Calculator Cycle
Period calculator
Enter your last period and your usual cycle length to predict your next few periods, plus your estimated ovulation and fertile window. Everything is calculated privately on your device.
Your next period is expected around
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By the numbers Context
How period prediction works
Your menstrual cycle is counted from the first day of one period to the first day of the next. A period calculator takes your last start date and your typical cycle length and simply counts forward, projecting when your next few periods are likely to begin. It also estimates ovulation and your fertile window, since those are tied to the cycle.
The key thing to understand is variability. Cycle length is normal anywhere from 21 to 35 days in adults, and most people’s cycles drift by a few days from month to month — affected by stress, sleep, travel, illness, and life stage. That is why calendar prediction is a helpful planning tool but not a precise one, and why it is unreliable as a form of birth control on its own.
21–35 days
the normal range for an adult menstrual cycle
ACOG
~28 days
the average cycle length
ACOG
~14 days
the typical luteal phase — from ovulation to the next period
Clinical
Sources: American College of Obstetricians and Gynecologists (ACOG); standard reproductive physiology.
Frequently asked questions
How do I calculate my next period?
Take the first day of your last period and add your average cycle length (the number of days from the start of one period to the start of the next — around 28 on average). This calculator does that and projects your next several periods.
What is a normal menstrual cycle length?
Anywhere from 21 to 35 days is considered normal for adults. Teenagers can have longer, more irregular cycles. What matters more than the exact number is whether your cycle is roughly consistent for you.
How accurate is a period calculator?
It is a good estimate for people with fairly regular cycles, but it cannot account for the natural month-to-month variation caused by stress, illness, travel, or hormonal changes. Treat the dates as a guide, not a certainty.
Can I use a period calculator for birth control?
No. Calendar-based prediction is unreliable for preventing pregnancy on its own, because ovulation timing shifts and sperm can survive several days. For contraception, use a proven method and talk to a clinician.
Why is my period irregular?
Occasional variation is normal. Persistent irregularity can be caused by stress, significant weight change, thyroid issues, PCOS, perimenopause, or certain medications. If your cycles are consistently unpredictable, very heavy, or absent, it is worth seeing a clinician.