Knowing when your period will arrive is one of those small acts of planning that disproportionately reduces life friction.
About this calculator
Knowing when your period will arrive is one of those small acts of planning that disproportionately reduces life friction. Booking travel, attending a wedding, scheduling a sports event, preparing for a long workday — having even a 1-2 day prediction radius makes things smoother. This calculator is a simple, doctor-recommended forward-projection tool that uses your average cycle length to estimate the next 3-12 periods.
The accuracy depends on how regular your cycles are. The medically defined 'normal' range is 21-35 days for adults and 21-45 days for teens within the first few years after menarche. If your cycles consistently fall in that range with variation under 7 days, the prediction is usually accurate to within 2 days. If cycles vary by more than a week, the prediction becomes a rough guide -- consider tracking for 3-6 months to find your true average, or use a basal body temperature method which detects ovulation directly.
Missed periods, persistent irregularity, very heavy or painful bleeding, or cycles shorter than 21 days or longer than 35 days warrant a check-up with a gynaecologist. Conditions like PCOS, thyroid disorders, perimenopause and hormonal imbalances often present as cycle changes. For pregnancy planning, pair this with the [[ovulation]] calculator to identify your fertile window, and the [[pregnancy]] calculator once you've conceived. The [[bmi]] calculator is also worth checking -- both very low and very high BMI can disrupt cycles.
Common uses
- Planning travel, weddings or events around expected period dates
- Tracking irregular cycles over 3-6 months to identify patterns
- Coordinating contraception, fertility tracking or birth control schedules
- Preparing supplies and pain medication ahead of time
- Spotting early signs of cycle changes from stress, weight or health shifts
Frequently asked questions
What counts as a 'normal' cycle length?
21 to 35 days from one period start to the next is considered normal in adults. Teens and women within 2 years of menopause can have normal variation up to 45 days. Variation of less than 7 days between consecutive cycles is also expected.
How do I track my cycle length accurately?
Mark the first day of bleeding (not spotting) for at least 3 consecutive cycles. Count from that first day to the first day of the next period -- that
Why is my period suddenly shorter or longer?
Common triggers are stress, travel across time zones, sudden weight loss or gain, intense exercise, breastfeeding, hormonal changes and starting or stopping birth control. Occasional one-off variation is normal; persistent change over 3+ cycles needs a doctor
Can this calculator predict ovulation too?
It predicts cycle start dates only. Use the dedicated [[ovulation]] calculator for fertile window predictions -- ovulation timing depends on luteal phase length, which is more nuanced than just cycle length.
What if my period is very late?
If you