Ovulation Calculator
Find your estimated ovulation day, fertile window, peak days and next period from your last period and cycle length — with a luteal-phase setting, irregular-cycle range and a table of your next six cycles.
How it works
next period = last period + cycle length · ovulation ≈ next period − luteal phase (14 days by default) · fertile window = ovulation − 5 days to ovulation + 1 day (widened by ± your cycle variation) · peak = the day before and the day of ovulation · due date if conceived = ovulation + 266 days
The luteal phase — from ovulation to the next period — is fairly constant, about 14 days for most people, so ovulation is estimated by counting back from the next expected period; if you know your own luteal phase from ovulation tests or temperature charts, enter it under More options. Because sperm can survive up to about five days, the fertile window opens five days before ovulation and closes about a day after, and the chance of pregnancy is highest from intercourse in the two days before ovulation and on the day itself. If your cycles vary, enter the variation and the window is widened to cover your shortest and longest cycle. The table repeats the pattern for your next cycles with period dates, fertile windows, ovulation days and the due date that would follow from each one.
Worked example
A last period on June 1, 2026 with a 28-day cycle: the next period is due June 29, so ovulation is estimated 14 days earlier, around June 15 (cycle day 15). The fertile window runs June 10 to June 16, with peak days June 14–15, and conceiving that cycle would give a due date around March 8, 2027. The next cycle starts June 29 with ovulation around July 13.
Sources: Wilcox, Weinberg & Baird (1995) — Timing of sexual intercourse in relation to ovulation, NEJM · ACOG — Fertility awareness-based methods of family planning
Frequently asked questions
How long is the fertile window?
About six days carry a real chance of pregnancy: the five days before ovulation and the day of ovulation itself, because sperm can live about five days and the egg about a day. This calculator adds the day after ovulation, since the exact day is never certain.
What if my cycles are irregular?
Enter how many days your cycle varies under More options and the window widens to cover the earliest and latest likely ovulation. Ovulation predictor kits, basal body temperature tracking or advice from your doctor give better timing.
Does ovulation always happen on day 14?
No — about 14 days before the next period is the average, so it lands later in longer cycles and earlier in shorter ones. The luteal phase itself can range from about 10 to 16 days; if you know yours, enter it.
When can I take a pregnancy test?
Home tests are most reliable from the day your next period is due. Testing earlier can give a false negative because the pregnancy hormone may still be too low.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.