What is Ovulation Calculator?
The ovulation calculator is an online estimator based on the standard cycle model. Enter your last menstrual period (LMP) and cycle length (default 28 days, range 21-45). It computes using the rule that ovulation occurs about 14 days before the next period: ovulation day = LMP + cycle length − 14 days; fertile window = 5 days before through 1 day after ovulation; next period = LMP + cycle length. It shows a countdown to ovulation and a visual fertile-window timeline. All calculation happens locally — nothing is uploaded. Useful for conception and contraception planning; because real ovulation shifts with stress, sleep, and health, combine it with ovulation tests or basal body temperature (BBT) for more accurate tracking.
▶Use Cases5 scenarios
- ◆Finding the best timing window when trying to conceive
- ◆Tracking the countdown to ovulation day
- ◆Scheduling ovulation-test monitoring
- ◆Better understanding an irregular cycle
- ◆Learning how ovulation timing works
▶How to Use5 steps
- 1Pick your last menstrual period (LMP) date
- 2Adjust the cycle length (21-45 days, default 28)
- 3See the predicted ovulation day and fertile window
- 4Check the countdown to ovulation
- 5Verify with ovulation tests or basal body temperature
▶Features8 features
- ✓Free
- ✓No signup
- ✓Ovulation day prediction
- ✓Fertile window calculation
- ✓Next period prediction
- ✓Countdown to ovulation
- ✓Visual timeline
- ✓100% local, nothing uploaded
▶FAQ7 questions
What is an ovulation calculator?
An online tool that predicts ovulation. Input: last period date and cycle length. Output: predicted ovulation day, fertile window (5 days before to 1 day after ovulation), and next period date. Used for conception and contraception planning. All calculation runs locally.
How is the ovulation day calculated?
Ovulation day = LMP + cycle length − 14 days. Example: LMP August 1 with a 28-day cycle gives an expected ovulation day of August 15.
Which days are the fertile window?
5 days before through 1 day after ovulation — about 7 days total. With ovulation on August 15, the fertile window is August 10-16.
Why is the fertile window so long?
Sperm can survive about 5 days in the body while an egg survives about 24 hours, so intercourse up to 5 days before ovulation can still result in conception.
How accurate is this?
It follows the standard model and is fairly accurate for average cycles, but real ovulation shifts with stress, sleep, and health. For conception, combine with ovulation tests, BBT, or a doctor.
Can I use it with irregular cycles?
You can enter your latest cycle length, but the estimate is less reliable the more irregular your cycle is. Persistent irregularity is worth a medical check.
Is my data safe?
Yes. Everything is calculated locally in your browser — your dates are never uploaded.