Pregnancy tools
Ovulation Calculator & Fertile Window
Trying to conceive or just want to understand your cycle? Enter your last period and average cycle length to find your estimated ovulation date and most fertile days.
Most cycles are 21 to 35 days. The average is 28.
General guidance from ACOG and the NHS. Please always check with your provider.
How Ovulation Timing Works
Ovulation is when an ovary releases a mature egg, and it typically happens about 14 days before your next period starts. This is called the luteal phase, and it stays relatively consistent even if your overall cycle length varies.
Your fertile window is roughly six days long: the five days leading up to ovulation (because sperm can survive in the reproductive tract for up to five days) plus the day of ovulation itself (the egg is viable for about 12 to 24 hours). The two or three days right before ovulation are usually your most fertile.
- 28-day cycle: ovulation around day 14, fertile window roughly days 9 to 14.
- 32-day cycle: ovulation around day 18, fertile window roughly days 13 to 18.
- 24-day cycle: ovulation around day 10, fertile window roughly days 5 to 10.
This calculator gives a helpful estimate, but ovulation can shift based on stress, sleep, travel, and other factors. For the most accurate tracking, your Garden OB/GYN provider may recommend ovulation predictor kits or ultrasound monitoring.
Common Questions
How does the ovulation calculator work?
It estimates your ovulation date by subtracting 14 days from your expected next period. This is based on the fact that the luteal phase (ovulation to period) is relatively consistent at about 14 days for most people, even when cycle length varies.
What is the fertile window?
Your fertile window is the roughly six-day stretch when conception is possible: the five days before ovulation (because sperm can survive up to five days) plus the day of ovulation itself (because the egg is viable for about 12 to 24 hours). The two days before and the day of ovulation are typically your most fertile.
How accurate is this calculator?
It gives a good estimate for people with regular cycles, but ovulation can shift from cycle to cycle due to stress, illness, travel, or hormonal changes. For more precise tracking, your provider may recommend ovulation predictor kits, basal body temperature charting, or ultrasound monitoring.
What if my cycles are irregular?
If your cycle length varies by more than a few days, this calculator becomes less reliable. Irregular cycles can sometimes signal conditions like PCOS. Talk with your Garden OB/GYN provider, who can help evaluate and support your fertility.
When should I see a fertility specialist?
Most providers recommend evaluation after 12 months of trying to conceive if you are under 35, or after 6 months if you are 35 or older. If you have irregular periods, known conditions like PCOS or endometriosis, or other concerns, it is reasonable to reach out sooner.
Thinking about starting a family?
Our board-certified OB/GYNs and fertility specialists across Brooklyn, NYC, and Long Island are here to support you. Explore our due date calculator and baby size tool, or book a preconception visit.
Book a visit with Garden OB/GYN