Period Calculator: Predict Your Next Period and Fertile Days
Knowing when your period is due helps you prepare, plan, and avoid surprises. This calculator predicts your next period start date and end date from your last period and average cycle length — giving you a clear picture of the weeks ahead.
The Formula
Next Period Start = LMP + Cycle Length (days)
Next Period End = Next Period Start + Period Length − 1 day
Worked Example
LMP: January 1. Cycle: 28 days. Period length: 5 days.
Next period: January 29 – February 2
Predict your next period with our Period Calculator.
Understanding Your Cycle
| Cycle Phase | Days (28-day cycle) | What Happens |
|---|
| Menstruation | Days 1–5 | Uterine lining sheds |
| Follicular | Days 1–13 | Follicles grow; estrogen rises |
| Ovulation | Day 14 | Egg released from ovary |
| Luteal | Days 15–28 | Progesterone rises; PMS symptoms |
Cycle Length Variations
- A "normal" cycle ranges from 21 to 35 days. 28 days is the average, not the norm for everyone.
- Cycle length can vary by 2–7 days from month to month in the same person — this is completely normal.
- Stress, illness, and travel can temporarily shift cycle length.
- Cycles may become less predictable in the years before perimenopause (typically late 30s to 40s).
When to Seek Medical Advice
- Cycles consistently shorter than 21 days or longer than 35 days
- Missing periods for 3 consecutive months (without pregnancy)
- Very heavy bleeding (soaking a pad or tampon every hour for 2+ hours)
- Severe pain that interferes with daily activities
- Spotting between periods
Practical Tips
Track your cycle for 3 months. A single cycle calculation is a rough estimate. Three months of data gives you a reliable personal average and reveals your natural variation.
Use a period tracking app. Apps let you log symptoms, flow, and mood alongside dates — building a comprehensive picture of your cycle health.
Note any significant changes. A sudden change in cycle length, flow volume, or pain level is worth discussing with a doctor.
Final Thoughts
Knowing your cycle helps you plan, prepare for your period, and identify any patterns that may be worth discussing with your healthcare provider.
Predict your next period with our free Period Calculator.