FFree Calculators Online
Menu
Calculator library/Clinical Estimates

Period Calculator

The tool lists your next period and the two following it. A 30-day cycle starting from the first date predicts periods 30, 60 and 90 days later. Use it to plan travel, events and supplies, and to notice changes: a period that is consistently late, very heavy or absent may be worth discussing with a clinician.

Pregnancy, stress, illness, weight change and medications can shift timing.

Period Calculator: inputs

Calculated result
Next periodOctober 8, 2026
The period after thatNovember 7, 2026
Two periods aheadDecember 7, 2026

Period Calculator: First day of your most recent period sensitivity

Period Calculator: First day of your most recent period and Next period5 calculations vary the “First day of your most recent period” input while holding the other entered inputs constant.August 25, 2026September 24, 2026September 1, 2026October 1, 2026September 8, 2026October 8, 2026September 15, 2026October 15, 2026September 22, 2026October 22, 2026

Cycles vary by a few days from month to month even in regular people, and the further ahead the prediction, the less reliable it is. Hormonal contraception changes cycle patterns. The tool does not diagnose anything, and it should not be used to avoid pregnancy.

Irregular cycles need a different approach.

Enter the first day of your most recent period and your typical cycle length, counted from the first day of one period to the first day of the next. Adult cycles commonly run from 21 to 35 days. Use your average of the last several cycles, and remember the estimates are less accurate if your cycles are irregular.

Tracking your dates in a calendar or app improves predictions over time.

Period: worked example

First day of your most recent periodSeptember 8, 2026
Typical cycle length (days)30
Next periodOctober 8, 2026
The period after thatNovember 7, 2026
Two periods aheadDecember 7, 2026

Period: formula

Next period = first day of last period + cycle length; each following period adds one more cycle length

MedlinePlus: Menstruation

Common error: 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.

Period: changing First day of your most recent period

First day of your most recent periodNext periodThe period after thatTwo periods ahead
August 25, 2026September 24, 2026October 24, 2026November 23, 2026
September 1, 2026October 1, 2026October 31, 2026November 30, 2026
September 8, 2026 (example)October 8, 2026November 7, 2026December 7, 2026
September 15, 2026October 15, 2026November 14, 2026December 14, 2026
September 22, 2026October 22, 2026November 21, 2026December 21, 2026

Period: individual input comparisons

InputValueNext periodThe period after thatTwo periods ahead
First day of your most recent periodAugust 25, 2026September 24, 2026October 24, 2026November 23, 2026
September 1, 2026October 1, 2026October 31, 2026November 30, 2026
September 8, 2026October 8, 2026November 7, 2026December 7, 2026
September 15, 2026October 15, 2026November 14, 2026December 14, 2026
September 22, 2026October 22, 2026November 21, 2026December 21, 2026
Typical cycle length (days)24October 2, 2026October 26, 2026November 19, 2026
27October 5, 2026November 1, 2026November 28, 2026
30October 8, 2026November 7, 2026December 7, 2026
33October 11, 2026November 13, 2026December 16, 2026
36October 14, 2026November 19, 2026December 25, 2026

Period: combined input changes

First day of your most recent period / Typical cycle length (days)243036
August 25, 2026September 18, 2026September 24, 2026September 30, 2026
September 1, 2026September 25, 2026October 1, 2026October 7, 2026
September 8, 2026October 2, 2026October 8, 2026October 14, 2026
September 15, 2026October 9, 2026October 15, 2026October 21, 2026
September 22, 2026October 16, 2026October 22, 2026October 28, 2026

Period: values and definitions

Quantity and definitionValue used
First day of your most recent periodSeptember 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 35.30
Next periodOctober 8, 2026
The period after thatNovember 7, 2026
Two periods aheadDecember 7, 2026
Next period by First day of your most recent periodPeriod Calculator. Assumed First day of your most recent period values and their calculated Next period. Typical cycle length (days): 30August 25, 2026September 24, 2026September 1, 2026October 1, 2026September 8, 2026October 8, 2026September 15, 2026October 15, 2026September 22, 2026October 22, 2026
Next period by Typical cycle length (days)Period Calculator. Assumed Typical cycle length (days) values and their calculated Next period. First day of your most recent period: September 8, 202624October 2, 202627October 5, 202630October 8, 202633October 11, 202636October 14, 2026

Clinical Estimates calculators: GFR Calculator, BAC Calculator, Creatinine Clearance Calculator, Ovulation Calculator, Pregnancy Week Calculator, Cholesterol (LDL) Calculator.

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.

Use your own average cycle length from several months. A single unusual cycle makes the whole series shift.

See fertile timing on the ovulation calculator, and pregnancy dating on the due date and pregnancy week pages. The conception date calculator relates dates back to a last period. Talk to a clinician about cycles that are unusually irregular, painful or heavy.

Why is my period late?

Stress, illness, travel, weight change, hormonal changes and pregnancy are common causes. A test or a clinician visit can help if it is unusual for you.

Can I use this to prevent pregnancy?

No. Predictions are only estimates and cannot reliably prevent pregnancy, so use a proven contraceptive method if you want to avoid it.

What is a normal cycle length?

Cycle length is counted from the first day of one period to the first day of the next. It can vary between people and across time. Use an individual’s recorded history for a calendar estimate; a prediction is not a diagnosis, a confirmation of ovulation or a reliable contraceptive method.