Period Calculator
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What Your Result Means
- Next period date: Estimated start of your next menstruation based on your cycle length.
- Ovulation date: Approximate date of egg release, typically the midpoint of your cycle.
- Fertile window: The 5–7 day window when pregnancy is most likely (sperm can survive up to 5 days; egg is viable 12–24 hours after ovulation).
- Cycle projections: Six upcoming cycle dates assume your cycle length remains regular. Irregular cycles make projections less reliable.
How This Calculator Works
Enter your last period start date and cycle length (typically 21–35 days; 28 is average). The calculator adds your cycle length to find the next period date. Ovulation is estimated as the midpoint of the cycle, and the fertile window spans 5 days before to 1 day after ovulation. Six future cycles are projected by adding multiples of your cycle length. The model assumes regular, predictable cycles and does not account for natural variation, stress, or hormonal changes.
Quick Questions
How reliable is this for predicting ovulation?
This calendar method is rough; it works well for regular 28-day cycles but becomes less accurate with irregular cycles. For fertility tracking, consider ovulation predictor kits or basal body temperature charting.
Can I use this for birth control?
No. This calendar method is NOT a reliable form of contraception. Modern contraceptives (hormonal, barrier, or IUD methods) are much more effective. Consult a healthcare provider.
What if my cycle is irregular?
Cycle length varies by up to 7 days in many people. If yours varies more, predictions become less reliable. Track 3–6 months to find your average, or use more precise methods (ovulation predictor kits, ultrasound).
What's a normal cycle length?
The typical range is 21–35 days from the first day of one period to the first day of the next. 28 days is the population average, but 24–32 days is common.
Does stress affect my cycle?
Yes. Stress, illness, exercise intensity, weight changes, and travel can shift ovulation timing by several days. This calculator cannot predict those shifts.
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.