Ovulation Calculator

Predict your fertile window and most likely ovulation date based on your last period and average cycle length. Useful for family planning — whether trying to conceive or tracking your cycle.

Part of our Health Calculators collection.

Ovulation Calculator

Free online calculator

First Day of Last Period*

Average Cycle Length (days)

Most cycles are 21–35 days. 28 days is average.

Results update automatically as you type

How to Use This Calculator

1
Enter the first day of your last menstrual period.
2
Enter your average cycle length (track 2–3 months to get an accurate average).
3
See your predicted ovulation date and fertile window.
4
The fertile window spans roughly 6 days — 5 days before ovulation plus ovulation day itself.

How Ovulation Is Predicted

Ovulation typically occurs 14 days before the next period.

Ovulation Date = Last Period + (Cycle Length − 14)
Fertile Window = Ovulation Date − 5 days to Ovulation Date + 1 day
Next Period = Last Period + Cycle Length

Note: This is an estimate. Actual ovulation varies — apps like Flo or Natural Cycles with temperature tracking are more accurate.

Example Calculation

Example: Last period April 1, 28-day cycle

Inputs

lastPeriodDate: 2025-04-01cycleLength: 28

Result

Ovulation: ~April 15 | Fertile Window: April 10–16

With a 28-day cycle starting April 1: ovulation is predicted around day 14 (April 15). The fertile window spans April 10–16. Next period expected April 29.

Frequently Asked Questions

How accurate is this calculator?

Calendar-based methods like this are roughly 75–80% accurate for women with regular cycles. Irregular cycles make prediction less reliable. For higher accuracy, combine with basal body temperature (BBT) tracking or LH ovulation test strips.

What are my most fertile days?

The two days before ovulation and ovulation day itself have the highest conception probability. However, sperm can survive up to 5 days, so the fertile window extends several days before ovulation.

I have irregular cycles — can I use this?

You can use this as a rough estimate but it will be less accurate. For irregular cycles, LH surge test strips (ovulation predictor kits) give more reliable real-time feedback.