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Ovulation Calculator

Estimate your ovulation day and fertile window from your last period and cycle length.

Most cycles run 20 to 45 days. The default is 28.

Your estimate

Enter the first day of your last period to see your fertile window and estimated ovulation date.

These are estimates based on a regular cycle and a fixed 14-day luteal phase. Actual ovulation varies from month to month, so this is not a contraceptive method and not medical advice. For questions about fertility or birth control, talk with a qualified healthcare provider.

Worked examples

LMP 30 March 2025, 28-day cycle
Ovulation is estimated around April 13, 2025 (about cycle day 14), with a fertile window from April 8 through April 13.
LMP 1 June 2025, 32-day cycle
A longer cycle pushes ovulation later, to about June 19, 2025 (around cycle day 18), with a fertile window of June 14 through June 19.
LMP 1 April 2025, 26-day cycle
A shorter cycle brings ovulation earlier, to about April 13, 2025 (around cycle day 12), with a fertile window of April 8 through April 13.
28-day cycle, general rule
Ovulation lands roughly in the middle of the cycle (about day 14), and the six most fertile days are the five days before ovulation plus ovulation day itself.

How the ovulation calculator works

This calculator estimates the days in your cycle when you are most likely to be fertile. You give it two things: the first day of your last period (your LMP) and your average cycle length. From there it works out your estimated ovulation date, the fertile window around it, and the same figures for the next few cycles.

  • Ovulation day = LMP + (cycle length − 14 days)
  • Fertile window = 5 days before ovulation through ovulation day (6 days total)
  • Next period = LMP + cycle length

The cycle and the fixed 14-day luteal phase

A menstrual cycle has two halves. The follicular phase runs from the first day of your period up to ovulation, and its length can vary quite a bit from person to person and month to month. The luteal phase runs from ovulation to the start of the next period, and it is much steadier, averaging about 14 days for most people.

That stability is the key idea behind this tool. Rather than assume ovulation always falls on day 14, it counts backward from your next expected period. Because the luteal phase is roughly fixed, ovulation lands about 14 days before your next period regardless of how long your cycle is.

Calculating your ovulation day (cycle length minus 14)

To turn that into a date, the calculator subtracts the 14-day luteal phase from your cycle length and adds the result to your last period date:

  • A 28-day cycle: ovulation is about 28 − 14 = 14 days after your period starts, so around cycle day 14.
  • A 32-day cycle: ovulation is about 32 − 14 = 18 days in, so around cycle day 18.
  • A 26-day cycle: ovulation is about 26 − 14 = 12 days in, so around cycle day 12.

So a longer cycle does not usually mean a longer luteal phase; it means the first half of the cycle is longer and ovulation happens later.

The six-day fertile window and sperm survival

You are not only fertile on ovulation day. Sperm can survive inside the reproductive tract for up to about five days, while the egg itself lives for roughly 12 to 24 hours after ovulation. Put together, that creates a fertile window of about six days: the five days before ovulation plus ovulation day itself. The two days just before ovulation and the day of ovulation are typically the most fertile. This calculator highlights that six-day span for the current cycle and the next three cycles.

Why estimates vary

Every part of this relies on averages, so treat the dates as a guide rather than a guarantee. Real luteal phases range from about 11 to 17 days, not a flat 14. Cycles are rarely identical month to month, and stress, illness, travel, sleep changes, and coming off hormonal birth control can all move ovulation earlier or later. If you want signals from your own body instead of a calendar, ovulation predictor kits detect the hormone surge before ovulation, basal body temperature rises slightly after ovulation, and cervical mucus becomes clear and stretchy near your fertile days.

Worked example

Say your last period started on 30 March 2025 and your cycles average 28 days:

  • Ovulation: 30 March + (28 − 14) = 30 March + 14 days = 13 April 2025 (about cycle day 14).
  • Fertile window: five days before through ovulation, so 8 April to 13 April 2025.
  • Next period expected: 30 March + 28 days = 27 April 2025.

If your cycles were 32 days instead, ovulation would move to about 18 April, since the extra four days fall in the first half of the cycle before ovulation.

A note on accuracy and medical advice

These figures are estimates for general information and family-planning awareness only. They are not a diagnosis or a fertility assessment. Just as important: this calculator is not a contraceptive method. Because ovulation timing shifts and sperm can survive for days, you should never rely on a calendar estimate to prevent pregnancy. If you are trying to conceive, want to avoid pregnancy, or have any concerns about your cycle, please talk with a qualified healthcare provider who can give advice based on your full history.

Frequently asked questions

When do you ovulate in your cycle?
Ovulation usually happens about 14 days before your next period starts, not necessarily in the exact middle of the cycle. That is because the luteal phase (the time from ovulation to the next period) is fairly consistent at around 14 days, while the first half of the cycle can vary. This calculator estimates ovulation as your last period date plus your cycle length minus 14. For a 28-day cycle that is about day 14; for a 32-day cycle it is closer to day 18.
What is the fertile window?
The fertile window is the span of days in each cycle when sex can lead to pregnancy. It runs from about five days before ovulation through the day of ovulation, so roughly six days total. Sperm can survive for several days inside the body, so intercourse in the days leading up to ovulation can still result in conception. The two days before ovulation and ovulation day itself are usually the most fertile.
How accurate is an ovulation calculator?
It is a useful estimate, not a precise prediction. The math assumes your cycles are regular and that your luteal phase is exactly 14 days, which is only an average. Stress, illness, travel, and natural variation can all shift ovulation earlier or later. If you want more precise timing, ovulation predictor kits, basal body temperature tracking, and cervical mucus changes give real-time signals that a calendar cannot.
Can I use this calculator for birth control?
No. This tool is not a contraceptive method and should never be relied on to prevent pregnancy. Because ovulation timing varies and sperm can survive for several days, the fertile window can be wider or shift unexpectedly. If you want to avoid pregnancy, use a proven method of contraception and talk with a healthcare provider about the options that fit you.
Is this medical advice?
No. This calculator is for general information and family-planning awareness only. It is not a diagnosis, a fertility assessment, or a substitute for professional care. If you are trying to conceive, having trouble conceiving, or have questions about your cycle or contraception, please speak with a qualified healthcare provider.

Related tools

Pregnancy Due Date CalculatorEstimate your due date and gestational age from your last period.Due Date CalculatorEstimate your due date from LMP, conception or IVF, with gestational age and trimester.Period CalculatorPredict your next periods and cycle dates from your last period and cycle length.

Last updated: 2026-07-02