Period Calculator
Estimate when your next period will start based on your last period and typical cycle length.
Cycle variability
Cycle length can vary month to month even for people with generally regular cycles — treat this as an estimate, and track over a few months for a more personalized pattern.
Tracking cycle patterns over time
While this calculator uses a single average cycle length, actual cycles often vary by a few days from month to month even in people with generally regular patterns. Tracking your actual start dates over several months tends to produce more accurate predictions than a single average assumption.
Significant or sudden changes in cycle regularity are worth discussing with a healthcare provider, since they can sometimes signal underlying factors worth understanding.
A worked example
With a last period starting July 1 and a 29-day average cycle, the next period is estimated around July 30, with the following one estimated around August 28.
For someone with a shorter 24-day cycle starting on the same date, the next period would be estimated around July 25 instead — 5 days earlier, showing how meaningfully cycle length affects these predictions.
Tracking your cycle over time
A single-cycle estimate is a reasonable starting point, but tracking actual period start dates over 3-6 months typically produces a more personalized and accurate prediction than a generic 28-day assumption, especially if your cycle tends to run shorter or longer than average.
Cycle length can also shift due to stress, travel, illness, or hormonal changes, so even a well-tracked pattern can vary — treating predictions as an estimate rather than a certainty is generally the most realistic approach.
Frequently asked questions
What if my cycle is irregular?
This estimate assumes a consistent cycle length, so it will be less accurate if your cycles vary significantly month to month. Tracking over several cycles can improve accuracy.
What counts as an irregular cycle?
Cycles that vary by more than about 7-9 days from month to month, or fall well outside the typical 21-35 day range, are generally considered irregular and worth tracking closely or discussing with a provider.
Can stress affect period timing?
Yes — stress, significant weight changes, travel, and illness can all shift cycle timing temporarily, which is part of why calendar predictions are estimates rather than guarantees.
How much can cycle length normally vary month to month?
Some natural variation (a few days) is common even for people with generally regular cycles — larger or more frequent variation may be worth discussing with a healthcare provider.