Period and Ovulation Calculator

Features

• The next period and the three after it
• Estimated ovulation (the next period minus the luteal phase)
• Fertile window: five days before to one day after ovulation
• Due date by Naegele’s rule (last period + 280 days)
• Cycle length, bleeding days and luteal phase are all adjustable
• A note when the cycle length is outside the usual range
• A prominent “not for contraception or diagnosis” row
• Computed locally; the date is never uploaded

How to use

1. Pick the first day of your last period
2. Adjust cycle length (28), bleeding days (5) and luteal phase (14) as needed
3. Results appear at once and can be copied row by row
4. Note down the due date and fertile window you need

FAQ

Can this be used for contraception?

No. So-called “safe period” planning rests on exactly this kind of estimate, and it fails often — ovulation shifts with stress, illness and lost sleep, and sperm survive for several days. That warning is a row in the results, not a formality.

Why is the due date last period plus 280 days?

That is Naegele’s rule. The common phrasing “9 months and 7 days” goes wrong at month ends, because month arithmetic clamps the day first and then offsets. Adding 280 days is always exact; the two agree on most dates and differ by a day or two at month ends.

How is ovulation worked out?

As “next period − luteal phase”. The luteal phase is treated as relatively stable (about 14 days), which is why counting back from the next period is the usual method. It still varies between people, and stress or illness can move ovulation.

What if my cycle is irregular?

This tool assumes a fixed cycle. If your cycle is outside 21–35 days, or varies a lot, the projected dates are only a rough range. For accurate ovulation you need basal body temperature, ovulation tests, or a clinician.