Period Calculator
Estimate your next period, fertile window, and ovulation date.
Period predictions are estimates and should not be used as the only method of contraception.
Period calculator
Enter your last period date, average cycle length, and period length.
Cycle day 1 is the first day of menstrual bleeding.
Count from period start to next period start.
Period length is the number of bleeding days.
Future dates use the same average.
Estimated next period start
Friday, August 14, 2026
Based on your average 28-day cycle.
Estimated period window
Aug 14–Aug 18
Based on a 5-day average period length.
Days until next period
13 days
Current cycle day
Day 16
Period predictions are estimates. This calculator does not diagnose menstrual disorders, replace professional care, or provide contraception guidance.
Personalized interpretation
Based on your last period and average cycle length, your next period may start around Friday, August 14, 2026.
Your estimated period window is Friday, August 14, 2026 to Tuesday, August 18, 2026.
Cycle timing can shift due to stress, illness, travel, sleep changes, medications, or hormonal changes.
This calculator should not be used as the only method of contraception. If your cycles become very irregular or symptoms concern you, consider speaking with a qualified healthcare professional.
Cycle timeline overview
Last period start
Jul 17
Current cycle day
Day 16
Estimated next period
Aug 14
Estimated window
Aug 14–Aug 18
Forecast cycle 1
Aug 14–Aug 18
Forecast cycle 2
Sep 11–Sep 15
Forecast cycle 3
Oct 9–Oct 13
This timeline is based on average cycle length. Actual dates can vary.
How period prediction works
Menstrual cycle phases explained
Menstrual phase
Cycle day 1 is the first day of menstrual bleeding.
Follicular phase
This phase can vary by person and from cycle to cycle.
Ovulation
Ovulation timing can vary and is not guaranteed by a calculator.
Luteal phase
The phase after ovulation can vary, though it is often more consistent.
Not every cycle follows a perfect 28-day pattern.
Cycle length, period length, and variability
Average cycle length commonly varies between people. A 28-day cycle is often used as an example, but it is not universal.
Cycles can vary slightly month to month, and period length can also vary. Tracking multiple cycles provides a better average.
Irregular periods and when to seek advice
Predictions may be less reliable with irregular cycles. Consider professional guidance for very heavy bleeding, severe pain, missed periods without clear reason, bleeding between periods, sudden major cycle changes, possible pregnancy, or symptoms that feel concerning.
Period tracking tips
Formula / date explanation
Next Period Start = Last Period Start Date + Average Cycle LengthEstimated Period End = Next Period Start + Period Length − 1 dayFuture Period Start = Last Period Start Date + Average Cycle Length × Number of CyclesThese calculations use your last period start date, average cycle length, average period length, and forecast count. Estimates can shift because individual cycles vary.
Frequently Asked Questions
Related tools
More tools to try
Estimate only. This calculator is for general education and planning. It is not medical advice, and not medical, nutrition, fitness, diagnosis, treatment, or emergency advice. Personal factors can change results. Dates are estimates based on the inputs provided. Cycle length, ovulation timing, pregnancy dating, and medical factors can vary. Speak with a qualified professional for personal guidance.