Period Calculator
Two numbers give you the next six months, including the PMS week before each period.
Your next six periods
PMS symptoms track the drop in progesterone after ovulation, so the window sits in the days before bleeding starts.
| Period starts | Ends | PMS window |
|---|
Your calendar
Period
Fertile
Ovulation
What the calculator assumes
It adds your cycle length to the first day of your last period, then repeats. That is the whole method. It holds up while your cycles stay close to the number you entered and drifts as soon as they do not.
A cycle between 21 and 35 days is normal. Varying by a few days from month to month is normal. If yours swing by more than about eight days, take the dates below as a rough middle rather than a schedule, and expect the sixth month to be furthest off.
Where the PMS window comes from
After ovulation the corpus luteum produces progesterone. If there is no pregnancy it breaks down, progesterone falls, and bleeding follows. Premenstrual symptoms track that fall, which is why they cluster in the five or six days before your period rather than at a fixed point in the month.
Symptoms that start at ovulation and run right through to bleeding, or that stop you working, are worth raising with a doctor. Premenstrual dysphoric disorder is a real diagnosis with real treatment, and it gets missed a lot.
Questions people actually ask
How late can a period be before it counts as late?
Most clinicians treat it as late once it is more than seven days past the expected date. A few days either way is ordinary. If you have missed one entirely and pregnancy is possible, take a test rather than waiting.
Why is my period early this month?
You probably ovulated early. Stress, illness, travel across time zones, sharp changes in weight or training, and starting or stopping hormonal contraception all stretch or compress the first half of the cycle. The stretch from ovulation to bleeding stays close to 14 days regardless.
How many days should a period last?
Two to seven days. Soaking a pad or tampon every hour for several hours in a row, bleeding past seven days, or passing clots larger than a coin are all reasons to book an appointment. Heavy bleeding is common and it is also treatable.
Can I move my period?
On combined hormonal contraception you can run packs back to back and skip the withdrawal bleed. Without it, there is no reliable way to shift a cycle already under way. Talk to a pharmacist or doctor before trying anything you read on a forum.
Can I use this to avoid getting pregnant?
No. Fertile windows sit outside the textbook days for most women, and a calendar cannot see the month your ovulation moves. Use contraception you can rely on.