Period Calculator
Predict your next period date, ovulation day, and fertile window based on your cycle length.
Next Period Starts
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Ovulation Date
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Fertile Window
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Period Ends
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Days Until Next Period
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What is Period Calculator?
The period calculator predicts your next menstrual period, ovulation date, and fertile window based on the first day of your last period and your average cycle length. It uses standard menstrual cycle arithmetic to estimate your most fertile days.
How to use
- 1 Enter the first day of your last period using the date picker.
- 2 Enter your average cycle length (typically 21–35 days; default is 28).
- 3 Enter your average period duration (typically 3–7 days; default is 5).
- 4 Click Calculate to see your results.
- 5 Results show next period start date, ovulation date, and your 5-day fertile window.
Formula
Example calculation
If your last period started on June 1 with a 28-day cycle, your next period is June 29. Ovulation is June 15 (June 1 + 28 − 14). Fertile window: June 10–June 20.
Frequently asked questions
How accurate is period prediction?
The calculator is accurate for women with regular cycles. Irregular cycles, stress, illness, or hormonal changes can shift dates. Always track your actual cycle for better precision.
What is the fertile window?
The fertile window spans approximately 5 days before ovulation through the day of ovulation. Sperm can survive up to 5 days, so intercourse in this window can result in conception.
What is a normal cycle length?
A normal menstrual cycle ranges from 21 to 35 days, with 28 days being the average. Anything outside this range on a consistent basis should be discussed with a healthcare provider.
Can I use this to avoid pregnancy?
This calculator is for informational purposes only and should not be used as a contraceptive method. Consult your doctor for family planning advice.
Why does ovulation occur 14 days before the next period?
The luteal phase (from ovulation to the next period) is consistently about 14 days in most women, regardless of total cycle length. The follicular phase varies, which is why ovulation timing differs across cycle lengths.