Health & Fitness
Period Calculator
Predict your next period, ovulation date, and fertile window based on your last period and average cycle length.
Enter your details and click Calculate
Next Period
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How is this calculated?
Formula: Cycle-based period and ovulation prediction.
Next Period = Last Period Start + Cycle Length
Period End = Next Period + Duration − 1
Ovulation = Last Period + Cycle Length − 14
Fertile Window:
Start = Ovulation − 5 days
End = Ovulation + 1 day
Days Until Next Period = Next Period − TodayCalculation history
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Medical Disclaimer: This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice or a reliable method of contraception. If you experience irregular periods, severe pain, or heavy bleeding, consult your healthcare provider.
FAQ
Frequently asked questions
How accurate is a period calculator?
Period calculators work best for regular cycles (consistent within 1–2 days each month). If your cycle varies significantly (more than 7 days between cycles), predictions become less reliable. Track at least 3–6 cycles to establish your average. Even with regular cycles, stress, travel, illness, and lifestyle changes can shift timing by a few days.
What is a normal cycle length?
Normal menstrual cycles range from 21–35 days in adults (most commonly 28–30 days). Cycles shorter than 21 days or longer than 35 days may indicate hormonal imbalances and are worth discussing with a healthcare provider. Cycle length is counted from the first day of one period to the first day of the next.
Why is my period late?
Common causes of late periods include: pregnancy, stress (physical or emotional), significant weight changes, excessive exercise, hormonal contraception changes, polycystic ovary syndrome (PCOS), thyroid disorders, perimenopause, and illness. A period that's 1–5 days late is usually not concerning for those with regular cycles. See your doctor if periods are regularly irregular or absent.
How long should a period last?
A normal period lasts 2–7 days, with most people bleeding for 3–5 days. The heaviest flow typically occurs in the first 2 days. If your period regularly lasts longer than 7 days, involves very heavy bleeding (soaking through a pad/tampon hourly), or causes severe pain that interferes with daily life, consult your healthcare provider.
Can I get pregnant during my period?
While less likely, pregnancy during menstruation is possible, especially with shorter cycles. Sperm can survive up to 5 days in the reproductive tract. If you have a 21-day cycle, ovulation might occur as early as day 7 — meaning sperm from intercourse on day 2–3 could still be viable. No days are completely "safe" without other contraception methods.