The Ovulation Calculator estimates the next period, likely ovulation date, fertile window, and an estimated due date from the first day of the last menstrual period and an average cycle length. It also projects the same dates for the next three cycles.
Calculation method
The model assumes ovulation occurs 14 days before the next period, regardless of cycle length.
The calculator accepts cycle lengths from 20 to 45 days and requires a valid YYYY-MM-DD date.
Worked example
If the last period began on 1 January and the cycle length is 28 days:
- Next period estimate: 29 January.
- Ovulation estimate: 15 January.
- Fertile window estimate: 10 through 16 January.
- Due-date estimate if conception occurred: 8 October using 266 days after ovulation.
Interpreting the dates
These are calendar estimates based on an average cycle. Ovulation can vary between cycles, and the fertile window is not a guarantee of conception or contraception. Track changes in cycle length and use clinician-recommended methods when timing matters.
Questions
Is ovulation always 14 days before a period?
No. Fourteen days is a common planning assumption, but the luteal phase and ovulation timing vary. An irregular cycle can make calendar estimates especially uncertain.
Does the fertile window guarantee pregnancy?
No. It identifies an estimated window in which conception may be more likely; it is not a diagnosis or a fertility guarantee.
Can I use this as birth control?
No. Calendar estimates alone are not reliable contraception. Use appropriate medical guidance and an evidence-based contraceptive method.
Health notice and limitations
This tool is educational and does not replace medical advice, pregnancy testing, or care from a qualified clinician. It does not account for ovulation tests, basal temperature, postpartum cycles, hormonal contraception, medical conditions, or irregular periods.