How should I calculate average cycle length?

Count first-day-to-first-day intervals across several recent cycles and average those intervals. Use recorded dates rather than memory when possible.

Does this predict when bleeding will end?

No. It projects start dates only and makes no assumption about period duration.

Can it tell whether my cycles are irregular?

No. One average hides variation. A clinician can assess persistent changes using your history and other relevant information.

Can a late projected period confirm pregnancy?

No. A calendar projection cannot confirm or exclude pregnancy. Follow appropriate testing guidance and seek care when needed.

Why are inputs limited to 21–45 days?

The range keeps this general planning tool bounded. Values outside it deserve individualized context rather than automatic extrapolation.

Project Your Next Period Dates

Build a simple planning calendar from your last period and your own average cycle length, with uncertainty kept visible.

Up to 12 future cycles

Leap-year-safe date arithmetic

No false precision

First day of your last periodUse the first day bleeding began, which is cycle day 1.Your average cycle lengthEnter your observed average from first day to first day (21–45 whole days).Future cycles to showChoose 1–12 projections.

How to Use the Period Calculator

Use dates from cycles you have actually observed. The NHS defines cycle length from the first day of one period to the day before the next, so first-day-to-first-day spacing is the number entered here.

Enter the first day of your most recent period.

Average does not mean exact

Even regular cycles vary. A repeated average creates a clean calendar, not a confidence interval or biological forecast.

A Date-Only Cycle Projection

The calculator adds the same user-provided average interval to cycle day 1 for each future row. It does not infer period duration, symptoms, pregnancy status, or ovulation.

Period Projection Examples

Four 28-day projections

Cycle day 1 and average interval

lastPeriodStart:2026-01-01

averageCycleLength:28

cyclesToProject:4

Projected starts

2026-01-29, 2026-02-26, 2026-03-26, and 2026-04-23

Each date is exactly 28 calendar days after the previous projected start.

Projection across leap day

Cycle day 1 and average interval

lastPeriodStart:2024-02-01

averageCycleLength:28

cyclesToProject:2

Projected starts

2024-02-29 and 2024-03-28

Gregorian date-only arithmetic includes February 29 in a leap year and is unaffected by daylight-saving time.

Update the average

Recalculate when several recent cycles show that your observed average has changed.

Frequently Asked Questions

No. It is the result of adding your average cycle length. Individual cycles can begin earlier or later.

Still have questions about this calculation?

Try the Calculator

Cycle Projection Formula

Cycle day 1 is the first day of bleeding. Future starts are spaced by the average entered by the user.

Formula

projected start n = last period start + n × average cycle length

Scientific Background

NHS guidance explains that cycle length varies and that regular adult cycles can be shorter or longer than 28 days. This tool does not substitute a population average for the user's value and does not turn a calendar average into medical certainty.

Educational planning tool

Dates alone cannot diagnose cycle irregularity or explain bleeding changes. Contact a qualified healthcare professional for individualized assessment, especially for severe pain, very heavy bleeding, pregnancy concerns, or persistent changes.