Ovulation Calculator
Predict your fertile window and ovulation dates for the next 3 months.
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Understanding Your Fertile Window
An egg is viable for only 12-24 hours after ovulation. However, sperm can survive in the reproductive tract for up to 5 days. This means intercourse in the 5 days before ovulation can still result in fertilization. The combined "fertile window" is approximately 6 days.
Signs of Ovulation
Physical signs include cervical mucus becoming clear and slippery (egg-white consistency). Basal body temperature rises slightly (0.2-0.5 F) after ovulation. Some women feel mild one-sided pelvic pain (mittelschmerz). An LH surge (detected by ovulation predictor kits) occurs 24-36 hours before ovulation.
Irregular Cycles
This calculator assumes regular cycles. Irregular cycles (varying by more than 7-9 days) make ovulation prediction less reliable. PCOS, stress, weight changes, and thyroid issues can cause irregular cycles. Track your cycles over several months for a better baseline. If trying to conceive, use our due date calculator once you have a positive test.
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How this is calculated
The calculator uses the calendar method: it counts back from your next expected period to estimate ovulation, then adds the days before it as the fertile window.
Formulas
The next period starts one cycle length after the first day of your last period.
next period = first day of last period + cycle length
Ovulation is assumed 14 days before the next period.
ovulation = first day of last period + (cycle length − 14)
The fertile window is the day of ovulation and the five days before it, six days in all.
window = ovulation − 5 days … ovulation
Sources
- Office on Women's Health (HHS), Your menstrual cycle. Explains that pregnancy is most likely from sex in the days before and on the day of ovulation, that sperm can survive 3 to 5 days, and that the time between ovulation and the next period varies.
How we test this
This tool is checked against independent formulas every time the site is released, and a release does not ship if a check fails. Example from the test suite: Period starting 1 February 2024, 28-day cycle gives Ovulation on 15 February, fertile window 10 to 15 February, next period 29 February. The test adds the days with its own calendar arithmetic, including the leap day in 2024.
The test is “ovulation: 28-day cycle ovulates on day 14 with a six-day fertile window” in tests/golden-calculators.test.mjs. More on how all tools are tested: our testing approach.
Assumptions and limits
- The 14-day rule is an assumption. The time between ovulation and the next period can range from about a week to nearly three weeks, so your own ovulation can fall earlier or later.
- The fertile window is longer than the source's "three days before" because sperm can survive up to 5 days; this calculator uses the wider six-day window.
- It cannot detect ovulation. It is not birth control and not a way to confirm or rule out pregnancy.
- An estimate for general information, not medical advice. Talk to a health professional about your own situation.
Last reviewed , the date of the latest change to this tool's calculation or content.
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When to use this
Whether you are trying to conceive or simply want to understand your cycle better, knowing when you ovulate is the single most important piece of the fertility puzzle. An egg is viable for only about 12–24 hours after release, but sperm can survive in the reproductive tract for up to five days. That creates a fertile window of roughly six days per cycle — the five days before ovulation and the day of ovulation itself. This calculator estimates that window based on your cycle length and the date of your last period.
The calculator uses the standard clinical assumption that ovulation occurs about 14 days before your next period is expected. For a 28-day cycle, that is day 14. For a 32-day cycle, that is day 18. It projects three months of estimated fertile windows so you can plan ahead rather than recalculating every month. Many couples use this alongside ovulation predictor kits (OPKs) and basal body temperature tracking for a more complete picture.
This tool provides estimates based on averages, and individual cycles can vary. Stress, travel, illness, and hormonal shifts can all move your ovulation day earlier or later. If you have been trying to conceive for 12 months (or 6 months if you are over 35) without success, consult a fertility specialist who can use ultrasound and bloodwork to pinpoint ovulation precisely.
Good to know
The fertile window is narrower than most people think. You can only conceive during about six days per cycle. Timing intercourse within this window — particularly the two to three days leading up to ovulation — gives you the highest probability of conception. The day after ovulation, the window is effectively closed for that cycle.
Cycle length varies, and that is normal. A "normal" cycle ranges from 21 to 35 days. If your cycle length fluctuates by a few days each month, your ovulation day shifts accordingly. The luteal phase (time from ovulation to your period) is relatively stable at 12–16 days, so most variation happens in the follicular phase before ovulation.
Ovulation predictor kits detect the LH surge. A sudden spike in luteinizing hormone (LH) typically occurs 24–36 hours before ovulation. OPKs detect this surge in urine and can confirm that the calendar estimate from this calculator is in the right ballpark for your body.
Basal body temperature confirms ovulation after the fact. Your resting temperature rises by about 0.5–1.0°F after ovulation and stays elevated until your next period. This confirms that ovulation occurred but does not predict it in advance — which is why the calendar method and OPKs are better for planning.
Quick Reference
| Cycle Length | Estimated Ovulation Day | Fertile Window |
|---|---|---|
| 21 days | Day 7 | Days 2 – 7 |
| 24 days | Day 10 | Days 5 – 10 |
| 26 days | Day 12 | Days 7 – 12 |
| 28 days | Day 14 | Days 9 – 14 |
| 30 days | Day 16 | Days 11 – 16 |
| 32 days | Day 18 | Days 13 – 18 |
| 35 days | Day 21 | Days 16 – 21 |