// predict your fertile window from cycle data
Calculate your ovulation date and fertile window based on your last period and cycle length. Free, private, browser-based ovulation predictor.
Ready to calculate
Fill in your cycle details and click CalculateSelect the first day your last menstrual period started.
Enter your average cycle length. Not sure? 28 days is the most common.
See your ovulation date, fertile window, and upcoming cycles at a glance.
This free ovulation calculator uses your last period date and average cycle length to estimate when you'll next ovulate and which days you're most fertile. It predicts multiple cycles ahead so you can plan with confidence. All calculations happen in your browser — no data is stored or sent anywhere.
Ovulation typically occurs 14 days before the start of your next period. For a 28-day cycle, that's day 14. For a 30-day cycle, it's day 16. This calculator subtracts 14 from your cycle length and counts forward from your last period start date.
The fertile window is the 6-day period during which conception is possible. It includes the 5 days before ovulation and the day of ovulation itself. Sperm can survive up to 5 days in the reproductive tract, so having intercourse before ovulation can still result in pregnancy.
This calculator uses the standard Ogino-Knaus method, which works best for women with regular cycles. Actual ovulation can shift due to stress, illness, travel, or hormonal changes. For higher accuracy, combine this with basal body temperature (BBT) tracking or ovulation predictor kits (OPKs).
Enter your average cycle length measured from the first day of one period to the first day of the next. If your cycles vary, use an average of the last 3–6 months. Cycles typically range from 21 to 35 days; the most common length is 28 days.
No. All calculations are performed entirely in your browser using JavaScript. No data is sent to our servers or stored anywhere. Your cycle information remains completely private on your device.
This calculator is for informational purposes only and should not be used as a contraceptive method. Natural family planning methods like the rhythm method have a higher failure rate than other contraception options. Please consult a healthcare provider for family planning advice.
Our free ovulation calculator helps you identify the days you're most likely to conceive by estimating your ovulation date and fertile window based on your menstrual cycle data. Whether you're trying to get pregnant or simply want to better understand your body's natural rhythms, this tool provides clear, actionable insights — all computed privately in your browser.
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A typical menstrual cycle lasts between 21 and 35 days, with 28 days being the most commonly cited average. Each cycle is divided into distinct phases: the menstrual phase (days 1–5), the follicular phase (days 1–13), ovulation (around day 14), and the luteal phase (days 15–28). The timing of ovulation is the key event in the cycle — it marks the moment an egg is released from the ovary and becomes available for fertilization.
It's important to remember that cycle lengths vary naturally from person to person, and even month to month within the same individual. Factors such as stress, illness, travel, significant weight changes, and hormonal fluctuations can all shift ovulation timing. This is why tracking multiple cycles and using physical signs (like basal body temperature and cervical mucus changes) alongside a calculator gives the most reliable picture.
The fertile window is the period during which sexual intercourse can result in pregnancy. It spans approximately six days: the five days leading up to ovulation and the day of ovulation itself. This window exists because sperm can survive in the female reproductive tract for up to five days, waiting for an egg to be released. The egg itself, once released, is only viable for 12–24 hours.
Our calculator determines the fertile window by:
A calculator based on average cycle data is a great starting point, but combining it with other methods significantly improves accuracy:
If you've been trying to conceive for 12 months (or 6 months if you're over 35) without success, it's time to consult a reproductive specialist or OB-GYN. Similarly, if your cycles are highly irregular (varying by more than 7–9 days), very short (fewer than 21 days), very long (more than 35 days), or if you experience significant pain or very heavy bleeding, seek medical advice. Conditions like PCOS, endometriosis, or thyroid disorders can affect cycle regularity and fertility, and are diagnosable and treatable with professional support.
This ovulation calculator is completely free, requires no account or sign-up, and processes all data locally in your browser. We do not store, transmit, or analyze any personal health information you enter. It is designed as an educational and planning aid, not a medical device. Always consult a qualified healthcare provider for personal medical advice related to fertility and reproductive health.