Period Calculator

Predicting from an average

The prediction adds the average cycle length to the start date of the last period. The average is more useful than the most recent cycle, since single cycles vary.

Three to six recorded cycles give a reasonable average. Fewer than three is guesswork, and someone whose cycles vary by more than a few days will find the prediction is a centre point rather than a date.

The cycle is counted from the first day of one period to the day before the next begins. Counting from the end of bleeding is a common error that shortens every recorded cycle by several days.

What counts as normal

Adult cycles typically run 21 to 35 days, with 28 being an average rather than a norm. Bleeding usually lasts two to seven days.

Variation of a few days between cycles is entirely normal. The first years after menarche and the years approaching menopause are both characterised by more variability, which is expected rather than concerning.

Consistently shorter than 21 days, longer than 35, or varying by more than about eight to nine days between cycles is worth mentioning to a doctor, not because it is necessarily a problem but because it is the kind of pattern worth a conversation.

What shifts a cycle

Significant stress can delay ovulation and therefore the period. So can illness, travel across time zones, shift work and sleep disruption.

Weight change in either direction affects it, and low body fat from restriction or high training volume can stop periods altogether, which is a recognised part of relative energy deficiency in sport and is not benign.

Hormonal contraception changes the picture entirely. Bleeding on combined pills is a withdrawal bleed rather than a period, and many hormonal methods reduce or stop bleeding, at which point cycle prediction has no meaning.

Symptoms across the cycle

Premenstrual symptoms affect a large proportion of women in the week or two before bleeding: mood changes, bloating, breast tenderness, headaches and food cravings.

Premenstrual dysphoric disorder is a distinct and more severe condition affecting a small percentage, involving marked mood disturbance that interferes with daily life. It is treatable and it is frequently dismissed as ordinary premenstrual symptoms for years.

Tracking symptoms alongside dates is genuinely useful for distinguishing cyclical patterns from constant ones. A clinician assessing possible PMDD needs symptom records across at least two cycles, and having them already is a considerable advantage.

When a period is late

Pregnancy is the first consideration for anyone sexually active, and a home test is accurate from around the date the period was due.

Beyond that, a single late or missed cycle is common and usually resolves. Three consecutive missed cycles without pregnancy, called secondary amenorrhoea, is a reason to see a doctor.

Other patterns worth raising: bleeding between periods, bleeding after sex, unusually heavy bleeding that soaks through protection hourly or passes large clots, and any bleeding after menopause. Each of these has causes worth ruling out and none should be tracked and ignored.

One thing worth saying about tracking apps, since most people use one rather than a calculator. Cycle data is sensitive, and the privacy policies of period tracking apps vary considerably in what they collect, retain and share. Several have faced regulatory action over data handling. If you use one, it is worth knowing whether the data is stored on the device or on a server, and whether it is sold to third parties, because that information is in the policy and is rarely what people assume.