A standard 28-day cycle
A last period on January 1 with a 28-day cycle and a 14-day luteal phase puts ovulation around January 15, with a fertile window from January 10 to January 16.
Ovulation & fertility calculator
This is an estimate for information only. It is not a method of contraception and does not replace advice from a healthcare professional, especially if you are trying to conceive or avoid pregnancy.
Result
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How it works
Unlike the follicular phase (before ovulation), which varies a lot between people and cycles, the luteal phase (after ovulation, before the next period) is fairly consistent for a given person, typically 12–16 days. Working backward from the next expected period by that luteal length pinpoints the most likely ovulation day more reliably than simply assuming ovulation always falls on day 14.
The fertile window spans the five days before ovulation through the day after, reflecting how long sperm can survive in the body and the short lifespan of the egg after release.
Formulas used
Ovulation day (from last period)
Ovulation day = Cycle length − Luteal phase length
Fertile window
Fertile window = Ovulation day − 5 days, through Ovulation day + 1 day
Worked examples
A longer cycle
The same last period with a 32-day cycle shifts ovulation later, to around January 19, without changing the luteal phase assumption.
Assumptions and limits
- The luteal phase length stays constant across cycles, even if the total cycle length varies.
- Ovulation occurs once per cycle, on the estimated day.
- Cycle length is assumed to repeat at the average entered for all six projected cycles.
Frequently asked questions
Why does a longer cycle push ovulation later but not the luteal phase?
Cycle-length variation almost always comes from the first half of the cycle (before ovulation) taking longer or shorter — the second half, from ovulation to the next period, stays close to the same length for a given person.
What is the best day to start ovulation testing?
Testing typically starts a few days before the estimated ovulation day, since ovulation predictor kits detect a hormone surge that happens roughly a day or two beforehand.
Can I use this as birth control?
No — cycle timing varies too much, and sperm can survive several days, making a purely calendar-based method unreliable for preventing pregnancy.
What if my cycles are irregular?
This calculator assumes a fairly consistent cycle length; with irregular cycles, the projections for cycles further in the future become less reliable, and tracking actual cycle data or a fertility-monitoring method may serve better.
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