Menstrual Cycle Calculator

Q: What causes an irregular menstrual cycle?

Irregular cycles (varying by more than 7 days month to month, or cycle lengths outside 21–35 days) can be caused by: polycystic ovary syndrome (PCOS) — one of the most common causes; thyroid disorders (hypothyroidism, hyperthyroidism); hyperprolactinemia; significant weight changes or low body fat (common in athletes); high physical or emotional stress; perimenopause; recent hormonal contraceptive use; chronic illness; or certain medications. If your cycles are consistently irregular, a healthcare provider can evaluate hormonal levels and identify the cause.

Q: When should I see a doctor about my periods?

See a healthcare provider if: your periods are absent for 90+ days without pregnancy; cycles are consistently shorter than 21 days or longer than 35 days; periods last more than 7 days consistently; you experience severe cramping (dysmenorrhea) that interferes with daily activities; you soak through a pad or tampon hourly for several hours (possible menorrhagia); you experience bleeding between periods or after sex; periods suddenly become much heavier, lighter, or more irregular than before; or you have been trying to conceive for 12 months (6 months if over 35) without success.

Predict your next period, ovulation date, and fertile window. Get a 3-cycle forecast based on your average cycle length.

🌸 What is a Menstrual Cycle?

The menstrual cycle is a monthly hormonal process that prepares the female reproductive system for potential pregnancy. It begins on the first day of your period (day 1) and ends the day before your next period starts. The average cycle length is 28 days, but normal cycles range from 21 to 35 days. Understanding your cycle timing helps with family planning, predicting your next period, identifying your fertile window, and recognizing patterns that may indicate hormonal imbalances.

The cycle has two main phases separated by ovulation. The follicular phase (days 1–14 in a 28-day cycle) begins with menstruation and continues as follicle-stimulating hormone (FSH) drives the growth of a dominant follicle in the ovary. Rising estrogen levels thicken the uterine lining (endometrium) in preparation for a fertilized egg. Near the end of the follicular phase, a surge of luteinizing hormone (LH) triggers ovulation — the release of an egg from the dominant follicle.

After ovulation, the luteal phase begins (typically lasting 12–16 days). The empty follicle transforms into the corpus luteum, which produces progesterone to maintain the uterine lining. If fertilization does not occur, progesterone and estrogen levels drop, the corpus luteum degenerates, and the uterine lining sheds — triggering the next menstrual period. The luteal phase duration is remarkably consistent at approximately 14 days across most people, which is why ovulation is estimated as "cycle length minus 14 days before the next expected period."

Your fertile window spans the 6 days ending on ovulation day: the 5 days prior (because sperm can survive in the female reproductive tract for up to 5 days) plus the day of ovulation itself (the egg survives 12–24 hours post-release). Having intercourse during this window maximizes the chance of conception. For contraception purposes, avoiding unprotected intercourse during this window reduces risk, though calendar methods alone have significant failure rates and should be combined with other indicators such as basal body temperature and cervical mucus monitoring.

📐 Formula

Ovulation Date = Last Period Start + (Cycle Length − 14)

Next Period = Last Period Start + Cycle Length

Ovulation Date = Next Period − 14 days (luteal phase = 14 days)

Fertile Window Start = Ovulation Date − 5 days

Fertile Window End = Ovulation Date (day of ovulation)

Period End = Last Period Start + Period Duration − 1

Example (28-day cycle, started May 1): Ovulation = May 1 + (28−14) = May 15. Fertile window = May 10 – May 15. Next period = May 1 + 28 = May 29.

Example (35-day cycle, started May 1): Ovulation = May 1 + (35−14) = May 22. Fertile window = May 17 – May 22. Next period = May 1 + 35 = June 5.

📖 How to Use This Calculator

Period Predictor Mode

Enter the first day of your last period — select the date when your most recent period began (the day bleeding started, not when it ended).

