Ovulation & Fertility Calculator
Predicts how many days until ovulation and your fertile window from how many days ago your last period started, your cycle length and luteal phase — and can project a later cycle.
Last updated: May 2026
About this calculator
Ovulation occurs at the end of the follicular phase, which varies in length, and the start of the luteal phase, which is relatively fixed for each person (typically 12–16 days). The formula calculates the ovulation day within the cycle as: ovulationDay = cycleLength − lutealLength. That day number is added to the first day of the last period to give the ovulation date. The fertile window spans from 5 days before ovulation (because sperm can survive up to 5 days in the reproductive tract) to 1 day after ovulation (the approximate lifespan of a released egg): fertilityStart = ovulationDate − 5 days; fertilityEnd = ovulationDate + 1 day. This 6-day window is supported by large prospective studies of conception probability. For subsequent cycles, the same offsets are applied from each projected period start date, assuming a consistent cycle length.
How to use
Suppose your last period started 8 days ago, your average cycle is 30 days, and your luteal phase is 14 days. Ovulation falls on cycle day 30 − 14 = 16, which is 16 − 8 = 8 days away, with a fertile window of cycle days 11–17. Result: "8 days until ovulation in cycle 1 (cycle day 16; fertile window cycle days 11–17)". Set “Cycles to Predict” to 3 to project ovulation three cycles ahead.
Frequently asked questions
How do I know if my luteal phase length is normal?
The luteal phase is the time between ovulation and the first day of the next period. A typical luteal phase lasts 12–16 days, with 14 days being the most common value. A luteal phase shorter than 10 days is sometimes called luteal phase deficiency and may make it harder for a fertilized egg to implant successfully. You can estimate your luteal phase by tracking basal body temperature (BBT) — it rises by about 0.2°C after ovulation — or by using ovulation predictor kits in combination with period tracking. Consistent measurements over 3 or more cycles give the most reliable estimate.
What is the most fertile day in the menstrual cycle?
The day of ovulation itself and the day immediately preceding it are statistically the two highest-probability days for conception. Research published in the New England Journal of Medicine found that the probability of conception peaks at about 33% on the day before ovulation. The fertile window extends back 5 days because sperm can remain viable in cervical mucus for up to 5 days, meaning intercourse on those earlier days can still result in fertilization. Pinpointing ovulation precisely is difficult from calendar methods alone; combining this calculator with BBT tracking or LH surge testing improves accuracy.
Why does my ovulation date change from cycle to cycle?
Ovulation timing is driven primarily by the length of the follicular phase — the time from menstruation to egg release — which can vary due to stress, illness, travel, changes in body weight, or hormonal fluctuations. The luteal phase, by contrast, is more stable because it is governed by the corpus luteum's fixed lifespan. If your cycles are irregular, relying solely on an average cycle length may produce inaccurate predictions. Tracking physical signs such as cervical mucus changes, ovulation pain (mittelschmerz), and BBT shifts alongside this calculator gives a more complete picture of your individual cycle pattern.