How the conception estimate works
Conception can only happen around ovulation, when an egg is released and available to be fertilised. In a typical cycle the gap between ovulation and the following period — the luteal phase — stays close to 14 days, even when the overall cycle is longer or shorter than average. That is the anchor this calculator relies on.
Starting from the first day of your last period, the tool counts forward by your cycle length and then steps back 14 days to land on the likely ovulation date. Conception is then estimated as the same window, because fertilisation happens within a day or so of the egg being released.
Reading the fertile window
The result shows a single most-likely day plus a fertile window of several days around it. The window is wider on the early side because sperm can survive in the reproductive tract for up to about five days, so intercourse before ovulation can still lead to conception once the egg arrives.
The egg itself only lives for roughly 12 to 24 hours, which is why the window closes quickly after ovulation. In practice the day of ovulation and the one or two days before it are the most fertile.
Getting a more reliable estimate
This method assumes a fairly regular cycle. If yours varies from month to month, the predicted day can drift, so combining it with direct signs of ovulation gives a sharper picture.
- Track basal body temperature, which rises slightly after ovulation.
- Watch for clear, stretchy cervical mucus in the days beforehand.
- Use ovulation predictor kits that detect the luteinising hormone surge.
- Log several cycles so you can use your own average length rather than 28 days.
Medical disclaimer
This calculator provides general estimates for educational purposes only and is not a substitute for professional medical advice. It cannot confirm pregnancy, diagnose fertility issues, or be relied on for contraception. Speak with a doctor or qualified healthcare provider for guidance about conception, fertility, or any concerns.
Formula
ovulation = lastPeriod + (cycleLength − 14); fertile window = ovulation − 5 … ovulation + 1Frequently asked questions
- When is conception most likely?
- On the days leading up to and including ovulation. The day of ovulation and the day before are typically the most fertile.