Answer first
What this calculator tells you
Predict the next three period start dates from the last period and your cycle length. Plan around the expected dates, knowing that real cycles can vary by 2 to 3 days from month to month. Formula: Next period = first day of last period + cycle length; each following period adds one more cycle length. At the worked-example inputs, the next period is October 8, 2026. Holding every other input steady, moving first day of your most recent period from August 25, 2026 to September 22, 2026 moves the result from September 24, 2026 to October 22, 2026.
Transparent method
The formula
Plan around the expected dates, knowing that real cycles can vary by 2 to 3 days from month to month.
Worked example
Example inputs
How to interpret the result
A period predictor adds the cycle length to the first day of the last period, over and over. With a 28 day cycle and a period that started on September 1, the next starts on September 29, then October 27 and November 24. Real cycles drift by a few days, so each further step is less certain than the one before. The first estimate is the useful one, and the third is a rough guide. A shorter cycle also means more periods in a year: a 21 day cycle gives about 17, a 28 day cycle about 13 and a 35 day cycle about 10. Counting from the first day of one period to the first day of the next keeps the length consistent from month to month.
This is a calendar estimate from the numbers you enter, not a medical test, and it is not a method of birth control. Cycles vary from month to month, so the real date can land days away from the estimate. A clinician can confirm timing with a test or an ultrasound.
Before you rely on it
What to check
Use your own average cycle length from several months. A single unusual cycle makes the whole series shift.
The common error
Where people go wrong with period calculator
Trusting the dates as fixed. Stress, illness, travel and hormonal changes move a period by days, and the further ahead the date, the less reliable it is.
Sensitivity evidence
How first day of your most recent period changes the next period
Holding every other input at the worked-example value, moving first day of your most recent period from August 25, 2026 to September 22, 2026 moves the next period from September 24, 2026 to October 22, 2026: a spread of 28.0 days.
| First day of your most recent period | Next period | The period after that | Two periods ahead |
|---|---|---|---|
| August 25, 2026 | September 24, 2026 | October 24, 2026 | November 23, 2026 |
| September 1, 2026 | October 1, 2026 | October 31, 2026 | November 30, 2026 |
| September 8, 2026worked example | October 8, 2026 | November 7, 2026 | December 7, 2026 |
| September 15, 2026 | October 15, 2026 | November 14, 2026 | December 14, 2026 |
| September 22, 2026 | October 22, 2026 | November 21, 2026 | December 21, 2026 |
Two variables at once
Next period by first day of your most recent period and typical cycle length (days)
Across the grid the next period runs from September 18, 2026 to October 28, 2026. Moving first day of your most recent period from August 25, 2026 to September 22, 2026 shifts it by January 29, 1970 at the middle column, and moving typical cycle length (days) from 24 to 36 shifts it by January 13, 1970 at the middle row, so first day of your most recent period is the bigger lever here.
| First day of your most recent period \ Typical cycle length (days) | 24 | 30 | 36 |
|---|---|---|---|
| August 25, 2026 | September 18, 2026 | September 24, 2026 | September 30, 2026 |
| September 1, 2026 | September 25, 2026 | October 1, 2026 | October 7, 2026 |
| September 8, 2026 | October 2, 2026 | October 8, 2026 | October 14, 2026 |
| September 15, 2026 | October 9, 2026 | October 15, 2026 | October 21, 2026 |
| September 22, 2026 | October 16, 2026 | October 22, 2026 | October 28, 2026 |
The highlighted cell is the worked example: October 8, 2026.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| First day of your most recent period | September 8, 2026 | Enter the first day of your most recent period used in this calculation. |
| Typical cycle length (days) | 30 | Days from the first day of one period to the first day of the next. The typical range is 21 to 35. |
| Next period | October 8, 2026 | |
| The period after that | November 7, 2026 | |
| Two periods ahead | December 7, 2026 | |
Inputs, definitions and assumptions
First day of your most recent period
Enter the first day of your most recent period used in this calculation. The prefilled worked-example value is September 8, 2026.
Typical cycle length (days)
Days from the first day of one period to the first day of the next. The typical range is 21 to 35. The prefilled worked-example value is 30.
How to use this calculator
- 1Verify the inputs. Gather first day of your most recent period and typical cycle length (days) from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the next period at October 8, 2026. Store your own version of it as Scenario A.
- 3Test one change. Change one assumption at a time so the driver stays visible.
- 4Check the boundary. Read the interpretation boundary above before acting on the result.
People also ask
Frequently asked questions
How do you calculate period?
Next period = first day of last period + cycle length; each following period adds one more cycle length. At the worked-example inputs the next period is October 8, 2026.
What does the period result mean?
Plan around the expected dates, knowing that real cycles can vary by 2 to 3 days from month to month.
How much does first day of your most recent period change the next period?
Holding every other input at the worked-example value, moving first day of your most recent period from August 25, 2026 to September 22, 2026 moves the next period from September 24, 2026 to October 22, 2026, a spread of 28.0 days.
What are the limits of this period calculator?
This is a calendar estimate from the numbers you enter, not a medical test, and it is not a method of birth control. Cycles vary from month to month, so the real date can land days away from the estimate. A clinician can confirm timing with a test or an ultrasound. The tables on this page test first day of your most recent period only from August 25, 2026 to September 22, 2026; a value outside that range is not tabulated here.
Can a calculator replace a lab test or a doctor's visit?
No. It gives an educational estimate from the numbers you enter. Only a clinician can read them alongside your history, symptoms and other tests.
Why can a calculated value differ from what my lab reports?
A lab may measure directly where a formula estimates, use a different method or reference range, or round differently. When the two disagree, go with the lab and ask your clinician.
What is a normal range?
A range that most healthy people fall inside, usually the middle 95 percent of a reference group. Sitting just outside it is not automatically a problem, and sitting inside it is not a guarantee.
Is an estimated due date reliable?
Only a small share of babies arrive on the estimated date. Most arrive within a couple of weeks either side, so the date is a midpoint to plan around, not a deadline.
Is the fertile window the same every month?
No. Ovulation timing moves with stress, illness, travel and cycle length, so the window shifts from month to month, even for people with regular cycles.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.