How do I calculate my menstrual cycle length?
Count the calendar days between the first day of one period and the first day of the next. January 3 to January 31 is 28 days.
Calculate your average menstrual cycle length
Cycle Length Calculator
Enter the start dates of your recent periods to find your recorded cycle lengths, average, shortest and longest cycle.
This tool describes the dates you enter. It does not diagnose a condition or determine whether your cycles are healthy.
Quick answers
What counts as day 1?
Day 1 is the first day of your period. Enter its start date, not the last day of bleeding.
How many periods give an average?
Three consecutive period starts provide two completed cycle intervals. Two starts measure just one cycle.
Why can cycle length change?
Cycle timing can vary with life stage, stress and hormonal contraception. Dates alone cannot explain why a particular cycle changed.
How we calculate your cycle length
Start with consecutive dates
Enter the most recent period first, then work backward. Add earlier starts when you have them. Do not insert an estimated date for a period you did not record.
Measure every interval
Later period start minus earlier period start equals cycle length in calendar days. The next period starts a new cycle; do not add an extra day.
Calculate the average
Add every measured interval and divide by the number of intervals. We show up to one decimal place and a separate nearest whole-day value for planning.
Describe the range
Shortest and longest are the smallest and largest recorded intervals. Observed variation is longest minus shortest. No interval is automatically removed.
Understanding cycle length
Cycle length and period length are different
Cycle length includes the days between periods, not just bleeding days. Period length describes how long bleeding lasts. This tool asks only for start dates.
A cycle does not have to be exactly 28 days
Twenty-eight days is an example, not a requirement. Age, life stage, hormonal contraception and your own history matter when interpreting a pattern. This calculator does not apply a universal healthy range.
Build a useful record
Use a consistent record of consecutive period starts. Several completed cycles give more context than one. If you missed recording a period, the gap spans missing information and cannot be treated confidently as one cycle.
Read your shortest and longest cycles
These are the extremes of the history you entered, not limits for your next cycle. Two people can have the same average and very different individual intervals. Look at the bars as well as the headline.
Changes need context
Stress, weight changes, exercise, contraception and transitions such as puberty or perimenopause may affect timing. A recent illness or other life change is useful context to discuss; this tool cannot identify a cause.
When to ask for advice
If your usual pattern changes, you have bleeding between periods, or you are concerned, speak with a healthcare professional. If pregnancy is possible after a missed period, consider a pregnancy test. Seek advice if you miss three periods in a row.
Cycle Length Calculator: Quick Facts
Your period starts
Enter the FIRST DAY each period started, most recent first. Include consecutive periods; no end dates needed.
Your recorded cycle length
Three consecutive period starts provide two completed cycle intervals. Two starts measure just one cycle.
Your average cycle length
Add every measured interval and divide by the number of intervals. We show up to one decimal place and a separate nearest whole-day value for planning.
Observed variation
Shortest and longest are the smallest and largest recorded intervals. Observed variation is longest minus shortest. No interval is automatically removed.
Recent cycle history
This tool describes the dates you enter. It does not diagnose a condition or determine whether your cycles are healthy.
Does Selin store the dates I enter here?
No. Web calculator dates and results remain in memory inside an isolated browser frame. They are not sent to the server, analytics, app stores or shared links. Logging dates in the Selin app is a separate choice.
Frequently asked questions
Do I count from the first or last day?
Use the first day each period started. End dates are not used in the calculation.
How is the average worked out?
For intervals of 27, 29 and 30 days, the total is 86. Dividing by 3 gives 28.7 days to one decimal, or about 29 days.
How many dates can I enter?
Begin with two. Add earlier starts one at a time, up to twelve dates, giving at most eleven intervals.
Is a 28-day cycle required?
No. The tool measures the dates you enter without grading or classifying them.
Why was my last cycle longer?
The dates show the size of the gap, not its cause. Keep a record and discuss an unexpected change with a healthcare professional.
What if I missed a period or a record?
Do not invent a start date. A gap may include a missed record or a missed period. The tool will show the full gap, so interpret it with that context.
Can I use this if my cycles vary?
Yes, it describes the entered intervals and their variation. It cannot diagnose irregular periods or explain variation.
Is this my next cycle length?
No. A historical average is a summary of recorded intervals, not a guaranteed future duration.
Is cycle length the same as period duration?
No. A five-day period can be part of a much longer cycle. Only period starts are needed here.
Does Selin store the dates I enter here?
No. Web calculator dates and results remain in memory inside an isolated browser frame. They are not sent to the server, analytics, app stores or shared links. Logging dates in the Selin app is a separate choice.
Medically reviewed by
Elena VasilievaGynecologist and sexologist, Selin
Reviewed:
Sources and references
Keep tracking your cycle with Selin
Log your period starts in Selin to build an ongoing cycle history and track symptoms over time.
- Log actual period starts
- Record your symptoms
- Set reminders
