CalcVitalsPeriod Calculator — Next Period & the Next Three
Your next period is expected one cycle length after the last one started: with a 28-day cycle, a period starting January 1 points to the next on January 29, then February 26. This page projects the next three, with end dates and ovulation estimates.
How it works
Period prediction is simple addition — last period plus your typical cycle length — and genuinely useful: for planning trips, events, training blocks and knowing whether a late period is late-yet-normal or late enough to test. This page projects three cycles ahead so you can see the rhythm across a season, and adds the estimated ovulation day in each cycle (14 days before each expected period).
Accuracy depends entirely on your regularity. Research on cycle tracking shows most women vary by a few days month to month, and a variation of up to about a week across a year is unremarkable. Stress, illness, travel across time zones, weight change and perimenopause all stretch or compress individual cycles, which is why trackers work in probabilities and this page speaks of expected dates.
Two patterns deserve a clinician rather than a calculator: cycles consistently shorter than 21 or longer than 35 days, bleeding longer than 8 days or soaking through protection hourly, severe pain, or a period that stops for more than 90 days without pregnancy. Those are symptoms with causes worth finding, not variations to track.
The method
Next periods = LMP + k × cycle length for k = 1, 2, 3. Period end = start + duration − 1. Ovulation estimate = 14 days before each expected period start (fixed luteal-phase assumption).
Frequently asked questions
How accurate is the prediction?
Within a few days for most people with regular cycles — your own variation is the limit. If your last six cycles spanned 26–30 days, expect predictions to be off by up to four days in either direction.
Is a 35-day cycle normal?
Yes — adult cycles from 21 to 35 days are within the normal range, and teenagers often run longer. What matters medically is a change from your own pattern, or cycles outside that range persistently.
When is a late period worth a pregnancy test?
If your period is a week past its expected date — or past the longest cycle you typically have — a home test is accurate and worth taking. Tests are most reliable from the day of the missed period onward.