Set your average cycle length — enter how many days typically pass from the start of one period to the start of the next. Use 28 if you're unsure. Track 3+ cycles and average them for greater accuracy.

Set period duration — enter how many days your period typically lasts (2–10 days). This populates the period end date in the forecast table.

Click Calculate — see your next period date, ovulation day, fertile window, and a 3-cycle forecast table with all dates.

💡 Example Calculations

Example 1 — Standard 28-Day Cycle

Last period started May 1, cycle length 28 days, period lasts 5 days

Period end: May 1 + 5 days − 1 = May 5. Current period runs May 1–5.

Ovulation: May 1 + (28 − 14) = May 1 + 14 = May 15.

Fertile window: May 15 − 5 = May 10 through May 15 (6 days).

Next period: May 1 + 28 = May 29. Following period: June 26. Third period: July 24.

Result = Next period: May 29 | Ovulation: May 15 | Fertile: May 10–15

Try this example →

Example 2 — Long 35-Day Cycle

Last period started May 1, cycle length 35 days, period lasts 7 days

Period end: May 1 + 7 − 1 = May 7. Current period runs May 1–7.

Ovulation: May 1 + (35 − 14) = May 1 + 21 = May 22 (day 21 of the cycle, not day 14!).

Fertile window: May 22 − 5 = May 17 through May 22.

Next period: May 1 + 35 = June 5. Following: July 10. Third: August 14.

Result = Next period: June 5 | Ovulation: May 22 | Fertile: May 17–22

Try this example →

Example 3 — Short 24-Day Cycle

Last period started May 8, cycle length 24 days, period lasts 4 days

Period end: May 8 + 4 − 1 = May 11. Current period runs May 8–11.

Ovulation: May 8 + (24 − 14) = May 8 + 10 = May 18 (day 10 of the cycle — earlier than typical!).

Fertile window: May 18 − 5 = May 13 through May 18. This begins just 2 days after the period ends.

Next period: May 8 + 24 = June 1. Short cycles mean ovulation occurs soon after the period — unprotected intercourse even during late period days carries conception risk.

Result = Next period: June 1 | Ovulation: May 18 | Fertile: May 13–18

Try this example →

❓ Frequently Asked Questions

How do I calculate when my next period is due?+

Add your average cycle length to the first day of your last period. If your last period started on May 1 and your cycle is 28 days, your next period is due May 29. For a 30-day cycle: May 31. The formula is: Next Period = First Day of Last Period + Cycle Length. This calculator does this automatically and also shows the fertile window and ovulation date.

When do I ovulate in my cycle?+

Ovulation typically occurs 14 days before the start of your next period (the luteal phase is consistent at ~14 days). On a 28-day cycle: ovulation = day 14. On a 35-day cycle: ovulation = day 21. On a 21-day cycle: ovulation = day 7. The formula is Ovulation = Last Period + (Cycle Length - 14). Ovulation can shift by 2–3 days due to stress, illness, or disrupted sleep.

What is the fertile window?+

The fertile window is the 6-day period during which intercourse can result in pregnancy: the 5 days before ovulation plus the day of ovulation itself. An egg survives only 12–24 hours after release, but sperm can survive in the reproductive tract for up to 5 days. Having intercourse in the days leading up to ovulation is most effective for conception. This calculator marks the fertile window as ovulation day minus 5 through ovulation day.

What is a normal menstrual cycle length?+

A normal menstrual cycle ranges from 21 to 35 days, with an average of 28 days. The cycle begins on the first day of your period (day 1) and ends the day before your next period starts. Cycles consistently shorter than 21 days or longer than 35 days, or varying by more than 8–10 days each month, may warrant evaluation by a healthcare provider.

How accurate are period and ovulation predictions?+

Calendar-based predictions are most accurate for women with regular cycles (varying by less than 3 days month to month). For regular cycles: next period prediction is typically accurate within 1–3 days. For irregular cycles (varying by 7+ days): predictions can be off by a week or more. Confirm ovulation with LH strips or basal body temperature (BBT) tracking for greater accuracy.

