NumberTally How the Fertile Window Works
Ovulation usually happens about 14 days before your next period, so for a 28-day cycle it lands around day 14. The fertile window covers the five days before ovulation through the day after: sperm can survive in the reproductive tract for up to five days, while the egg itself is viable for roughly 24 hours after release. That six-day span is when conception is possible — the two to three days right before ovulation are the most fertile.
Ovulation day = LMP + (cycle length − 14)
Fertile window = ovulation − 5 days · through · ovulation + 1 day
Why Ovulation Estimates Are Approximate
The “14 days before the next period” rule is an average — real luteal phases range roughly from 11 to 17 days, and stress, illness, travel, and age can shift ovulation from month to month. If your cycles vary by more than a few days, treat the window as a guide rather than a guarantee. Ovulation predictor kits, basal body temperature charting, and cervical mucus tracking all pin down the day more precisely than any calendar estimate. And if you are trying not to conceive, do not rely on this calculator — talk to your provider about contraception.
Trying to conceive? Our pregnancy due date calculator picks up where this one leaves off.
Frequently Asked Questions
How is my ovulation day estimated?
By subtracting 14 days from your expected next period. With a 28-day cycle starting January 1, ovulation lands around January 15. Longer or shorter cycles shift it later or earlier.
Why does the fertile window start five days before ovulation?
Sperm can survive up to five days waiting for the egg, so intercourse in the days leading up to ovulation can still result in conception. The window closes the day after ovulation, when the egg is no longer viable.
What if my cycle length varies month to month?
Use your average over the last 3–6 cycles, and treat the window as wider than shown. For irregular cycles, ovulation predictor kits are far more reliable than calendar math.
Can I get pregnant outside the fertile window?
It is very unlikely, but not impossible — cycles can surprise you, and sperm occasionally survives longer than five days. The window shown covers the realistic range.
Can I use this as birth control?
No. Calendar-based estimates are not reliable contraception on their own. Talk to a healthcare provider about options that fit your needs.
Is my data stored anywhere?
No. Every calculation happens in your browser; nothing is sent to a server or stored.