Begin when the period begins
The first day of your period is cycle day 1. Counting does not start when bleeding ends. If you are unsure whether bleeding is your period, ask a clinician before treating it as a cycle start.
← All free tools
01 02 03
Periods
Calendar counting labHow to count menstrual cycle length: interactive calendar
Learn how to count menstrual cycle length from two period start dates. Explore numbered calendar examples, distinguish missing records, and print a retrospective count.
Use two period start dates that have already happened. The count stops on the day before the next period begins. This tool does not predict a future period or assess whether a cycle is normal.
A date and a cycle day are different numbers
See exactly which days count
Try an example, or use two recorded starts. The large calendar numbers are dates. The small numbers below them show the count.
Your recorded starts
Enter the start of one period and the start of the very next one. If a record may be missing, select the gap-only option.
Entries stay in this open page. Reloading clears them. Download or print if you want a copy.
Worked learning example
28calendar days
28 days in this completed cycle
Count January 28, 2026 as day 1. The last day counted is February 24, 2026. February 25, 2026 starts a new cycle, so it is not an extra day in the previous one.
- Start counting: day 1January 28, 2026
- Last day in this countFebruary 24, 2026
- Next cycle: new day 1February 25, 2026
First start Included in the count Next start, excluded
Small numbers are cycle days. “New 1” is the first day of the next cycle.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|
The count continues into February 2026. Both endpoint months are included in the printed example.
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|
| Mon | Tue | Wed | Thu | Fri | Sat | Sun |
|---|---|---|---|---|---|---|
All calendar months between these two starts are shown.
Second start minus first start = 28 calendar days. Include the first start; exclude the second.
A count from two dates is not an average, an ovulation estimate or a contraceptive method. The calendar does not mark bleeding days.
No current count: date entries are incomplete or have changed. Create a new count before printing.
Three counting details
Stop before the next start
The next period begins a new cycle. Including that date in the previous count would add an extra day. Days still count when the calendar changes months or years.
Keep bleeding days separate
Cycle length is the time between consecutive period starts. It is different from the number of days you bleed. Missing a recorded start also makes the gap different from a known cycle length.
Speak with a clinician if your usual period pattern changes, periods are irregular, or you have bleeding between periods. A date count cannot explain the cause or replace advice about your circumstances.
Make the most of this tool
Count menstrual cycle days from one period start to the next
Choose a worked example to see how calendar dates and cycle days differ. For your own count, enter two known period start dates and say whether they are consecutive. Review the numbered calendar, then print or download the count.
Do I include the first day of the next period in cycle length?
The next period starts a new cycle. Count the first period start as day 1 and stop on the day before the next start. For example, March 3 to March 31 is a 28-day cycle, with March 30 as day 28.
Can this calendar count a cycle if a period start is missing?
It can calculate the gap between two recorded dates, but a missing or uncertain start means the gap is not a known single-cycle length. Select the gap-only option. The tool does not guess missing dates or predict your next period.
Behind this tool
Research checked October 9, 2026. Original calendar and independently tested date arithmetic. The guidance below supports the counting convention and the distinction between periods and cycles; it does not validate this tool as a medical assessment.