What causes an irregular menstrual cycle?+

Irregular cycles can be caused by: polycystic ovary syndrome (PCOS) — one of the most common causes; thyroid disorders; hyperprolactinemia; significant weight changes or low body fat (common in athletes); high physical or emotional stress; perimenopause; recent hormonal contraceptive use; chronic illness; or certain medications. If your cycles are consistently irregular, a healthcare provider can evaluate hormonal levels and identify the cause.

Can I use this calculator to avoid pregnancy?+

This calculator can help you understand your cycle, but calendar-based methods alone are not reliable contraception. The fertility awareness method (FAM) has a typical-use failure rate of 24% per year. Calendar methods work best as part of a comprehensive fertility awareness program that also includes BBT tracking and cervical mucus observation. If avoiding pregnancy is important, use a more reliable contraceptive method or consult a healthcare provider.

What is the luteal phase and why does it matter?+

The luteal phase is the second half of your menstrual cycle — from ovulation until your next period. It typically lasts 12–16 days (commonly 14 days). The luteal phase length is more consistent across cycles than the follicular phase (first half). This is why ovulation is calculated as 'cycle length minus 14.' A short luteal phase (under 10 days) may indicate a luteal phase defect, which can affect fertility and implantation.

How do I track my cycle if it is irregular?+

For irregular cycles: track your last 3–6 periods to calculate your average cycle length. Use the shortest cycle in your recent history to estimate the earliest possible ovulation. Pair calendar tracking with LH ovulation tests (they detect the LH surge that precedes ovulation by 24–48 hours) and basal body temperature (BBT) charting — a temperature rise of 0.2°C after ovulation confirms it occurred.

When should I see a doctor about my periods?+

See a healthcare provider if: periods are absent for 90+ days without pregnancy; cycles are consistently shorter than 21 days or longer than 35 days; periods last more than 7 days consistently; you experience severe cramping that interferes with daily activities; you soak through a pad or tampon hourly for several hours; you experience bleeding between periods or after sex; or you have been trying to conceive for 12 months (6 months if over 35) without success.

What does it mean if my period is late?+

A period is considered late if it has not started by 7 days after your predicted date. Common causes: pregnancy (most important to rule out with a home test); stress or major life changes; illness or fever during your cycle; significant weight gain or loss; changes in exercise habits; travel across time zones; thyroid imbalance; and perimenopause. Occasional late periods (1–2 per year) are normal. If your period is consistently late or absent, see a healthcare provider.

How long does a period typically last?+

A typical period lasts 3 to 7 days, with 5 days being most common. The heaviest flow usually occurs in the first 1–3 days. Periods lasting fewer than 2 days or more than 7 days consistently may indicate hormonal imbalance, structural issues like fibroids or polyps, or thyroid disorders. Using more than 6–7 pads or tampons per day for multiple days may indicate heavy menstrual bleeding (menorrhagia), which warrants medical evaluation.

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📌 Quick Tips

💡The average menstrual cycle is 28 days, but normal cycles range from 21 to 35 days. If your cycles vary by more than 7 days each month, you have an irregular cycle and ovulation predictions will be less reliable.

💡Ovulation typically occurs 14 days before your next period — not 14 days after your last period. On a 28-day cycle these are the same, but on a 35-day cycle, ovulation falls on day 21, not day 14.

💡You are most fertile in the 5 days before ovulation and on the day of ovulation itself. Sperm can survive in the fallopian tubes for up to 5 days, so having intercourse before ovulation is effective for conception.

💡Period tracking apps use the same formula as this calculator. For greater accuracy, track your basal body temperature (BBT) or use ovulation predictor kits (OPKs) which detect the LH surge that precedes ovulation by 24–36 hours.

💡This calculator uses averages based on your cycle length. If you are trying to conceive or avoid pregnancy, consult a healthcare provider — calendar methods alone have higher error rates than other contraceptive methods.