Period Calculator

Add your average cycle length to the first day of your last period. For a 28-day cycle starting January 1, your next period is expected January 29. For a 30-day cycle starting January 1, expect February 1. This calculator does this automatically for the next 12 cycles. Remember that cycle length naturally varies by a few days — treat the predicted dates as a window of a few days around the calculated date, not an exact day.

The average cycle is 28 days, but a normal cycle ranges from 21 to 35 days. A 2019 study of 600,000 menstrual cycles found the average was 29.3 days, with significant variation: only about 13% of cycles were exactly 28 days. Cycle length tends to be most variable in the teens and in the years approaching menopause, and most consistent during the 20s and 30s. What matters most is consistency within your own pattern.

A period 1–5 days late is usually normal variation. Common causes of a late period: pregnancy (most important to rule out if sexually active); stress — cortisol can disrupt the hormonal cascade needed for ovulation; significant weight change; intense exercise; illness or travel; thyroid dysfunction; PCOS; or perimenopause. If more than a week late and pregnancy has been ruled out, consult a healthcare provider, especially if this is a recurring pattern.

Yes — a 21-day cycle is at the shorter end of the normal range (21–35 days) and is medically considered normal as long as it's consistent for you. With a 21-day cycle, you'll have approximately 17 periods per year instead of the average 13. The key factor is consistency: if your cycle has always been 21 days, that's your normal. If it suddenly shortened to 21 days from 28+, that warrants investigation — particularly for thyroid issues or perimenopause in women 40+.

Yes — stress is one of the most common causes of delayed periods. The hypothalamus (which regulates the hormone cascade that triggers ovulation) is sensitive to cortisol and other stress hormones. High stress can delay or prevent ovulation, which pushes back the expected period date. A stressful event (exam week, grief, major life change) can delay a period by days to weeks. Once the stress resolves, the cycle usually normalizes. Chronic stress can cause ongoing cycle irregularity.

The number of periods per year depends on your cycle length: a 28-day cycle produces about 13 periods/year; a 21-day cycle produces about 17; a 35-day cycle produces about 10. The average is 11–13 periods per year. Fewer than 8 periods per year (oligomenorrhea) or more than 17 may indicate hormonal imbalance and warrants evaluation. Missing periods entirely for 3+ months (amenorrhea) should always be evaluated medically.

A period arriving 1–4 days early is within normal cycle variation. Consistently early periods (cycles becoming progressively shorter) can indicate: declining ovarian reserve (the egg supply is diminishing, common in the 30s–40s); stress; thyroid issues; or PCOS. Spotting that you mistake for an early period could be implantation bleeding (a sign of early pregnancy), mid-cycle spotting around ovulation, or a cervical issue. If your period consistently comes significantly earlier than expected, consult your OB/GYN.

Hormonal contraceptives significantly alter the menstrual cycle. Combined oral pills typically produce a withdrawal bleed during the placebo week, which is not a true period — it's lighter and more predictable. Progestin-only pills (minipill), hormonal IUDs, and implants often cause irregular spotting or stop periods entirely. After stopping hormonal birth control, cycles usually normalize within 1–3 months, though it can take 6 months or longer. The return of natural cycle variation is normal and expected after stopping hormonal contraception.

See a healthcare provider if: periods are consistently less than 21 or more than 35 days apart; you miss 3 or more consecutive periods (not due to pregnancy); periods are extremely heavy (soaking a pad/tampon every hour for 2+ hours); severe cramping that interferes with daily activities; periods that last longer than 7 days; spotting between periods or after sex; or any sudden significant change in your cycle pattern. These may indicate conditions like PCOS, endometriosis, fibroids, or thyroid dysfunction that benefit from early diagnosis and treatment.

Period prediction calculators are estimates within a 2–5 day window for women with regular cycles. Accuracy decreases with irregular cycles. A 2019 study found that even sophisticated app predictions were off by an average of 2–3 days. Predictions improve significantly when based on multiple cycles of tracked data rather than a single cycle input. Period tracking apps that use your actual logged data consistently outperform single-input calculators. For health decisions, always allow a ±5 day window around any predicted date.