Ovulation Calculator

What this ovulation calculator does

Ovulation is the release of an egg from the ovary. This calculator estimates the most likely ovulation date, the ovulation window, the intercourse window for pregnancy, a pregnancy test date, the next period, and a due date if pregnancy occurs.

The estimate assumes a regular cycle and a 14-day luteal phase. Real cycles vary, and ovulation can happen earlier or later than the estimate.

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.

Ovulation formulas

Next period = last period + cycle length
Ovulation = next period − 14 days
Ovulation window = ovulation − 2 to +2 days
Intercourse window = ovulation − 5 to +2 days
Pregnancy test = next period − 5 days
Due date if pregnant = ovulation + 266 days

The 14-day luteal phase is an average. The fertile window includes the days before ovulation because sperm can survive in the reproductive tract.

What the results mean

• Most probable ovulation: the estimated release date.
• Ovulation window: a few days around the estimate.
• Intercourse window: the days with the highest chance of conception.
• Pregnancy test: an estimated date after the next period would be due.
• Next period: the expected start of the next cycle.
• Due date if pregnant: an estimated date based on ovulation.

Assumptions and limitations

The calculator assumes a regular cycle and does not confirm ovulation. It does not use LH tests, basal body temperature, cervical mucus, or ultrasound. Stress, illness, travel, and hormonal conditions can shift ovulation.

It is not a contraceptive method. If you are trying to avoid pregnancy, use a method your clinician recommends rather than relying on calendar estimates.

Frequently asked questions

How accurate is the ovulation estimate?

It is a calendar estimate based on an average luteal phase. It can be off by several days, especially if your cycles are irregular.

When is the fertile window?

The calculator uses ovulation −5 to +2 days. The days just before ovulation are generally the most fertile.

Can I use this to avoid pregnancy?

No. Calendar-based estimates are not reliable contraception. Use a method recommended by a clinician.

What if my cycle is irregular?

The estimate is less reliable. Consider tracking ovulation with a validated method and discussing irregular cycles with a clinician.

Reference table

Example ovulation windows

The fertile window is estimated from the next expected period minus 14 days, with a few days of sperm survival on either side. Actual ovulation 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 cycle tracking and fertility with a qualified clinician.