Ovulation Calculator
Estimate when you are likely to ovulate and the days around it when conception is most probable.
How it works
A compact workflow from input to download.
Enter your figures
Fill in first day of last period and average cycle length (days). Every value stays in your browser — nothing is sent to a server.
Read the result
The result updates as soon as your inputs are valid, with the headline figure highlighted and the supporting numbers broken out beneath it.
Change the inputs and compare
Adjust any value to see immediately how it moves the result — the quickest way to understand which input the outcome is actually most sensitive to.
Frequently asked questions
When does ovulation happen?
What is the fertile window?
Is this reliable as contraception?
Are my numbers sent anywhere?
The two phases behave very differently
A menstrual cycle has two halves that vary in quite different ways, and understanding that resolves most of the confusion around prediction. The follicular phase runs from the first day of your period until ovulation, and its length is genuinely variable — it can stretch or compress in response to stress, illness, travel, weight change and sleep. The luteal phase runs from ovulation to the next period and is remarkably stable, usually 12 to 16 days, because it is governed by the lifespan of the corpus luteum. This is why 'day 14' is such a poor rule of thumb: it only holds for a 28-day cycle. Counting fourteen days back from your next expected period is far more robust, because you are anchoring to the phase that does not move.
Why the calendar is only a starting point
Calendar prediction works from averages, and your body works from hormones. If you want to know what is actually happening rather than what is typical, the physical signs are considerably more informative. Basal body temperature rises by a few tenths of a degree after ovulation, confirming it has happened. Cervical mucus becomes clear, slippery and stretchy in the fertile days, which is a forward-looking sign rather than a retrospective one. Ovulation predictor kits detect the luteinising hormone surge that precedes release by a day or so. Used together, these tell you where you are in this cycle. A calendar tells you where you were, on average, in previous ones.
This is an estimate, not medical advice
A calculator works from a formula and a handful of numbers, and a formula cannot examine you. The results here are population-level estimates derived from studies of large groups, and any individual can sit far from the average for entirely healthy reasons. They are a useful starting point for a conversation and a reasonable way to track a trend over time, but they are not a diagnosis and they cannot account for your medical history, medication, body composition or anything else a clinician would consider. If a result concerns you, or if you are making a decision about your health, speak to a doctor rather than to a web page.
Related tools
Continue the workflow with adjacent tools.