Period Calculator
Predict your next period and track future cycles
⚠️ Medical Disclaimer: This calculator is for informational and educational purposes only. It is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before making health-related decisions.
About Menstrual Cycles
This period calculator helps you estimate upcoming period start dates and visualize a forecast of future cycles. A menstrual cycle begins on the first day of bleeding (menstruation) and ends the day before the next period starts. While the average cycle is 28 days, normal cycle length varies between individuals and across life stages.
Tracking your cycle over several months improves the accuracy of predictions because the calculator can use your typical cycle length and period duration rather than a single measurement. This is valuable whether you're planning travel, managing symptoms, or tracking fertility. The tool produces a simple calendar-style forecast for the next year based on your inputs so you can prepare in advance.
How predictions are made
The calculator takes your last menstrual period (LMP), average cycle length, and typical period duration to project future start dates. It advances the LMP by your cycle length to estimate the next start date, and repeats that step to build a 12‑month forecast. Remember that this is an estimate — natural variability, stress, illness, medication, and lifestyle changes can cause cycles to shift.
When to consult a clinician
If you experience very irregular cycles, very heavy bleeding, missed periods, severe pain, or sudden changes in your cycle, consult a healthcare professional. Causes of irregularity include hormonal disorders (like PCOS), thyroid dysfunction, extreme weight changes, and some medications. A clinician can perform tests and recommend suitable management options.
This calculator is for informational use and runs solely in your browser. It does not record or transmit personal data. For personalized care, speak with your healthcare provider or a reproductive health specialist.
Frequently Asked Questions
For most adults, normal cycle length ranges from 21 to 35 days; averages around 28 days.
Accuracy improves with consistent cycle data; this tool uses your average cycle length to provide estimates but cannot account for sudden changes.
The tool helps identify likely fertile windows using cycle timing, but it should not replace clinical fertility advice or ovulation testing.
Stress, weight change, exercise, medications, and hormonal conditions can affect regularity.
See a clinician for very heavy bleeding, severe pain, or markedly irregular cycles that affect quality of life.
Yes — hormonal contraception can alter cycle length and bleeding patterns, reducing the usefulness of calendar-based predictions.
Similar Tools
Reviewed by: Health & Nutrition Research Team
Last updated: December 2025