Period and Cycle Calculator
Estimate your next period and upcoming cycle schedule from average timing.
Cycle Details
Enter your last period start date and average timing.
Use the first day bleeding started, not the final day.
Days from one period start to the next period start.
Number of bleeding days in a typical period.
Estimate Context
Use this as a planning calendar, not a medical diagnosis.
Period timing can vary from cycle to cycle. This calculator uses your average pattern to project dates and flags inputs that are outside common adult ranges.
Your Period Estimate
Estimated period window, current cycle day, and upcoming schedule.
Cycle Timing
The estimate rolls your average cycle forward from the last period start date.
Upcoming Periods
Six projected period windows using the same average cycle length.
Calendar Projection Only
This tool helps with planning. Sudden changes, bleeding between periods, very heavy bleeding, missed periods, or severe pain are reasons to consider medical advice.
How The Period Calculator Works
The calculator counts cycle length from period start to period start.
Cycle day 1
The first day of bleeding is counted as cycle day 1. The next cycle begins on the first day of the next period.
Average timing
The calculator advances by your average cycle length, then uses your average period length to show the estimated bleeding window.
Rolling forecast
If the last period date is old, the calculator rolls forward to the current cycle before showing upcoming period windows.
Period Tracking Guide
Calendar estimates improve when they are based on several recent cycles.
Track start dates
Record the first day of each period for several months. Your average cycle length becomes more useful when it reflects your own pattern instead of a single cycle.
Track changes
Note changes in flow, pain, missed periods, bleeding between periods, or cycles that shift suddenly. Those patterns can matter more than one projected calendar date.
Period Calculator FAQ
Common questions about period and cycle estimates.
How is cycle length counted?
Cycle length is counted from the first day of one period to the first day of the next period.
Why does the calculator roll old dates forward?
If the last period date is more than one cycle ago, the calculator advances by your average cycle length until it reaches the current or next estimated cycle.
Can this predict irregular periods exactly?
No. This is a calendar projection from average cycle timing. Irregular cycles, birth control changes, pregnancy, postpartum cycles, stress, illness, and medical conditions can change timing.
Sources and Clinical Context
The calculator follows standard menstrual-cycle counting conventions: cycle length runs from the first day of one period to the first day of the next. Source links are provided for context, not as personal medical advice.