Ovulation Calculator
Your estimated ovulation day and the six days when conception is possible — from two numbers you already know.
Find your fertile window
The math of the fertile window
Ovulation happens about 14 days before your next period — the luteal phase is the steady half of the cycle, which is why the calculation counts backward from the expected next period rather than forward from the last one. The egg lives 12–24 hours, but sperm survive up to five days, creating a six-day fertile window: the five days before ovulation plus ovulation day itself. The two days right before ovulation carry the highest probability.
A worked example
A 30-day cycle starting June 1: next period expected July 1, ovulation around June 17, fertile window June 12–18. The common mistake is assuming everyone ovulates on day 14 — that's only true for 28-day cycles. Longer cycles ovulate later, shorter ones earlier; the luteal 14 days is the constant.
Improving on calendar math
Calendar estimates work best with regular cycles. Bodies add precision: ovulation predictor kits detect the LH surge 24–36 hours before ovulation, cervical mucus turns clear and stretchy in the fertile days, and basal body temperature rises ~0.5°F after ovulation (confirming it happened). Couples trying to conceive typically aim for every day or two through the window. And the flip side matters: this calculator is for conception planning, not contraception — cycles vary too much for calendar math to prevent pregnancy reliably. Irregular cycles, or six-plus months of well-timed trying without success (or three if over 35), are good reasons to talk with a doctor.
Frequently asked questions
My cycles vary by several days — which length do I enter?
Use your average, but treat the window as wider than shown — recalculate with your shortest and longest recent cycles and consider the full span fertile. OPK strips add real precision when cycles wander.
Can I use this to avoid pregnancy?
No. Calendar-based estimates are not reliable contraception — ovulation shifts with stress, illness, and normal variation, and sperm survive five days. Talk with a healthcare provider about effective methods.
When should I take a pregnancy test?
Around 14 days after ovulation — the calculator shows the date. Testing earlier risks false negatives because hCG hasn't risen yet; if the result is negative and your period doesn't arrive, retest in two or three days.
Does ovulation cause symptoms?
Often: clear stretchy mucus, a mild one-sided twinge (mittelschmerz), slightly higher libido, and a post-ovulation temperature rise. They're useful confirmations, but kits beat sensations for timing.