Calculate your ovulation & fertile days
Everything updates instantly as you change the inputs. Dates are computed with the calendar method shown below.
Your results
Estimated ovulation day
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Fertile window
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Next period
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Upcoming periods
3-month fertility calendar
Conception likelihood across your fertile window
Relative likelihood per day — sperm survive up to 5 days, the egg about 12–24 hours, so the days before ovulation carry the highest chances.
| Date | Cycle day | Day vs ovulation | Likelihood |
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How the calculation works (the exact formula)
No black box. Every date above comes from these three steps — you can check the arithmetic by hand.
- Ovulation day. The luteal phase (ovulation → next period) is the most stable part of the cycle, so we count backward from the expected next period: ovulation falls on cycle day (cycle length − luteal phase), counting the first day of your last period as day 1.
- Fertile window. The six days ending on ovulation day: the 5 days before ovulation plus ovulation day itself. Sperm can survive up to 5 days waiting for the egg; the egg lives only 12–24 hours after release.
- Next periods. Each upcoming period is projected as last period start + cycle length (repeated for the next 3 cycles). The next cycle's ovulation and fertile window are computed the same way from each projected start.
ovulation date = last period + (cycle length − luteal phase) − 1 dayfertile window = ovulation − 5 days … ovulation daynext period = last period + cycle length
Honest limitations (read before relying on this)
- This is the calendar method: it assumes your next cycle resembles your last. Ovulation commonly shifts by a few days even in regular cycles.
- Stress, illness, travel, intense exercise, PCOS, thyroid conditions, and some medications can move ovulation or suppress it (anovulatory cycle).
- With irregular cycles (varying by more than ~7 days), dates alone are unreliable — combine with ovulation predictor kits, basal body temperature, or cervical mucus tracking.
- Not for contraception. Calendar estimates are among the least reliable ways to avoid pregnancy.
Why show the formula? A note on app-based trackers
Popular trackers and apps rarely show their math — and independent testing has found most of them misidentify the fertile window. Here every result carries its cycle-day arithmetic, the luteal phase is adjustable instead of hard-coded, and three future cycles are projected at once instead of one. If you disagree with an assumption, you can see exactly which one to change.
Worked examples (check the math yourself)
Three hand-computed examples using the same formulas as the calculator above.
Example 1 — textbook 28-day cycle
Last period: 1 January 2026, cycle 28 days, luteal phase 14 days.
Result: ovulation 14 January, fertile 9–14 January, next period 29 January.
Example 2 — longer 32-day cycle (ovulation is later, not "day 14")
Last period: 1 March 2026, cycle 32 days, luteal phase 14 days.
Result: ovulation 18 March, fertile 13–18 March. This is why "everyone ovulates on day 14" is a myth — cycle length moves ovulation, not the luteal phase.
Example 3 — short 24-day cycle with a 12-day luteal phase
Last period: 10 May 2026, cycle 24 days, luteal phase 12 days.
Result: ovulation 21 May, fertile 16–21 May, next period 3 June.
Ovulation day by cycle length
Same formula, different cycle lengths — all anchored at a last period of 1 January 2026 with a 14-day luteal phase. Notice ovulation moves with cycle length; the luteal phase does not.
| Cycle length | Ovulation (cycle day) | Ovulation date | Fertile window |
|---|---|---|---|
| 21 days | Day 7 | 7 January 2026 | 2–7 January |
| 24 days | Day 10 | 10 January 2026 | 5–10 January |
| 28 days | Day 14 | 14 January 2026 | 9–14 January |
| 30 days | Day 16 | 16 January 2026 | 11–16 January |
| 32 days | Day 18 | 18 January 2026 | 13–18 January |
| 35 days | Day 21 | 21 January 2026 | 16–21 January |
With a 21-day cycle, the fertile window can begin while bleeding from the previous period is still ending — one reason pregnancy from intercourse during a period is possible with short cycles.
Signs of ovulation (beyond the calendar)
Dates estimate; your body confirms. These are the most reliable physical signs, roughly in order of usefulness:
- Egg-white cervical mucus. Clear, stretchy, slippery discharge — considered the most reliable physical sign. It peaks right around ovulation because it helps sperm travel.
- LH surge (ovulation predictor kit). A positive OPK detects the luteinizing hormone surge that precedes ovulation by about 24–36 hours — the best tool for timing.
- Basal body temperature shift. A slight dip before ovulation, then a rise of roughly 0.2–0.5 °C that stays elevated until the next period. Confirms ovulation after it happened, so it is better for pattern-tracking than timing.
- Mittelschmerz. A brief one-sided pelvic twinge at ovulation, felt by about one in five women. Lasts minutes to hours.
- Secondary signs. Increased libido, breast tenderness, bloating, or light spotting around mid-cycle.
Looking for a different calculator?
This tool estimates fertile windows before conception from your last period and cycle length.
- Already pregnant and want a due date? Use our pregnancy due date calculator (Naegele's rule, conception date, IVF and ultrasound methods).
- Only want upcoming period dates, no fertile window? Our period calculator predicts period dates only.
Frequently asked questions
Straight answers to the questions people ask most about ovulation and fertile windows.
How do I calculate my ovulation date?
