Period Calculator
- Updated
- Formula shown
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Results
Next period expected Oct 29 – Nov 2, 2026; fertile window this cycle Oct 10–15, 2026.
| Period | Starts | Ends | Fertile window after it | Ovulation |
|---|---|---|---|---|
| 1 | Thu, Oct 29, 2026 | Nov 2, 2026 | Nov 7–12, 2026 | Nov 12, 2026 |
| 2 | Thu, Nov 26, 2026 | Nov 30, 2026 | Dec 5–10, 2026 | Dec 10, 2026 |
| 3 | Thu, Dec 24, 2026 | Dec 28, 2026 | Jan 2–7, 2027 | Jan 7, 2027 |
| 4 | Thu, Jan 21, 2027 | Jan 25, 2027 | Jan 30 – Feb 4, 2027 | Feb 4, 2027 |
| 5 | Thu, Feb 18, 2027 | Feb 22, 2027 | Feb 27 – Mar 4, 2027 | Mar 4, 2027 |
| 6 | Thu, Mar 18, 2027 | Mar 22, 2027 | Mar 27 – Apr 1, 2027 | Apr 1, 2027 |
| 7 | Thu, Apr 15, 2027 | Apr 19, 2027 | Apr 24–29, 2027 | Apr 29, 2027 |
| 8 | Thu, May 13, 2027 | May 17, 2027 | May 22–27, 2027 | May 27, 2027 |
| 9 | Thu, Jun 10, 2027 | Jun 14, 2027 | Jun 19–24, 2027 | Jun 24, 2027 |
| 10 | Thu, Jul 8, 2027 | Jul 12, 2027 | Jul 17–22, 2027 | Jul 22, 2027 |
| 11 | Thu, Aug 5, 2027 | Aug 9, 2027 | Aug 14–19, 2027 | Aug 19, 2027 |
| 12 | Thu, Sep 2, 2027 | Sep 6, 2027 | Sep 11–16, 2027 | Sep 16, 2027 |
Predictions assume every cycle is the same length; real cycles vary by a few days. See a doctor if your periods are often less than 21 or more than 35 days apart, last more than 7 days, vary in length by more than 7 to 9 days, stop for 3 to 6 months, are very heavy, or if you bleed between periods or after sex (ACOG, NHS).
Do not use these dates as contraception. Ovulation can come earlier or later than predicted, even in regular cycles, and sperm can survive in the body for several days.
These dates are estimates from an average cycle, not medical advice. Talk to your doctor or midwife about your own cycle, fertility or pregnancy.
How to use the period calculator
- Enter the first day of your last period: the first day of proper bleeding, not spotting beforehand.
- Enter your usual cycle length (from the first day of one period to the first day of the next) and how many days you usually bleed.
- Read your next period dates and scroll the table for the next 12. If your cycle varies, use your average and allow a few days either side.
Period formula
period ends = start + period length − 1 day
ovulation ≈ next period start − 14 days
fertile window = ovulation − 5 days to ovulation
Each period is predicted by adding your cycle length again and again, so the predictions are only as regular as your cycle. Ovulation usually happens about two weeks before the next period (the NHS gives 12 to 16 days), and the fertile window is the five days before it plus the day itself, the days on which sex can lead to pregnancy.
Worked example
Your last period started on Oct 1, 2026, your cycle is 28 days and you usually bleed for 5 days. The next period should start on Thursday, October 29, 2026 and end on Nov 2, 2026. The fertile window in the current cycle is Oct 10–15, 2026. Twelve cycles on, the 12th period is due on Sep 2, 2027: with a 28-day cycle there are about 13 periods a year, so one month will have two.
Next periods by cycle length
Predicted start dates after a period on Oct 1, 2026, and about how many periods that cycle length gives in a year.
| Cycle length | Next period | Second | Third | Periods a year |
|---|---|---|---|---|
| 21 days | Oct 22, 2026 | Nov 12, 2026 | Dec 3, 2026 | 17.4 |
| 24 days | Oct 25, 2026 | Nov 18, 2026 | Dec 12, 2026 | 15.2 |
| 28 days | Oct 29, 2026 | Nov 26, 2026 | Dec 24, 2026 | 13 |
| 30 days | Oct 31, 2026 | Nov 30, 2026 | Dec 30, 2026 | 12.2 |
| 32 days | Nov 2, 2026 | Dec 4, 2026 | Jan 5, 2027 | 11.4 |
| 35 days | Nov 5, 2026 | Dec 10, 2026 | Jan 14, 2027 | 10.4 |
What is a normal cycle?
ACOG says a normal menstrual cycle is typically 21 to 35 days long and a normal period lasts up to 7 days. The NHS describes periods as usually coming every 21 to 35 days and lasting 2 to 7 days, about 5 on average. Cycles are often irregular in the first years after periods start and in the years before menopause.
- See a doctor if your periods are often less than 21 or more than 35 days apart, last more than 7 days, vary by more than 7 to 9 days from one cycle to the next, or stop for 3 to 6 months.
- Also get checked for very heavy bleeding (soaking a pad or tampon every hour), bleeding between periods or after sex, or any bleeding after menopause.
- A late period can have many causes, including pregnancy, stress, illness and weight change. If you could be pregnant, take a pregnancy test.
A period calendar is not contraception: ovulation can happen earlier than predicted and sperm can survive for several days. This is not medical advice; talk to your doctor or midwife. For a closer look at fertile days, use the ovulation calculator.
Sources
- ACOG FAQ, Abnormal Uterine Bleeding.
- NHS, Periods (reviewed January 2023) and Irregular periods (reviewed June 2026).
- Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333(23):1517–1521.
Frequently asked questions
How do I calculate when my next period is due?
Add your cycle length to the first day of your last period. If your last period started on October 1, 2026 and your cycle is 28 days, the next should start on October 29 and, if it usually lasts 5 days, end on November 2.
What is a normal cycle length?
ACOG says a normal menstrual cycle is typically 21 to 35 days, counted from the first day of one period to the first day of the next, and a normal period lasts up to 7 days. The NHS gives 28 days as the average.
When should I see a doctor about my periods?
If your periods are often less than 21 or more than 35 days apart, last more than 7 days, vary in length by more than 7 to 9 days, stop for 3 to 6 months, or are very heavy, or if you bleed between periods, after sex or after menopause.
Can I use a period calculator to avoid pregnancy?
No. The dates assume every cycle is the same length, but ovulation can come earlier or later and sperm can survive for several days, so pregnancy is possible on days the calendar marks as safe. Ask a doctor or nurse about reliable contraception.
Why do I sometimes get two periods in one month?
A cycle shorter than the month means a period can start near the beginning and again near the end. With a 28-day cycle you get about 13 periods a year, so at least one calendar month has two.