How accurate is a period tracker?

A calendar-based period tracker makes an educated guess. It can tell you roughly when your next period is due. It can't tell you the exact day, and it can't tell you when you ovulate.

What the tracker works out

Almost every period tracker starts the same way: take the first day of your last period and add your usual cycle length. That's exactly what Period Tracker Free does, with nothing hidden.

The cycle length you enter matters most. Say your last four cycles were 27, 29, 30 and 28 days. Entering 29, your average, will land closer than the default 28, and it's worth planning around a few days either side.

Why predictions miss

Cycles move around. A period that turns up a few days earlier or later than last month is completely ordinary. The NHS says cycles anywhere from about 21 to 40 days are normal. Illness, stress, travel, changes in weight or exercise, breastfeeding, starting or stopping hormonal contraception, and the years around puberty and perimenopause can all shift your timing.

The calculator doesn't know about any of that. It only sees the date and lengths you entered, so it keeps predicting the same pattern until you enter something different.

Ovulation is the least reliable part

Trackers usually place ovulation about 14 days before your next expected period. That's a reasonable average, but the NHS notes that the exact day is hard to pin down. If the period prediction is four days out, the ovulation estimate will be too. Tracking body signs like temperature or cervical mucus, or using ovulation test strips, tells you more than a calendar, though those methods take practice and have limits of their own.

Don't rely on a calendar estimate to avoid pregnancy. You can get pregnant outside the predicted fertile window.

Getting better predictions

• Enter the real first day. Day 1 is the first day of proper bleeding. Spotting before that doesn't count.
• Use your own average. Count the days between your last few periods and enter that as your cycle length instead of 28.
• Come back each period. Enter the new date when it starts so the next cycle counts from the right day.
• Plan around a window. If your cycles vary by a week, plan around the whole week.

When to talk to a doctor

If your tracker shows periods suddenly getting much more irregular, very far apart or very close together, or much heavier or more painful than usual, mention it to a GP or nurse. Printing the calculator page from your browser gives you three months to take along.