Period Calculator
Enter the first day of your last period, your average cycle length and how many days your period lasts to see estimated dates for your upcoming periods.
Last updated November 2, 2026
From the first day of one period to the first day of the next
Result
Next period expected: November 26, 2026
| Measure | Value |
|---|---|
| Next period starts | Thursday, November 26, 2026 |
| Next period ends | November 30, 2026 |
| Period | Start | End |
|---|---|---|
| Period 1 | November 26, 2026 | November 30, 2026 |
| Period 2 | December 24, 2026 | December 28, 2026 |
| Period 3 | January 21, 2027 | January 25, 2027 |
| Period 4 | February 18, 2027 | February 22, 2027 |
| Period 5 | March 18, 2027 | March 22, 2027 |
| Period 6 | April 15, 2027 | April 19, 2027 |
Dates repeat your average cycle length from the first day of your last period. A late or unusually irregular period can have many causes, so talk to a health professional if you are worried. These dates are estimates. Cycles and pregnancies vary from person to person, and this tool cannot confirm a pregnancy or replace care from a health professional.
Show the work
Formula: Next period start = last period + cycle length; end = start + period length - 1 day.
- Last period starts 2026-10-29; cycle 28 days; period lasts 5 days.
- Next start: 2026-10-29 + 28 days = 2026-11-26.
- Next end: 2026-11-26 + 4 days = 2026-11-30.
- Each later period adds another 28 days (period 6 starts 2027-04-15).
Result: Next period expected: November 26, 2026
How period dates are estimated
next period start = last period start + cycle length
end = start + period length − 1
Each upcoming period is dated by adding your average cycle length to the previous start date. With a 28-day cycle and a first day of January 1, periods are expected on January 29, February 26 and March 26, each lasting the number of days you enter (5 by default).
Many people have cycles that vary by a few days from month to month, so real dates can drift from this schedule. Cycles outside about 21 to 35 days, missed periods and very heavy or painful periods are worth discussing with a health professional.
See how we calculate for the formulas, sources and review schedule behind every calculator.
This is an estimate, not medical advice
The dates on this page are estimates. Cycle length and pregnancy length vary from person to person and from month to month, so real dates can differ by days or weeks. The tool cannot confirm a pregnancy, ovulation or fertility, is not a method of birth control, and does not replace care from a doctor or midwife. This page has not been reviewed by a medical professional.
Sources
Official and primary sources for the statements on this page. Their authors have not reviewed or endorsed this page.
- Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. J Obstet Gynecol Neonatal Nurs 2006;35(3):376-384. In 141 regularly cycling women, cycle length averaged 28.9 days (SD 3.4) with 95% of cycles between 22 and 36 days, so real dates differ from a fixed-length estimate
- American College of Obstetricians and Gynecologists, Committee Opinion No. 651: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign, Obstetrics & Gynecology 2015;126(6):e143-e146. Context only: treats the menstrual cycle as a vital sign and says normal cycle length should be explained to patients. The abstract gives no numeric range, so this page’s 21 to 45 day input limit is not taken from it
Future periods here are the last start date plus your cycle length, repeated. Real cycles vary from month to month, and a late or missed period is a reason to ask a doctor, not to trust this estimate.
How to use the Period Calculator
- Enter the first day of your last period.
- Enter your average cycle length and period length in days.
- Choose how many periods to show and read the dates, then use Copy, Print or Share.
Worked example
Last period began January 1, 2026, with a 28-day cycle and a 5-day period. The next periods are estimated as follows.
| Period | Start | End |
|---|---|---|
| Period 1 | January 29, 2026 | February 2, 2026 |
| Period 2 | February 26, 2026 | March 2, 2026 |
| Period 3 | March 26, 2026 | March 30, 2026 |
Frequently asked questions
How do I count my cycle length?
Count from the first day of one period to the day before the first day of the next. Use the average of a few cycles.
Why is my period late?
Stress, illness, travel, hormones and pregnancy are some of many possible causes. Ask a health professional if you are concerned.
Is a 28-day cycle normal?
28 days is a common average, but cycles from about 21 to 35 days are common, and they can vary month to month.
Does this predict ovulation too?
No. Use the Ovulation Calculator for the estimated fertile window.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser and nothing you enter is saved or uploaded.