About Koyomia

About the name

Koyomi (暦) is Japanese for “calendar”. Koyomia is a private calendar for your cycle: the calculators on this site now, and an app for iPhone and Android later.

How the calculators work

Every calculator starts from the first day of a period (day 1) and your usual cycle length. The next period is expected one cycle length later.

Ovulation is estimated as 14 days before the next period. The NHS describes it as usually 12 to 16 days before, so 14 is the middle of that range.

The fertile window is the 5 days before ovulation plus the day itself, because sperm can survive up to 5 days and an egg about a day (ACOG).

Every result is a range. Choosing how much your cycle varies widens it by 1 to 4 days either side. With several start dates, we average your three most recent cycles and widen the range to cover your shortest and longest of them.

We chose ranges over single dates because cycles move by a few days from month to month. A single date looks precise but is often wrong.

What the calculators can’t do

They can’t see what your body is doing this month. Ovulation tests, cervical mucus and basal body temperature can confirm ovulation; a calendar can only estimate it.

They are less reliable for very irregular cycles, after stopping hormonal contraception, while breastfeeding, and around puberty or perimenopause.

They are not medical advice and not birth control. Never use them to avoid pregnancy.

Sources and standards

We base every page on guidance from the NHS and the American College of Obstetricians and Gynecologists (ACOG), and link the sources on each page.

We write in plain language and avoid claims we can’t back up. Each page shows the date it was last checked. When guidance changes, we update the page and the date.

Contact and corrections

Questions, or spotted a mistake? Email us at hello@koyomia.app. We fix errors quickly and update the page’s date when we do.