How This Calculation Works
The period calculator predicts your next menstrual period by adding your average cycle length to the first day of your last period. A typical cycle ranges from 21–35 days, with 28 days being average.
Ovulation is estimated to occur about 14 days before your next period starts. The fertile window spans approximately 5 days before ovulation through 1 day after, as sperm can survive up to 5 days in the body.
Common Mistakes to Avoid
- Assuming every cycle is the same length. Cycles naturally vary by 1–5 days. Track several months for a more accurate average.
- Using this for contraception. Calendar-based predictions are not reliable enough for birth control. Consult a healthcare provider for family planning.
- Counting from the wrong day. Cycle day 1 is the first day of full bleeding, not spotting.
Worked Example
Scenario: Last period started August 1, cycle length is 30 days.
Next period: August 1 + 30 = August 31.
Ovulation: August 31 − 14 = August 17.
Fertile window: August 12–18.