Period Calculator
Use this period calculator to predict your next period dates from your last period and average cycle length.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Predicted Dates Are Calculated
This calculator projects your upcoming period start dates forward from the most recent cycle start date you’ve entered, using either a default average cycle length or one calculated from your own logged history. A large real-world study of over 600,000 cycles found an average length of 29.3 days¹, slightly longer than the traditional 28-day textbook figure, which is part of why logging your own history produces a more personal estimate than any single default number can.
What This Calculator Measures
This estimates when your next several periods are likely to start. It does not diagnose cycle irregularity, pregnancy, or any medical condition, and it cannot account for factors that shift ovulation timing month to month.
The Formula
Next start date = most recent known start date + average cycle length
How This Calculator Computes Your Result
Enter the start date of your last period. By default we project forward using a 28-day average cycle length, or the average you enter. If you log 3 or more past cycle start dates, we instead calculate your own rolling average from the gaps between your last 3 entries and use that for every projection going forward, clearly labeling the switch.
Handling Irregular Cycles
If the gaps between your logged cycles vary by more than about 7 to 8 days, we surface a gentle note rather than a confident prediction. This threshold tracks closely with the clinical definition of cycle regularity used by FIGO (the International Federation of Gynecology and Obstetrics), which defines a regular cycle as varying by no more than about 7 to 9 days between your shortest and longest cycle, depending on age². Larger swings meaningfully reduce forecast accuracy and may be worth discussing with a provider.
Why Cycle Length Isn’t Fixed
The same large-scale study found average cycle length decreases gradually with age, by roughly 0.18 days per year between ages 25 and 45, and that cycle-to-cycle variability differs meaningfully between individuals even when each person’s own cycles are broadly regular¹. This is part of why a personal rolling average, built from your own logged history, tends to outperform a single fixed default the longer you track.
A Note on Accuracy
Normal menstrual cycles typically range from about 21 to 35 days³ and can vary month to month even when generally regular. This tool projects forward using simple averages, it does not diagnose irregular cycles or any underlying condition.
Frequently Asked Questions
How does this calculator predict my next period?
It takes the start date of your most recent period and adds your average cycle length to project when your next period is likely to start, then repeats that forward for several cycles.
Why did the calculator switch away from the 28-day default?
Once you've logged 3 or more cycle start dates, the calculator quietly switches from the generic 28-day default to your own rolling average, calculated from the gaps between your last 3 logged cycles, and labels the result accordingly. Real-world data shows the average across a large population is closer to 29.3 days¹, but your own personal average is a better predictor of your own cycles than any population figure.
What if my cycle lengths vary a lot between entries?
If your logged cycle lengths differ by more than about 7 to 8 days from each other, close to the clinical threshold for regularity², the calculator shows a gentle note that irregularity reduces prediction accuracy, which may be worth mentioning to a healthcare provider.
Is this accurate if I have PCOS, perimenopause, or recently changed birth control?
No, not reliably. Irregular or absent cycles from conditions like PCOS or perimenopause, or recent changes to hormonal contraceptives, make cycle-based predictions much less reliable. Treat every result here as a rough estimate only.
Sources
Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles, Bull et al., 2019
The Two FIGO Systems for Normal and Abnormal Uterine Bleeding Symptoms and Classification of Causes of Abnormal Uterine Bleeding in the Reproductive Years: 2018 Revisions, Munro et al., 2018
Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign, ACOG Committee Opinion No. 651, 2015
Period Calculator
Use this period calculator to predict your next period dates from your last period and average cycle length.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Predicted Dates Are Calculated
This calculator projects your upcoming period start dates forward from the most recent cycle start date you’ve entered, using either a default average cycle length or one calculated from your own logged history. A large real-world study of over 600,000 cycles found an average length of 29.3 days¹, slightly longer than the traditional 28-day textbook figure, which is part of why logging your own history produces a more personal estimate than any single default number can.
What This Calculator Measures
This estimates when your next several periods are likely to start. It does not diagnose cycle irregularity, pregnancy, or any medical condition, and it cannot account for factors that shift ovulation timing month to month.
The Formula
Next start date = most recent known start date + average cycle length
How This Calculator Computes Your Result
Enter the start date of your last period. By default we project forward using a 28-day average cycle length, or the average you enter. If you log 3 or more past cycle start dates, we instead calculate your own rolling average from the gaps between your last 3 entries and use that for every projection going forward, clearly labeling the switch.
Handling Irregular Cycles
If the gaps between your logged cycles vary by more than about 7 to 8 days, we surface a gentle note rather than a confident prediction. This threshold tracks closely with the clinical definition of cycle regularity used by FIGO (the International Federation of Gynecology and Obstetrics), which defines a regular cycle as varying by no more than about 7 to 9 days between your shortest and longest cycle, depending on age². Larger swings meaningfully reduce forecast accuracy and may be worth discussing with a provider.
Why Cycle Length Isn’t Fixed
The same large-scale study found average cycle length decreases gradually with age, by roughly 0.18 days per year between ages 25 and 45, and that cycle-to-cycle variability differs meaningfully between individuals even when each person’s own cycles are broadly regular¹. This is part of why a personal rolling average, built from your own logged history, tends to outperform a single fixed default the longer you track.
