Ovulation Calculator
Enter the first day of your last period and your average cycle length to find your ovulation date and fertile window.
How Ovulation Timing Works
Ovulation — the release of an egg from the ovary — occurs approximately 14 days before the start of your next period, regardless of cycle length. This means the timing of ovulation relative to your last period depends on how long your cycle is. For a 28-day cycle, ovulation occurs around day 14. For a 35-day cycle, it occurs around day 21.
The fertile window spans the five days before ovulation plus the day of ovulation itself — six days in total. This is because sperm can survive in the fallopian tubes for up to five days, and an egg is viable for 12–24 hours after ovulation. The two most fertile days are the day before ovulation and ovulation day itself.
These calculations assume a regular cycle. Irregular cycles make predicting ovulation harder; tracking basal body temperature (BBT) or using ovulation predictor kits (OPKs) alongside this calculator gives a more accurate picture.
Cycle Length and Ovulation Day
| Cycle length | Ovulation day | Fertile window | Next period |
|---|---|---|---|
| 24 days | Day 10 | Days 5 – 10 | Day 25 |
| 26 days | Day 12 | Days 7 – 12 | Day 27 |
| 28 days | Day 14 | Days 9 – 14 | Day 29 |
| 30 days | Day 16 | Days 11 – 16 | Day 31 |
| 32 days | Day 18 | Days 13 – 18 | Day 33 |
| 35 days | Day 21 | Days 16 – 21 | Day 36 |
Worked Examples
Example 1: LMP 1 April 2026, 28-day cycle. Ovulation: day 14 = 15 April. Fertile window: 10–15 April. Next period expected: 29 April.
Example 2: LMP 1 April 2026, 35-day cycle. Ovulation: day 21 = 22 April. Fertile window: 17–22 April. Next period expected: 6 May.
Example 3: LMP 1 April 2026, 24-day cycle. Ovulation: day 10 = 11 April. Fertile window: 6–11 April. Next period expected: 25 April. Note the short fertile window early in the cycle — easy to miss without tracking.
Frequently Asked Questions
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Calendar-based ovulation calculators are most accurate for women with regular cycles. Studies suggest calendar methods predict ovulation within a 2-day window about 80% of the time for regular cycles. For irregular cycles, BBT tracking and OPK tests are significantly more reliable.
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Common signs include: a small rise in basal body temperature (0.2–0.5°C) after ovulation; cervical mucus becoming clear, stretchy, and egg-white-like in the days before ovulation; mild one-sided pelvic pain (mittelschmerz); increased libido; and a positive ovulation predictor kit (OPK) test detecting the LH surge.
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It is very unlikely but not impossible if your cycle length varies. If ovulation occurs earlier than predicted due to cycle variation, sperm from intercourse before the expected fertile window could still be present. This is why "safe days" based on calendar methods alone are unreliable for contraception.
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An OPK detects the luteinising hormone (LH) surge that triggers ovulation. LH rises 24–36 hours before ovulation, so a positive OPK indicates ovulation is imminent. OPKs are more accurate than calendar methods and are available cheaply online or from pharmacies. They are best used alongside a calendar estimate to know when to start testing.
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Yes. Significant physical or emotional stress can delay or suppress ovulation by disrupting the hormonal signals that trigger it. Illness, extreme exercise, rapid weight changes, and psychological stress can all cause ovulation to occur later than usual or not at all in a given cycle.
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Most pregnancy tests can detect the hCG hormone from around 12–14 days after ovulation (the equivalent of a missed period). Some sensitive early tests can detect pregnancy 10 days after ovulation, though results are less reliable this early. Testing at or after a missed period gives the most accurate result.