Ovulation Calculator

The ovulation calculator estimates ovulation from the first day of the last menstrual period and an average cycle length. It assumes ovulation occurs 14 days before the next period, then labels a fertile window from five days before through one day after that estimate.

Calendar prediction cannot establish whether or when ovulation occurred. Cycle timing varies even in people with usually regular periods. This tool must not be used as contraception or as proof of fertility or infertility.

The dates can support general cycle awareness or a conversation with a clinician. People trying to conceive may use broader evidence, but no calendar estimate guarantees conception.

Enter a valid LMP and cycle length from 20 to 45 days. The implementation calculates cycle length minus 14 days after LMP as estimated ovulation, then subtracts five days and adds one day for the displayed window.

Estimated ovulation date = LMP + (cycle length − 14) days. Window = estimated ovulation − 5 days through +1 day.

LMP

First day of the last menstrual period.

Cycle length

Average days from one period's first day to the next; accepted range is 20–45.

These are modeled dates, not clinical findings. Illness, stress, postpartum changes, perimenopause, endocrine conditions, and many other factors can shift or prevent ovulation. The window is deliberately approximate.

Relying on a simple calendar for pregnancy prevention can result in unintended pregnancy. Use an evidence-based contraceptive method and professional guidance. Seek care for persistent irregularity, severe pain, abnormal bleeding, or fertility concerns.

For a 28-day cycle, the model places ovulation 14 days after LMP and displays the period from five days before through one day after. Actual ovulation can occur outside these dates.

Use the dates only for general awareness. Do not rely on them for contraception, diagnosis, or fertility assurance; use qualified reproductive-health guidance.