About the Period Calculator
Cycle length is measured from the first day of one period to the first day of the next, not from the day bleeding stops. Get that wrong and every prediction is a week out.
The calculator maps the next six cycles so you can see holidays, exams or travel against them, and marks the approximate fertile window in each.
How it works
- Enter the first day of your last period.
- Enter your average cycle length. If it varies, take an average of the last three to six cycles.
- Enter how many days you usually bleed, so the end dates are right too.
What you get: the next period date, the day of the cycle you are on today, and a six-month table. The fertile window shown is counted back from each predicted period, on the same logic as the ovulation calculator.
Examples
With a 1 June start and a 29-day cycle:
- Next period 30 June, then 29 July, 27 August, 25 September.
- Ovulation lands around 16 June, so the fertile window is roughly 11–16 June.
- Over a year, a 29-day cycle gives about 12.6 periods — which is why one occasionally 'moves' into a month you were not expecting.
Period Calculator: frequently asked questions
- What counts as a normal cycle?
21 to 35 days for adults, 21 to 45 in the first few years after periods start. It is normal for length to vary by a few days from cycle to cycle.
- My period is late — am I pregnant?
Possibly, but lateness on its own is weak evidence: stress, illness, travel, heavy training and weight change all delay ovulation and therefore the period. A test is accurate from the day the period was due.
- Why did my cycle suddenly change?
Common causes are stress, illness, a new exercise routine, big weight changes, starting or stopping hormonal contraception, and approaching menopause. A single odd cycle is usually nothing; a persistent change is worth a doctor's visit.
- Can I move a period for a holiday?
Sometimes, with medication such as norethisterone, or by running contraceptive pill packs together. That is a conversation with a doctor or pharmacist, not something to improvise.