Find your most fertile days. Enter the first day of your last period and your usual cycle length to see your ovulation day, your fertile window and your next expected period.
This is an estimate only, based on average cycles. It is not medical advice and should not be used as a method of contraception.
1. Select the first day of your last period. 2. Enter your average cycle length in days (28 is the most common). 3. Click Calculate Ovulation to see your fertile window and next period.
Ovulation is the day an egg is released from the ovary, and it is the key to understanding fertility. A woman is most likely to conceive in the days around ovulation, called the fertile window. This free ovulation calculator estimates your ovulation day, your fertile window and your next expected period from two simple facts: the first day of your last period and your usual cycle length. It is a quick, private way to plan, whether you are trying to conceive or simply want to understand your cycle better.
The calculation uses a simple rule: ovulation usually happens about 14 days before the next period starts. So if your cycle is 28 days, ovulation is around day 14. If your cycle is 35 days, ovulation is around day 21. The fertile window is the 5 days before ovulation plus the day of ovulation itself, because sperm can live for up to 5 days inside the body, while an egg lives for only about 24 hours after it is released.
Suppose the first day of your last period was 1 November 2026 and your cycle is 28 days long. Your next period is expected on 29 November 2026. Ovulation is 14 days before that, which is 15 November 2026. Your fertile window runs from 10 November (5 days before ovulation) to 16 November (1 day after ovulation). Your most fertile days are 13 to 15 November. Try it in the calculator above: enter 1 November 2026 and 28 days, and check that your result matches.
Now take a longer cycle: the first day of the last period was 1 November 2026 and the cycle is 35 days. The next period is expected on 6 December 2026, so ovulation is 14 days before that, on 22 November 2026. The fertile window is 17 to 23 November, and the most fertile days are 20 to 22 November. Notice how a longer cycle pushes ovulation later — it is not always day 14. That is why entering your own cycle length matters so much. Enter 1 November 2026 and 35 days in the calculator and check your result.
The method has three steps. First, the next period date = first day of the last period + cycle length. Second, ovulation day = next period date - 14 days, because the second half of the cycle (the luteal phase) is usually about 14 days for most women. Third, the fertile window = ovulation day - 5 days to ovulation day + 1 day, because sperm can survive up to 5 days and the egg lives for about 1 day. For example, with a 28-day cycle starting 1 November, the next period is 29 November, ovulation is 15 November, and the fertile window is 10 to 16 November.
Track your cycles for 3 months first. Your average cycle length is more reliable than one month's length, so note your periods for a few months. Remember the fertile window is wider than one day. The most fertile days are the 2 days before ovulation and the day itself, but the full window is 6 days. Watch for body signs too. Changes in cervical mucus, a small rise in temperature and mild pain can all signal ovulation. Do not rely on this for contraception. Ovulation can shift by days, and sperm can live for 5 days, so this calculator cannot prevent pregnancy. See a doctor if cycles are very irregular. Cycles shorter than 21 days or longer than 35 days, or missing periods, are worth discussing with a doctor.
Assuming ovulation is always day 14. Day 14 is only true for a 28-day cycle; with a 35-day cycle, ovulation is around day 21. Using one month's cycle length. Cycles vary by a few days, so use the average of 3 months. Thinking the fertile window is one day. The window is 6 days, because sperm live for up to 5 days before ovulation. Using this calculator as contraception. It cannot prevent pregnancy, because ovulation can happen earlier or later than predicted. Ignoring irregular cycles. If your cycles are irregular, the estimate is much less reliable, so talk to a doctor.
Pro Tip: Keep a simple period diary for 3 months — just the start date of each period. Your average cycle length from those 3 months makes this calculator much more accurate.
Note: This calculator gives an estimate only and is not medical advice. It must not be used as a method of contraception. If you have concerns about fertility or your cycle, please see a doctor.
What is ovulation? Ovulation is the release of an egg from the ovary, usually about 14 days before the next period. It is the most fertile time of the menstrual cycle.
What is the fertile window? The fertile window is the 6 days when conception is possible: the 5 days before ovulation plus the day of ovulation itself. Sperm can live for up to 5 days, while the egg lives for about 24 hours.
How do I know my cycle length? Count the days from the first day of one period to the first day of the next period. Do this for 3 months and take the average for the most reliable number.
Is ovulation always on day 14? No. Day 14 is only true for a 28-day cycle. For a 35-day cycle, ovulation is around day 21, and for a 24-day cycle it is around day 10.
Can I use this calculator to avoid pregnancy? No. This calculator cannot be used as contraception, because ovulation can shift by days and sperm can live for up to 5 days. Use a proper contraceptive method instead.
What if my cycles are irregular? The estimate becomes much less reliable. Track your cycles for a few months and talk to a doctor if your cycles are shorter than 21 days, longer than 35 days, or often missed.
What are the signs of ovulation? Common signs are clear, stretchy cervical mucus, a small rise in body temperature after ovulation, mild pain on one side, and a higher sex drive. Ovulation test kits can also detect it.
How accurate is this ovulation calculator? It is an estimate based on averages. It is most accurate for women with regular cycles and least accurate for irregular cycles. For planning a pregnancy, combine it with ovulation tests and body signs.
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