Ovulation Calculator
The Ovulation Calculator estimates when you are most likely to ovulate and conceive.
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Enter the first day of your last period and your usual cycle length, and it projects your fertile window, ovulation day, next period and an estimated due date across your next three cycles.
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How to Use
Enter your last period date
Pick the first day of bleeding of your most recent period. This is day 1 of your cycle, not the day it ended.
Set your cycle length
Enter the number of days from the first day of one period to the first day of the next. If your cycles vary, use your average over the last few months.
Read your fertile window
The highlighted window spans the five days before ovulation and the day after, which is when conception is most likely.
Plan across three cycles
The table projects the next three cycles so you can plan ahead. Copy the whole set with one click.
Frequently Asked Questions
How does an ovulation calculator work?
It counts backwards from your next expected period, not forwards from your last one. Your luteal phase (ovulation to period) is roughly 14 days for most people and stays fairly constant, while the first half of the cycle varies. So the calculator adds your cycle length to your last period date to find the next one, then subtracts the luteal phase to estimate ovulation.
When is the fertile window?
The fertile window covers the five days before ovulation plus ovulation day and the day after. Sperm can survive up to five days in the reproductive tract, while the egg is only viable for about 12 to 24 hours after release, so the window opens well before ovulation and closes shortly after.
How accurate is this for irregular cycles?
Much less accurate. The calendar method assumes your cycle length is consistent, so if your cycles vary by more than a few days the estimate can be off by a week or more. If your cycles are irregular, track ovulation directly with basal body temperature, cervical mucus or an ovulation predictor kit, and speak to a healthcare professional.
Can I use this as birth control?
No. This calculator gives a statistical estimate, not a measurement of what your body is actually doing in a given cycle. Ovulation can shift due to stress, illness, travel, medication or no obvious reason at all. Do not rely on it to avoid pregnancy — use a proven contraceptive method and talk to a healthcare professional.