How this works
A menstrual cycle is the time from the first day of one period to the first day of the next. The population average is 28 days but normal cycles range from 21 to 35 days. To predict future periods, you simply add multiples of your cycle length to the LMP: cycle 1 = LMP + cycle_length, cycle 2 = LMP + 2 × cycle_length, and so on. The fertile window is the 5–6 day span around ovulation, which typically occurs 14 days before the next period (the luteal phase is more consistent than the follicular phase across women).
Cycle prediction is most reliable for people with consistent cycles. Variability of more than 7 days between consecutive cycles is clinically considered "irregular" and reduces the accuracy of any calendar-based prediction. Stress, illness, weight changes, sleep changes, and hormonal contraception transitions all affect cycle length. Use this tool as a rough planner — for fertility tracking, layer it with basal body temperature charting or ovulation predictor kits for higher precision.
The formula
LMP is the first day of your last period. cycle_length is your typical cycle length in days; track 3+ cycles for a meaningful average. The fertile window assumes ovulation 14 days before the next period (the standard luteal-phase model).
Example calculation
- LMP = January 1, cycle = 28 days. Next period = January 29; following = February 26; third = March 26.
- Ovulation around January 15 (LMP + 14); fertile window January 10–15.
Frequently asked questions
My cycle is irregular — how do I use this?
Use the average of your last 3–6 cycles as the input, but treat the predictions as wide windows rather than dates. Variability over 7 days between cycles makes calendar prediction unreliable; ovulation tests or temperature charting are more accurate for irregular cycles. If your cycles vary by more than 14 days between months, talk to a clinician — that's outside the normal range.
How is this different from the ovulation calculator?
Same underlying math, different headline output. The period calculator emphasises future menstruation dates (when am I going to bleed?). The ovulation calculator emphasises the current cycle's fertile window and ovulation day (when can I conceive?). Both are useful — pick the one that matches your immediate planning question. They share the same input fields.