Ovulation & Fertile Window Calculator
Estimate ovulation date and fertile window from your last period and cycle length. Useful for trying to conceive or natural cycle tracking.
What this calculates
Ovulation usually happens about 14 days before the next expected period — not on day 14 of the cycle, which is a common misunderstanding. The most fertile window is the 5 days leading up to and including ovulation. This calculator estimates both from your last period date and cycle length, with the usual caveat that natural cycles vary.
Formula & how it works
Ovulation date ≈ last_period_date + cycle_length − 14 days. Fertile window = ovulation date − 5 days through ovulation day (6 days total). Next period = last_period + cycle_length. This is a calendar estimate; actual ovulation timing varies and is best confirmed with LH testing or basal body temperature tracking.
Worked example
Last period started 2026-05-01, 28-day cycle. Ovulation ≈ 2026-05-01 + 28 − 14 = 2026-05-15. Fertile window: 2026-05-10 through 2026-05-15. Next period: 2026-05-29.
Frequently asked questions
How accurate is this?
Within ±2-3 days for most regular cycles. Stress, illness, travel, and life changes can shift ovulation. LH ovulation tests, basal body temperature, or cervical mucus tracking give better real-time confirmation.
What if my cycle isn't 28 days?
Set the actual length. Anything 21–35 days is generally considered normal. Cycles outside that range or wildly irregular ones warrant a clinic visit.
How long is sperm viable?
Up to 5 days in fertile cervical mucus. That's why the fertile window starts 5 days before ovulation — sperm can wait for the egg. The egg itself is viable for ~24 hours.
Is this contraception?
No. Calendar methods are unreliable for contraception (10–20 % failure rate per year). Use a real contraceptive method if pregnancy is not desired.
Disclaimer: Informational only. Talk to a doctor for fertility concerns.