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Ovulation Calculator

Enter the start of your last period and your usual cycle length to estimate when you ovulate and the days you are most likely to conceive.

How the window is estimated

  1. Your next period is projected as last-period date + cycle length.
  2. Ovulation is set 14 days before that (the luteal phase is fairly constant).
  3. The fertile window is the 5 days before ovulation through 1 day after.

This is why cycle length matters so much: a longer cycle does not lengthen the luteal phase, it delays ovulation. Tracking a few cycles to find your true average makes the estimate more useful.

Worked example

Last period started 1 January, cycle length 28 days. Next period is projected for 29 January; ovulation is 14 days earlier, on about 15 January; the fertile window runs roughly 10–16 January. On a 32-day cycle, ovulation moves to about 19 January — the extra four days are added before ovulation, not after.

Ovulation day by cycle length

Cycle lengthOvulation ≈ cycle day
24 daysDay 10
28 daysDay 14
32 daysDay 18
35 daysDay 21

The luteal phase (ovulation to next period) stays near 14 days; only the pre-ovulation phase changes.

Trying to conceive

Aim for intercourse every one to two days across the fertile window, especially the two to three days before ovulation, which have the highest conception odds. Once you have a due date in mind, the due date calculator projects it forward.

Related tools

Frequently asked questions

When do I ovulate?
Ovulation usually happens about 14 days before your next period starts — not 14 days after the last one. So on a 30-day cycle, ovulation is around day 16, and on a 26-day cycle around day 12.
What is the fertile window?
The roughly six days ending on ovulation day: the five days before, plus the day itself. Sperm can survive up to about five days in the reproductive tract, while the egg is viable for around 12 to 24 hours.
How accurate is a calendar estimate?
It is a starting point. Cycle length varies month to month for most people, and stress or illness can delay ovulation. Ovulation predictor kits (which detect the LH surge) and basal body temperature tracking pinpoint it better.
Can I use this to avoid pregnancy?
Not reliably. Calendar estimates are far too imprecise to use as contraception on their own — ovulation timing varies month to month. Fertility-awareness methods that combine basal body temperature, cervical mucus and calendar tracking do exist and can be effective, but they require proper training and consistent daily measurement.
What are the physical signs of ovulation?
Cervical mucus becomes clear, slippery and stretchy (like raw egg white) in the day or two before ovulation; basal body temperature rises about 0.3 °C just after ovulation and stays up; some people feel a one-sided twinge (mittelschmerz). An ovulation predictor kit detecting the luteinising-hormone surge is the most precise home method.
How long is the fertile window, really?
About six days: the five days before ovulation plus ovulation day itself. Sperm survive up to five days in fertile cervical mucus; the egg lives only 12 to 24 hours. Conception odds are highest in the two to three days immediately before ovulation.
My cycles are irregular — is this useful?
Less so. The calendar method assumes a fairly consistent cycle length. If yours varies by more than a few days month to month, track two or three cycles to find a realistic average, and lean on ovulation predictor kits or cervical-mucus observation for timing.

Last reviewed: September 2026. Figures and formulas are checked against their published sources; see the site's data notes.