Plan Your Fertile Days
Enter the first day of your last period and your average cycle details to estimate when ovulation occurs and which days are most fertile.
Count from day 1 of one period through the day before the next.
Most people have a luteal phase of 12 to 14 days.
How to Use This Calculator
Track period start dates
Log the first day of each period to determine your average cycle length.
Estimate the luteal phase
Most luteal phases are 12 to 14 days. Adjust if you know yours is consistently different.
Use the fertile window for timing
Intercourse every one to two days during the fertile window maximizes conception chances.
Formula
Ovulation day = Period start + (Cycle length - Luteal length)
Fertile window = Ovulation - 4 days through Ovulation + 1 day
Next period = Period start + Cycle length
Pregnancy testing = Ovulation + 10 to 12 days
Full Description
Ovulation typically occurs 12 to 16 days before the next period. Sperm can survive in the reproductive tract for up to five days, making the days leading up to ovulation the most fertile. This calculator provides a calendar-based estimate; combining it with ovulation predictor kits, cervical mucus monitoring, and basal body temperature tracking improves accuracy. Speak with a healthcare provider if conception has not occurred after guideline-recommended time frames based on age.
Frequently Asked Questions
How accurate is a calendar estimate?
It offers a reasonable starting point but biological variation means ovulation can shift by several days. Combine with ovulation predictor kits for better precision.
What if my cycles are irregular?
Irregular cycles reduce accuracy. Track ovulation signs each cycle and discuss evaluation for irregular menses with your clinician.
Can stress or illness change ovulation timing?
Yes. Stress, illness, travel, and significant weight changes can delay or advance ovulation. Continue tracking to capture shifts.
When should I seek medical advice?
Under age 35, seek help after 12 months of trying; ages 35 to 39 after 6 months; age 40 and older after 3 months or sooner if recommended.