Ovulation Calculator
Estimate your fertile window, ovulation date, and next period in seconds.
How the Ovulation Calculator Works
Enter the first day of your last menstrual period (LMP) and your average cycle length. The calculator estimates ovulation as cycle length minus 14 days after the LMP — based on the fact that the luteal phase (post-ovulation) is fairly constant at about 14 days for most people. Your fertile window is the 5 days before ovulation plus the day of ovulation itself, since sperm can survive up to 5 days inside the reproductive tract.
What Your Results Mean
The ovulation date is when an egg is released. The fertile window is your best chance to conceive. The next period is when your following cycle should begin if you don't conceive. The estimated due date uses Naegele's rule (LMP + 280 days) and is shown for reference if conception occurs this cycle.
Frequently Asked Questions
How accurate is this calculator?
It's an estimate based on cycle averages. Real ovulation can vary by several days, especially if your cycles are irregular. For higher accuracy, combine this with ovulation predictor kits (OPKs) or basal body temperature tracking.
What if my cycle is irregular?
Use your average length over the last 3–6 months. If your cycles vary by more than 7 days, consider tracking ovulation symptoms directly (cervical mucus, OPKs, BBT) instead of relying on calendar math.
What is the fertile window?
It's the 6-day span ending on the day of ovulation. Sperm can live up to 5 days, so intercourse during this window gives the highest chance of conception.
Is my data saved or sent anywhere?
No. This tool runs entirely in your browser. Nothing is uploaded, stored, or transmitted.
Should I use this for birth control?
No. Calendar-based methods are not reliable contraception. Talk to your doctor about effective birth control options.
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