.3 C, cervical mucus
Luteal	16-28	Progesterone dominant	PMS, bloating, mood swings

When does ovulation actually happen?

Ovulation typically occurs 14 days before the next period - not 14 days after the last one. This distinction matters for shorter and longer cycles.

Cycle length	Ovulation day	Fertile window
21 days	Day 7	Days 3-8
25 days	Day 11	Days 7-12
28 days	Day 14	Days 10-15
32 days	Day 18	Days 14-19
35 days	Day 21	Days 17-22

Practical examples

Example 1: Last period June 1, cycle 28 days, period 5 days.
Next period: Jun 29. Ovulation: Jun 15. Fertile window: Jun 11-16.

Example 2: Last period June 10, cycle 32 days.
Next period: Jul 12. Ovulation: Jun 28. Fertile window: Jun 24-29.

Example 3: Last period May 20, cycle 25 days.
Next period: Jun 14. Ovulation: May 31. Fertile window: May 27-Jun 1.

Example 4: Last period June 5, cycle 35 days, period 7 days.
Next period: Jul 10. Ovulation: Jun 26. Fertile window: Jun 22-27.

Example 5: Last period June 15, cycle 21 days (short).
Next period: Jul 6. Ovulation: Jun 22. Fertile window: Jun 18-23.

FAQ - Frequently asked questions

How accurate is this calculator?

The calculator assumes a fixed cycle length and a 14-day luteal phase. For regular cycles this gives predictions accurate to 1-2 days. If your cycle varies by more than 5 days month to month, use the average of your last 3 cycles as input. Irregular cycles need medical evaluation - no calculator can reliably predict them.

Can I use this as birth control?

No. Calendar-based methods have a failure rate of 12-24% per year with typical use. Sperm can survive up to 5 days inside the body, and ovulation can shift by several days due to stress, illness or travel. This tool helps you understand your cycle - it is not a contraceptive method.

What if my cycle is irregular?

Cycles varying by more than 7-9 days are considered irregular. Common causes: PCOS, thyroid disorders, stress, extreme weight changes, perimenopause. Enter the average of your last 3-6 cycles for the best estimate, but consider tracking basal body temperature or using ovulation test strips for more precise data.

Why does ovulation happen 14 days before the period, not after?

The luteal phase (from ovulation to period) is hormonally fixed at about 14 days - it does not change much between cycles. The follicular phase (from period to ovulation) is what varies. So a 35-day cycle has ovulation on day 21, not day 14. Counting backward from the next period is more accurate than counting forward.

What is the fertile window?

The fertile window spans roughly 5 days before ovulation plus 1 day after. The egg survives only 12-24 hours after release, but sperm can wait in the fallopian tubes for up to 5 days. Peak fertility is the 2 days before and the day of ovulation.

Does cycle length change with age?

Yes. Teens often have cycles of 21-45 days that stabilize in the 20s. Between ages 25-35, cycles are most regular (average 28 days). After 35, cycles tend to shorten slightly. In perimenopause (typically 40s), cycles become irregular again before stopping altogether at menopause (average age 51).

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