Find your average cycle length (first day of one period to the first day of the next), then subtract your luteal phase. Ovulation happens on cycle day (cycle length minus luteal phase), counting the first day of your last period as day 1. Example: a 28-day cycle with a 14-day luteal phase means ovulation on day 14. A 32-day cycle with the same luteal phase means ovulation on day 18, not day 14.
How many days after my period do I ovulate?
It depends on your cycle length, because the luteal phase (ovulation to next period) stays roughly constant while the first half of the cycle varies. With a 28-day cycle and 14-day luteal phase, ovulation falls about 14 days after your period starts. With a 21-day cycle it can be as early as day 7, and with a 35-day cycle as late as day 21. Counting back from your next expected period is more reliable than counting forward from your last one.
How long is the fertile window?
About six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in the reproductive tract under the right conditions, while a released egg stays viable for only 12 to 24 hours. This is why the days leading up to ovulation matter more than the days after it.
What are the physical signs of ovulation?
Common signs include clear, stretchy, slippery cervical mucus (often compared to raw egg whites), a slight rise in basal body temperature after ovulation, a positive ovulation predictor kit (LH surge, usually 24 to 36 hours before ovulation), mild one-sided pelvic pain (mittelschmerz), increased libido, and breast tenderness. No single sign is definitive on its own; combining two or more methods is more reliable than dates alone.
Can you get pregnant on ovulation day or the day after?
Yes, ovulation day is one of the most fertile days. The day after ovulation, chances drop sharply because the egg is only viable for about 12 to 24 hours after release, but pregnancy is still possible if intercourse happened in the days before. Two or more days after ovulation, the chance from that cycle's egg is essentially gone until the next cycle.
Can you get pregnant during your period?
You cannot ovulate during a true period, but pregnancy from sex during a period is still possible in some cases. Sperm can survive up to five days, so with a short cycle (for example 21 days), ovulation can arrive while sperm from intercourse during menstruation are still alive. What looks like a period can also occasionally be mid-cycle bleeding around ovulation.
How accurate is an ovulation calculator?
It is an estimate, not a diagnosis. The calendar method assumes your cycles are regular and your luteal phase is stable, but ovulation timing varies from month to month even in regular cycles. Independent testing of fertility apps found most of them misidentify the fertile window, especially for irregular cycles. Treat the result as a planning aid, and confirm with ovulation predictor kits, basal body temperature, or cervical mucus tracking if timing matters to you.
Can I use an ovulation calculator as birth control?
No. Calendar-based estimates are not a reliable contraceptive method. Studies of natural family planning find typical-use failure rates far above modern contraceptive methods, because ovulation can shift by several days due to stress, illness, travel, or hormonal changes. Never rely on this calculator, or any app estimate, to avoid pregnancy. Speak with a healthcare professional about contraception options.
What is the luteal phase?
The luteal phase is the second half of the menstrual cycle, from ovulation to the start of the next period. It is the most consistent part of the cycle, usually lasting 12 to 16 days (14 is the average), and it varies much less between cycles than the first half. That consistency is why ovulation calculators work backward from the next expected period rather than forward from the last one. This calculator lets you adjust it between 10 and 16 days if you know yours.
What if my periods are irregular?
The calendar method becomes unreliable when cycle length varies by more than about a week, because the estimate assumes the next cycle will resemble the last. With irregular cycles, track physical signs instead: daily basal body temperature, cervical mucus changes, or ovulation predictor kits. If your cycles are consistently shorter than 21 days or longer than 35 days, or you have been trying to conceive for 12 months (6 months if over 35) without success, consult a healthcare professional.
How soon after ovulation can I take a pregnancy test?
Most home tests are reliable from the first day of your missed period, roughly 14 days after ovulation. Testing earlier can give a false negative because the pregnancy hormone (hCG) may not yet be detectable. If you test early and get a negative result but your period still does not arrive, test again after a few days.
When is the best time to have sex to get pregnant?
During the fertile window, with the highest chances on the two days before ovulation and on ovulation day itself. Having sex every one to two days through the fertile window covers the uncertainty in the exact ovulation date without needing to pinpoint a single day. Sperm quality is generally better with regular intercourse than with saving up for one specific day.
Do I always ovulate on day 14 of my cycle?
No, this is a widespread myth. Day 14 only applies to a textbook 28-day cycle with a 14-day luteal phase. Research shows ovulation commonly falls anywhere between day 11 and day 21, and it shifts with cycle length: roughly day 7 for a 21-day cycle and day 21 for a 35-day cycle. Using your own cycle length, as this calculator does, is more accurate than assuming day 14.
How long does ovulation itself last?
The release of the egg is a brief event, and the egg remains viable for fertilization for about 12 to 24 hours afterward. The reason the fertile window spans six days is not the egg's lifespan but the sperm's: sperm can wait in the reproductive tract for up to five days for ovulation to occur.
How is this different from a period calculator or a pregnancy due date calculator?
An ovulation calculator estimates your fertile window before conception. A period calculator only predicts upcoming period dates without fertile-window detail. A pregnancy due date calculator is for after conception: it estimates the delivery date from the last period or conception date. If you are already pregnant and want a due date, use our pregnancy due date calculator instead.
Is my data private? Does this tool send anything anywhere?
Yes, it is private. This calculator runs entirely in your browser: your dates are processed by JavaScript on your own device and never sent to any server. There is no account, no tracking of your inputs, and nothing to delete afterward because nothing ever leaves your device.