Ovulation Calculator

Estimate your ovulation day and fertile window

Important: this is an estimate for general information only. It is not medical advice and it is NOT a method of contraception - never rely on calendar estimates to avoid pregnancy.

This free Ovulation Calculator estimates the key dates of your cycle from two inputs: the first day of your last period and your usual cycle length. It works on the standard calendar method used by most cycle trackers - your next period is expected one cycle length after your last one started, and ovulation is estimated at 14 days before that next period, because the second half of the cycle (the luteal phase) is about 14 days for most people. Your fertile window is shown as the five days before ovulation plus the day after, reflecting how long sperm can survive and how long the egg lives.

Please use these dates as a rough guide, nothing more. Real cycles vary - sometimes by several days, even in people who are usually regular - and stress, travel, illness and many other factors can move ovulation earlier or later. This estimate is provided for general information only. It is not medical advice, it cannot diagnose anything, and it must never be used as a method of contraception: the calendar method on its own is not a reliable way to avoid pregnancy. If you are trying to conceive, ovulation test kits and basal temperature tracking give far better timing signals. If your cycles are painful, very irregular, or you have been trying to conceive for a year or more (six months if you are over 35), a clinician can help in ways no calculator can. Your dates are calculated in your browser and never uploaded anywhere.

How to use

  1. Enter the first day of your last period (the day bleeding started).
  2. Enter your usual cycle length in days - count from the first day of one period to the first day of the next; 28 is the textbook average.
  3. Click "Calculate my dates".
  4. Read your estimated ovulation day, your fertile window, and the expected date of your next period.
  5. Remember these are estimates based on a regular cycle - see the important note on the card.

Frequently asked questions

How does the calculator work out ovulation?

It adds your cycle length to the first day of your last period to estimate your next period, then counts back 14 days, because ovulation typically happens about 14 days before a period starts. The fertile window is the five days before ovulation (sperm can survive up to about five days) plus one day after (the egg survives roughly a day).

How accurate is this estimate?

It is a reasonable first guide for regular cycles, but only a guide. Studies of real cycles show ovulation day varies considerably from person to person and month to month - even "regular" 28-day cycles do not always ovulate on day 14. For trying to conceive, ovulation predictor kits that test hormone levels are much more precise than any calendar estimate.

Can I use this calculator to avoid pregnancy?

No. This is stated plainly because it matters: calendar estimates are not a reliable contraception method, and this tool is not designed or validated for that purpose. Ovulation can shift without warning, and sperm survive for days, so the "safe" days from any calendar calculation can be wrong. Use a proper contraceptive method and talk to a clinician about options.

What if my cycle is irregular or outside 21-35 days?

The calculator accepts 21 to 35 days, the range where the calendar method is least unreasonable. If your cycles are often outside that range, vary by more than about a week, or have changed recently, calendar estimates will be especially unreliable for you - and persistently irregular cycles are worth mentioning to a doctor, as they can have treatable causes.

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