Conception Calculator

What this conception calculator does

This calculator estimates the days when conception was most likely, based on the first day of your last period and your average cycle length. It also shows the ovulation window, the best intercourse days, a pregnancy test date, the next period, and a due date if pregnancy occurs.

Conception is not a single guaranteed day. The estimate uses an average 14-day luteal phase and a fertile window around ovulation.

How to use this calculator

1. Enter the first day of your last period.
2. Enter your average cycle length.
3. Click Calculate.

The default example uses today's date and a 28-day cycle.

Conception formulas

Next period = last period + cycle length
Ovulation = next period − 14 days
Most probable conception days = ovulation − 2 to +3 days
Ovulation window = ovulation − 2 to +2 days
Best intercourse days = ovulation − 5 to +2 days
Pregnancy test = next period − 5 days
Due date if pregnant = ovulation + 266 days

What the results mean

• Most probable conception days: a five-day window around ovulation.
• Ovulation window: the estimated days of egg release.
• Best intercourse days: the fertile window, including days before ovulation when sperm can survive.
• Pregnancy test: an estimated date after the next period would be due.
• Next period and due date: related dates for planning.

Assumptions and limitations

The calculator assumes a regular cycle and a 14-day luteal phase. It does not confirm ovulation or conception. It is not a contraceptive method and should not be used to avoid pregnancy.

Irregular cycles, illness, travel, and fertility treatment can shift the window. If you are trying to conceive or avoid pregnancy, discuss a tracking method with a clinician.

Frequently asked questions

Can this tell me the exact conception day?

No. It estimates a window. The exact day is rarely known without tracking ovulation or fertility treatment.

Why is the intercourse window wider than the conception days?

Sperm can survive for several days, so intercourse before ovulation can still lead to conception.

What if my cycle is irregular?

The calendar estimate is less reliable. Consider ovulation testing or clinical guidance.

Is this a contraception method?

No. Calendar estimates are not reliable contraception.

Reference table

Example conception windows

The window includes the day of ovulation and several days before it, because sperm can survive for up to about five days. Actual timing varies by cycle.

Source

References:

• ACOG — Fertility awareness
• NICHD — Pregnancy

Disclaimer

This calculator is for informational and educational purposes only. It is not medical advice, a fertility diagnosis, or contraception. Discuss conception and fertility with a qualified clinician.