Ovulation & fertile-window calculator
Estimate when you ovulate and your most fertile days from your last period and cycle length. Everything is worked out in your browser — nothing is saved or sent anywhere.
The first day of bleeding.
First day of one period to the next. Default 28.
Days from ovulation to your period. Usually ~14.
How to use it
- Enter the first day of your last period. Use the first day of bleeding, not the last.
- Set your average cycle length. Count from the first day of one period to the first day of the next. Leave it at 28 if unsure.
- Adjust the luteal phase only if you know it. The gap from ovulation to your period is about 14 days for most people; change it only if a clinician or tracking told you otherwise.
- Read your window and act on it. You’ll see your estimated ovulation day, fertile window, most-fertile days, and next period — plus a one-click link to the due-date calculator if you conceive.
How the estimate works
Ovulation is estimated as your next expected period minus your luteal phase, since the luteal phase (ovulation to period) is fairly steady while the first half of the cycle varies. The fertile window then spans the five days before ovulation plus ovulation day — the span in which conception is possible, because sperm can survive several days and the egg lives roughly 12–24 hours. For the biology behind this, see ovulation timing and signs and menstrual cycle length.
Frequently asked questions
How do you calculate the fertile window?
Ovulation is estimated as your next period minus your luteal phase (about 14 days). The fertile window is the five days before ovulation plus ovulation day, because sperm can survive several days and the egg lives about a day.
When am I most likely to conceive?
The two days before ovulation and ovulation day itself are usually the most fertile. Timing isn't exact, though — ovulation shifts between cycles, so treat the window as a guide.
Does cycle length change my ovulation day?
Yes. Ovulation is tied to when your next period is due, so a longer cycle pushes ovulation later and a shorter one pulls it earlier. That's why the tool asks for your cycle length.
Can I use this as birth control?
No. Fertility-window estimates are not a reliable method of contraception. Ovulation timing varies, and this tool is for those trying to understand their cycle or trying to conceive.
What if my periods are irregular?
The estimate is least reliable with irregular cycles, because the ovulation day is harder to predict. Ovulation predictor kits or a conversation with your clinician will be more dependable.