Period Calculator
Predict Your Next 12 Periods
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Enter your last period date to predict the next 12 periods.
Next Period
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Cycle Length
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Period Duration
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Periods per Year
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Cycle Type
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Understanding Your Menstrual Cycle
Your next period should start one cycle length after the first day of your last period. With a 28-day cycle, a period that began on September 1, 2026 points to September 29, 2026, then October 27. Enter your last start date, cycle length and period length above to see the next 12 periods, and treat each date as the middle of a window of a few days.
The average menstrual cycle is 28 days, but a normal cycle can range from 21 to 35 days. The cycle begins on the first day of your period (day 1) and ends the day before your next period starts. Period duration typically lasts 3 to 7 days.
Cycle length can vary naturally from month to month by 2 to 7 days due to stress, illness, travel, changes in sleep, or exercise. Tracking your cycle over several months reveals your personal pattern and helps identify what's normal for you.
Short Cycles (21 to 24 days)
Cycles under 24 days may mean ovulation occurs early: sometimes while bleeding is still occurring. Short cycles can also indicate perimenopause in women over 40, or hormonal imbalances. Always rule out medical causes if your cycle suddenly shortens.
Long Cycles (35+ days)
Cycles over 35 days are considered irregular and may indicate PCOS, thyroid disorders, or elevated prolactin. Occasional long cycles due to stress or illness are normal. If consistently over 35 days, consult a healthcare provider.
Why Periods Are Irregular
Common causes: stress, significant weight change, intense exercise, hormonal birth control changes, thyroid issues, PCOS, or perimenopause. Occasional variation of a few days is normal. Consistent irregularity warrants medical evaluation.
When Is a Period Late?
A period is considered late if it hasn't started within 5 days of its expected date. A missed period (7+ days late) may indicate pregnancy, significant stress, hormonal changes, or medical conditions. Take a pregnancy test if sexually active and more than 5 days late.
Next 12 Periods by Cycle Length
Predicted start dates for a last period that began on September 1, 2026. Each date is simply the previous one plus the cycle length.
| Period | 26-day cycle | 28-day cycle | 30-day cycle | 32-day cycle |
|---|---|---|---|---|
| 1 | Sep 27, 2026 | Sep 29, 2026 | Oct 1, 2026 | Oct 3, 2026 |
| 2 | Oct 23, 2026 | Oct 27, 2026 | Oct 31, 2026 | Nov 4, 2026 |
| 3 | Nov 18, 2026 | Nov 24, 2026 | Nov 30, 2026 | Dec 6, 2026 |
| 4 | Dec 14, 2026 | Dec 22, 2026 | Dec 30, 2026 | Jan 7, 2027 |
| 5 | Jan 9, 2027 | Jan 19, 2027 | Jan 29, 2027 | Feb 8, 2027 |
| 6 | Feb 4, 2027 | Feb 16, 2027 | Feb 28, 2027 | Mar 12, 2027 |
| 7 | Mar 2, 2027 | Mar 16, 2027 | Mar 30, 2027 | Apr 13, 2027 |
| 8 | Mar 28, 2027 | Apr 13, 2027 | Apr 29, 2027 | May 15, 2027 |
| 9 | Apr 23, 2027 | May 11, 2027 | May 29, 2027 | Jun 16, 2027 |
| 10 | May 19, 2027 | Jun 8, 2027 | Jun 28, 2027 | Jul 18, 2027 |
| 11 | Jun 14, 2027 | Jul 6, 2027 | Jul 28, 2027 | Aug 19, 2027 |
| 12 | Jul 10, 2027 | Aug 3, 2027 | Aug 27, 2027 | Sep 20, 2027 |
Small errors add up. If your real average is 29 days and you enter 28, the twelfth prediction will be 12 days early, so re-enter your latest start date each month rather than relying on a year-old forecast.
Periods per Year by Cycle Length
| Cycle length | Periods per year |
|---|---|
| 21 days | 17.4 |
| 24 days | 15.2 |
| 26 days | 14.0 |
| 28 days | 13.0 |
| 30 days | 12.2 |
| 32 days | 11.4 |
| 35 days | 10.4 |
Calculated as 365 divided by the cycle length, the same way the calculator does it.
How to Find Your Average Cycle
Cycle length is counted from the first day of one period to the day before the next one starts. Averaging several cycles gives a much better prediction than using one.
- Write down the start dates of your last 4 to 6 periods.
- Count the days between each pair. Example: 27, 30, 29 and 28 days.
- Add them and divide by the number of cycles: 114 divided by 4 is 28.5 days. Enter 28 or 29 in the calculator.
- Note your shortest and longest cycle (here 27 and 30). Your next period will most likely start somewhere in that range after your last start date.
What counts as normal
For adults, cycles of 21 to 35 days with bleeding for up to 7 days are the usual range given by ACOG and the Office on Women's Health. In the first few years after periods begin, cycles of 21 to 45 days are normal. Talk to a doctor if your cycles fall outside these ranges, change suddenly, or if you go 3 months without a period and are not pregnant.
To see your fertile days in each cycle, use the ovulation calculator.
Method and sources. Next period = first day of last period + cycle length, repeated for 12 cycles; periods per year = 365 ÷ cycle length. Table dates were computed with the same day count as the calculator. Sources: ACOG patient guidance and Committee Opinion 651 (menstruation in adolescents); Office on Women's Health (womenshealth.gov); Bull and colleagues, npj Digital Medicine 2019 (about 600,000 cycles); CDC contraception effectiveness figures. Predictions are estimates, not medical advice.
Frequently Asked Questions
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 January 31. 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 to 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 to 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 to 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 to 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 to 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 to 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 to 5 day window for women with regular cycles. Accuracy decreases with irregular cycles. 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 Β±5 day window around any predicted date.
Note the first day of each period for at least 3 to 6 months, count the days between consecutive start dates, then divide the total by the number of cycles. Cycles of 27, 30, 29 and 28 days average 28.5 days. Enter the nearest whole number in the calculator, and keep an eye on your shortest and longest cycle, because your next period can arrive anywhere in that range.
No. A period calculator predicts dates from an average and cannot see when you actually ovulate, which can shift from cycle to cycle. Even fertility awareness methods that track temperature and cervical mucus have a typical-use failure rate of 2% to 23% a year according to the CDC. If you want to avoid pregnancy, use a reliable method of contraception and talk to your doctor about options.