Period Calculator guide
Estimate when your next period will start, how long it will last, and when you will likely ovulate, for the next six cycles. Add past cycle lengths for a more realistic range.
How the next period is predicted
A menstrual cycle is counted from the first day of one period (day 1, the first day of real flow, not spotting) to the day before the next period starts. The calculator adds your cycle length to the first day of your last period: next period = last period start + cycle length. It then repeats that for six cycles and marks each period's expected last day using your period length.
If you type in your last few cycle lengths, the calculator averages them and also shows a likely range from your shortest to your longest recent cycle. That range is more honest than a single date, because very few people run like a clock. Cycles that vary by a few days month to month are normal.
Ovulation is estimated as 14 days before the next period. The phase after ovulation (luteal phase) is fairly steady at around 12-14 days for most people, while the phase before it varies. That is why a 32-day cycle means later ovulation, around day 18, not day 14.
What counts as a normal cycle
According to ACOG, the average cycle is 28 days, and a normal cycle for adults runs 21 to 35 days. In the first few years after periods start, teens can have cycles from 21 up to 45 days. A normal period lasts up to 7 days.
The calculator flags cycles shorter than 21 days, longer than 35 days, and periods longer than 7 days. That flag is not a diagnosis. It is a prompt that the pattern is outside the typical range and worth mentioning to a clinician, because things like thyroid problems, PCOS, and perimenopause often show up first as cycle changes.
Worked example
Your last period started September 15, 2026. Your last four cycles were 27, 30, 29, and 28 days. Average = (27 + 30 + 29 + 28) ÷ 4 = 28.5, rounded to 29 days. Next period ≈ September 15 + 29 days = Wednesday, October 14. Likely range: September 15 + 27 = October 12 to September 15 + 30 = October 15. Estimated ovulation: October 14 − 14 = September 30.
With a period length of 5 days, the October period is expected October 14-18, and the following one around November 12.
Common mistakes
Counting from the last day of your period. Always count from the first day of full flow.
Using one unusual month. Stress, illness, travel, big training changes, and new medication can shift a single cycle. Averaging three to six cycles smooths that out.
Using it as birth control. Calendar predictions can be off by several days and sperm survive up to 5 days. This is not contraception. Use a proven method if you do not want to get pregnant.
Trusting predictions on hormonal birth control. Pills, patches, rings, and hormonal IUDs produce withdrawal bleeds or suppress periods. A cycle calculator does not model those.
Ignoring a late period. If you are sexually active and your period is more than a few days past the latest date in your range, take a pregnancy test.
Assuming the same pattern after childbirth or during perimenopause. Cycles are often irregular while breastfeeding and in your 40s. Predictions are rough during those stages.
When to see a clinician
Make an appointment if cycles are regularly under 21 or over 35 days, if you miss three or more periods in a row and are not pregnant, if a period lasts longer than 7 days, if you soak through a pad or tampon every hour for several hours, if you bleed between periods or after sex, or if pain stops you from normal activities. Seek urgent care for very heavy bleeding with dizziness or fainting.
Tracking pays off here: bring your list of start dates and period lengths to the visit. It is the first thing a gynecologist will ask for. For planning a pregnancy, the ovulation calculator shows the fertile window for your next three cycles.
How we calculate: sources
Frequently asked questions
How do I calculate my next period?
Add your average cycle length to the first day of your last period. With a 28-day cycle and a last period on the 1st, the next one is expected on the 29th.
What is a normal cycle length?
For adults, 21 to 35 days, with 28 days the average. Teens can have cycles up to 45 days in the first few years after periods start. A normal period lasts up to 7 days.
How accurate is a period calculator?
It is most accurate for regular cycles. Many people vary by a few days month to month, which is why entering past cycle lengths shows a likely range rather than one date.
How do I count my cycle length?
Count from the first day of one period (day 1) up to the day before your next period starts. Spotting before full flow does not count as day 1.
Can I use a period calculator as birth control?
No. Calendar predictions can be off by days, ovulation varies, and sperm can survive up to 5 days. Use a proven contraceptive method if you want to avoid pregnancy.
When should I see a doctor about my period?
If cycles are regularly shorter than 21 or longer than 35 days, a period lasts over 7 days, you miss 3 or more periods in a row, you bleed between periods, or bleeding is very heavy.
Is my data uploaded?
Everything runs in your browser. Nothing you enter is uploaded to a server or stored by us.
Why is my period late?
Common reasons include pregnancy, stress, illness, big changes in weight or exercise, travel, breastfeeding, some medications, thyroid problems, and PCOS. If there is any chance of pregnancy, take a test first.
How many days after my period will I ovulate?
Ovulation is usually about 14 days before your next period, not a fixed number of days after the last one. In a 28-day cycle that is around day 14; in a 32-day cycle, around day 18.