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Period Calculator
Predict your next periods and identify your fertile windows based on your cycle.
How it works
This calculator uses the calendar method: your next period starts after your cycle length. Ovulation occurs approximately 14 days before your next period. The fertile window spans the 5 days before ovulation and ovulation day itself.
Frequently Asked Questions
How long is a normal menstrual cycle?+
21–35 days is considered normal, with an average of 28. Variations of ±7 days between cycles are unconcerning. Source: ACOG.
How is cycle length counted?+
From the first day of one period to the day before the next period starts. Day 1 is the first day of bleeding.
When is a cycle considered irregular?+
If length varies by more than 7–9 days between cycles, or is shorter than 21 or longer than 35 days. Common causes: PCOS, stress, hormone disorders.
How accurate is the period calculator?+
For regular cycles, very accurate (±2 days). For variable cycles, less reliable — an app with symptom tracking gives more precise forecasts.
What are the four cycle phases?+
1. Menstruation (days 1–5), 2. Follicular (days 1–13), 3. Ovulation (day ~14), 4. Luteal (days 15–28). Hormone profiles and symptoms differ across phases.
When should I see a doctor?+
If your period is absent for > 90 days, bleeding is very heavy (> 80 mL), there is bleeding between periods, or severe pain — consult a clinician.
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Background
Period Calculator: Predict Your Next Period & Fertile Days
8 min