Ovulation Calculator
Estimate your ovulation date and fertile window from your last period and cycle length.
Formula last reviewed 4 August 2026 · How we verify our calculators
Estimated ovulation date
2024-01-15
- Fertile window starts
- 2024-01-10
- Fertile window ends
- 2024-01-16
- Next period (estimate)
- 2024-01-29
Updates live as you type
Frequently asked questions
Ovulation typically occurs about (cycle length − luteal phase) days after your period starts. For a 28-day cycle with a 14-day luteal phase, that is around day 14.
It spans roughly five days before ovulation through one day after, because sperm can survive several days and the egg is viable for about 24 hours.
Ovulation day is not fixed at 14 for everyone. Longer cycles ovulate later and shorter cycles earlier, so entering your real cycle length improves the estimate.
It is an estimate based on averages and assumes regular cycles. It should not be relied on alone for contraception; ovulation tests and medical advice are more reliable.
Not everyone ovulates on day 14 — this calculator doesn't assume you do
A 28-day cycle with a 14-day luteal phase, starting from an LMP of 1 January 2024, puts ovulation at 28 − 14 = day 14 — 15 January 2024 — with a fertile window spanning 10 to 16 January and the next period estimated for 29 January 2024. Change only the cycle length to 32 days, keeping the same 14-day luteal phase, and ovulation shifts to 32 − 14 = day 18 — four days later — carrying the entire fertile window and next-period estimate four days later with it.
The subtraction that most simple trackers skip
Many ovulation guides default straight to "day 14," but that number only holds for a 28-day cycle with a standard luteal phase. This calculator instead computes ovulation as cycle length minus luteal phase days after your period begins, which correctly pushes ovulation later for longer cycles and earlier for shorter ones — a meaningfully different answer for anyone whose cycle isn't exactly average, and the reason entering your real cycle length here matters more than it might seem.
Why the fertile window is wider than the ovulation day itself
The fertile window opens about five days before the estimated ovulation date and closes one day after, and that asymmetry isn't arbitrary: sperm can survive inside the body for several days waiting for ovulation to happen, while the egg itself is viable for only about 24 hours once released. That's why the days leading up to and including ovulation matter most for conception, not just the single estimated day. This calculation rests on population averages and assumes a reasonably regular cycle — ovulation predictor kits and day-to-day basal body temperature tracking can meaningfully sharpen the estimate for your specific cycle, especially if your cycle length tends to vary month to month.
This is an estimate based on standard assumptions, not medical advice. Pregnancy dating and growth can vary — consult your doctor for personalised assessment, especially if cycles are irregular.