How it works
Estimate your ovulation date, fertile window, and next period from your last menstrual period.
Enter the first day of your last period and your average cycle length. The calculator estimates ovulation about 14 days before the next period and marks the fertile window around it.
Formula
Ovulation = LMP + (cycle − 14) days. Fertile window = ovulation − 5 days through ovulation + 1 day. Next period = LMP + cycle days.
LMP- First day of last menstrual period (date)
cycle- Average cycle length (days)
Examples
- LMP 1 January, 28-day cycleLMP 2023-01-01 · cycle 28 daysOvulation ≈ 2023-01-15 · fertile window 2023-01-10 to 2023-01-16
Frequently asked questions
Is this method exact?
No. It is a calendar estimate based on averages. Ovulation timing varies with stress, illness, and cycle regularity.
Can I use this to avoid pregnancy?
No. Calendar methods alone are unreliable for contraception or family planning. Discuss reliable options with a healthcare provider.
How to read the result
The fertile window is the approximate interval when conception is possible. Results are planning estimates, not medical guarantees.
Limitations
Assumes a regular cycle and ovulation 14 days before the next period. Individual variation is significant. Not a substitute for ovulation tests or medical advice.
Source: Standard calendar method: ovulation ≈ cycle length − 14 days; fertile window of 6 days around ovulation.