A Note on Accuracy
Normal menstrual cycles typically range from about 21 to 35 days³ and can vary month to month even when generally regular. This tool projects forward using simple averages, it does not diagnose irregular cycles or any underlying condition.
Frequently Asked Questions
How does this calculator predict my next period?
It takes the start date of your most recent period and adds your average cycle length to project when your next period is likely to start, then repeats that forward for several cycles.
Why did the calculator switch away from the 28-day default?
Once you've logged 3 or more cycle start dates, the calculator quietly switches from the generic 28-day default to your own rolling average, calculated from the gaps between your last 3 logged cycles, and labels the result accordingly. Real-world data shows the average across a large population is closer to 29.3 days¹, but your own personal average is a better predictor of your own cycles than any population figure.
What if my cycle lengths vary a lot between entries?
If your logged cycle lengths differ by more than about 7 to 8 days from each other, close to the clinical threshold for regularity², the calculator shows a gentle note that irregularity reduces prediction accuracy, which may be worth mentioning to a healthcare provider.
Is this accurate if I have PCOS, perimenopause, or recently changed birth control?
No, not reliably. Irregular or absent cycles from conditions like PCOS or perimenopause, or recent changes to hormonal contraceptives, make cycle-based predictions much less reliable. Treat every result here as a rough estimate only.
Sources
Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles, Bull et al., 2019
The Two FIGO Systems for Normal and Abnormal Uterine Bleeding Symptoms and Classification of Causes of Abnormal Uterine Bleeding in the Reproductive Years: 2018 Revisions, Munro et al., 2018
Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign, ACOG Committee Opinion No. 651, 2015
Period Calculator
Use this period calculator to predict your next period dates from your last period and average cycle length.
Calculated instantly in your browser — nothing is sent to or stored on our servers.
Kimberly Langdon, MD
Kimberly Langdon, MD, is a retired, board-certified obstetrician/gynecologist with 19 years of clinical practice, trained at The Ohio State University.
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Predicted Dates Are Calculated
This calculator projects your upcoming period start dates forward from the most recent cycle start date you’ve entered, using either a default average cycle length or one calculated from your own logged history. A large real-world study of over 600,000 cycles found an average length of 29.3 days¹, slightly longer than the traditional 28-day textbook figure, which is part of why logging your own history produces a more personal estimate than any single default number can.
What This Calculator Measures
This estimates when your next several periods are likely to start. It does not diagnose cycle irregularity, pregnancy, or any medical condition, and it cannot account for factors that shift ovulation timing month to month.
The Formula
Next start date = most recent known start date + average cycle length
How This Calculator Computes Your Result
Enter the start date of your last period. By default we project forward using a 28-day average cycle length, or the average you enter. If you log 3 or more past cycle start dates, we instead calculate your own rolling average from the gaps between your last 3 entries and use that for every projection going forward, clearly labeling the switch.
Handling Irregular Cycles
If the gaps between your logged cycles vary by more than about 7 to 8 days, we surface a gentle note rather than a confident prediction. This threshold tracks closely with the clinical definition of cycle regularity used by FIGO (the International Federation of Gynecology and Obstetrics), which defines a regular cycle as varying by no more than about 7 to 9 days between your shortest and longest cycle, depending on age². Larger swings meaningfully reduce forecast accuracy and may be worth discussing with a provider.
Why Cycle Length Isn’t Fixed
The same large-scale study found average cycle length decreases gradually with age, by roughly 0.18 days per year between ages 25 and 45, and that cycle-to-cycle variability differs meaningfully between individuals even when each person’s own cycles are broadly regular¹. This is part of why a personal rolling average, built from your own logged history, tends to outperform a single fixed default the longer you track.
A Note on Accuracy
Normal menstrual cycles typically range from about 21 to 35 days³ and can vary month to month even when generally regular. This tool projects forward using simple averages, it does not diagnose irregular cycles or any underlying condition.
Frequently Asked Questions
How does this calculator predict my next period?
It takes the start date of your most recent period and adds your average cycle length to project when your next period is likely to start, then repeats that forward for several cycles.
Why did the calculator switch away from the 28-day default?
Once you've logged 3 or more cycle start dates, the calculator quietly switches from the generic 28-day default to your own rolling average, calculated from the gaps between your last 3 logged cycles, and labels the result accordingly. Real-world data shows the average across a large population is closer to 29.3 days¹, but your own personal average is a better predictor of your own cycles than any population figure.
What if my cycle lengths vary a lot between entries?
If your logged cycle lengths differ by more than about 7 to 8 days from each other, close to the clinical threshold for regularity², the calculator shows a gentle note that irregularity reduces prediction accuracy, which may be worth mentioning to a healthcare provider.
Is this accurate if I have PCOS, perimenopause, or recently changed birth control?
No, not reliably. Irregular or absent cycles from conditions like PCOS or perimenopause, or recent changes to hormonal contraceptives, make cycle-based predictions much less reliable. Treat every result here as a rough estimate only.
Sources
Real-World Menstrual Cycle Characteristics of More Than 600,000 Menstrual Cycles, Bull et al., 2019
The Two FIGO Systems for Normal and Abnormal Uterine Bleeding Symptoms and Classification of Causes of Abnormal Uterine Bleeding in the Reproductive Years: 2018 Revisions, Munro et al., 2018
Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign, ACOG Committee Opinion No. 651, 2015