Can a conception calculator tell the exact day?

No. It estimates a likely window from cycle timing, due date or birth date. Ovulation can vary, and fertilization timing is not visible on a calendar.

What is the fertile window?

The common fertile window includes the five days before ovulation and the day of ovulation, because sperm can survive for several days and ovulation timing is approximate.

Can this prove paternity?

No. Calendar dates cannot prove paternity. Use a medically and legally appropriate DNA test for paternity questions.

Conception Calculator

Estimate a likely conception date and fertile window from a birth date, due date, last menstrual period or confirmed ovulation date.

Calculation method

Child's birth dateEstimated due dateLast menstrual periodConfirmed ovulation

Child's birth date

Cycle length, days

Luteal phase, days

Use 14 days if you do not know your luteal phase.

Conception date is a range

A conception calculator estimates the likely ovulation or conception period. It cannot prove the exact day of fertilization, paternity or fertility status.

ovulation day \approx cycle length - luteal phase

Calendar estimate for ovulation day within a menstrual cycle.

fertile window = [ovulation - 5 days, ovulation]

Common 6-day fertile window ending on ovulation.

How to interpret the result

A birth date works backward from a typical 280-day pregnancy, so it is approximate.
A due date is a planning date, not proof of the actual conception day.
A known ovulation date is usually a stronger calendar anchor than LMP alone.
A DNA test, not an online calculator, is needed for paternity questions.

Not contraception

Do not use this calendar estimate as birth control. Fertility and contraception decisions need reliable methods and professional advice when needed.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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