Enter the first day of your last period and your average cycle length. See your estimated ovulation day, your fertile window, and your next three predicted periods.
The first day of bleeding from your most recent period.
Standard is 28 days. Count from the first day of one period to the first day of the next.
Days between ovulation and your next period. 14 is typical if you don't track this yourself.
In 10 days
This cycle
| Cycle | Ovulation | Fertile window | Period expected |
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This is a statistical estimate based on average cycle patterns, not a measurement of your actual hormones. Ovulation predictor kits (LH strips) or basal body temperature tracking are meaningfully more accurate for trying to conceive or avoid pregnancy.
Ovulation typically happens 14 days before your next period starts, not 14 days after your last one. So this calculator works backward: it adds your cycle length to your last period's start date to estimate your next period, then subtracts your luteal phase length to find your ovulation day.
The fertile window covers the five days before ovulation plus the day of ovulation itself — six days total. Sperm can survive in the reproductive tract for up to five days, while the egg is viable for roughly 24 hours after release, so intercourse anywhere in that window can result in conception.
A last period starting June 1, a 28-day cycle, and a 14-day luteal phase:
It's a reasonable starting estimate for people with regular cycles, but it assumes your luteal phase and cycle length stay consistent month to month, which isn't true for everyone. Studies comparing calendar prediction to ovulation test strips found the calendar method noticeably less precise. If you're actively trying to conceive, pairing this estimate with an LH ovulation predictor kit or basal body temperature tracking narrows the window considerably.
Because the luteal phase (time from ovulation to the next period) is far more consistent across people than the follicular phase (time from period to ovulation), which varies a lot. Counting backward from the predicted next period, using your luteal phase length, is the more reliable estimation method used in most clinical fertility guidance.
Use your average over the last 3–6 cycles, and treat the result as a range rather than a single fixed day. If your cycles vary by more than about 7 days in length, calendar-based prediction becomes considerably less reliable, and tracking ovulation directly with LH strips or basal body temperature will serve you better than a calendar estimate.
It's uncommon but not impossible, mainly because ovulation timing can shift slightly cycle to cycle even when your average is consistent. The six-day fertile window captures the vast majority of conception probability, with the two days before and the day of ovulation carrying the highest odds.
Calendar-based estimates are considerably less reliable with irregular cycles, since the core assumption of a predictable cycle length doesn't hold. If your cycles are irregular, ovulation predictor kits, basal body temperature charting, or a conversation with your doctor will give you far more useful information than a calendar calculation.
Most people have a luteal phase between 11 and 17 days, with 14 days being the average. Unlike the pre-ovulation phase, your luteal phase tends to stay fairly constant cycle to cycle for a given individual, which is exactly why it's the more reliable number to count backward from.
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