Ovulation Calculator

Enter your last period date and cycle length to predict your ovulation day and fertile window. Plan your conception or track your cycle with this free ovulation calculator.

How to Use

  1. Enter last period date

    Select the first day of your most recent menstrual period.

  2. Enter cycle length

    Input your average menstrual cycle length in days (typically 28 days).

  3. View results

    See your predicted ovulation date, fertile window, and next period date.

What is the ovulation day?

Ovulation is the day a mature egg is released from the ovary and travels into the fallopian tube — the point in your cycle when the chance of conception is highest. Ovulation happens about 14 days before your next period starts, because the luteal phase (the span from ovulation to your period) stays nearly constant at around 14 days for most people. The follicular phase, by contrast, varies with cycle length.

Why count back from the next period

  • With a 28-day cycle, ovulation falls on day 14; with a 35-day cycle, it shifts to day 21.
  • The released egg survives only about 12–24 hours.
  • That's why the fertile window, which includes the few days before ovulation, is key to planning conception.

This calculator works best when your cycle is regular and should be used as a guide for conception planning.

The formula

This calculator first finds your next expected period, then subtracts 14 days to estimate ovulation.

Next period = last period date + cycle length
Ovulation = next period − 14 days
Fertile window = ovulation −2 days to ovulation +2 days

Example

If your last period was on 2026-06-01 and your cycle is 28 days: next period = 6/1 + 28 days = 6/29, ovulation = 6/29 − 14 days = 6/15. The fertile window runs from 6/13 to 6/17. Change the cycle to 32 days and ovulation shifts 4 days later, to 6/19.

Frequently Asked Questions

How is the ovulation day calculated?
Subtract 14 days from your next expected period (last period date + cycle length), and that day is your ovulation. This method is used because the luteal phase stays roughly constant at about 14 days for most people.
When is the fertile window?
This calculator marks the fertile window as the 2 days before and after ovulation — that is, from ovulation −2 days to +2 days, a 5-day span. Because sperm survive 3–5 days inside the body, intercourse a few days before ovulation can still lead to pregnancy.
How long does the released egg stay alive?
Once released, the egg can be fertilized for only about 12–24 hours — a very short window. Sperm, on the other hand, can survive in cervical mucus for up to 5 days, so the earlier days of the fertile window matter more for actually conceiving.
What if my cycle is irregular?
Enter the average of your last few months' cycles to get a rough range, though accuracy will be lower. Tracking basal body temperature (BBT), using ovulation test strips (LH tests), and observing cervical mucus together can pinpoint your actual ovulation day more precisely.
Is the luteal phase always 14 days?
It usually ranges from 12 to 16 days, with 14 days as the average. If your luteal phase differs, your ovulation day shifts accordingly, so for precise planning, consult a gynecologist or use ovulation testing.
Is this calculator accurate?
It is fairly reliable for regular cycles, but it provides a prediction only and cannot replace a medical diagnosis. Stress, illness, and hormonal changes can move ovulation earlier or later.
Can knowing my ovulation day improve my chances of pregnancy?
Yes. Having intercourse during the fertile window — especially from 2–3 days before ovulation through the day of ovulation — gives the highest chance of conceiving. However, relying on this method alone for contraception is not recommended.
Updated 2026 — WHO standards

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