Menstrual Cycle Length Calculator

Enter recent period start dates (at least 2):

Enter at least two recent period start dates to calculate your average cycle length.

Average length

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Shortest

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Longest

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Regularity

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Predicted next periods

Predictions use the average interval from your entered dates. Real cycles vary and predictions are approximate.

What this calculator does

The Menstrual Cycle Length Calculator estimates your average cycle length, shows the shortest and longest recent cycles, gives a quick regularity indicator, and predicts your next few period start dates based on your entries.

Results depend on the dates you provide and are approximate. Cycle length naturally varies.

How the calculator works

Enter at least two recent period start dates (you can add up to six).
The tool computes day gaps between each consecutive pair of dates.
It reports the average, shortest, and longest gap.
Regularity is estimated from how close your shortest and longest cycles are.
Predictions add the average gap to your most recent start date for the next few months.

What to enter

Start dates only (first day of bleeding) — not ovulation or mid-cycle symptoms.
Use your most recent dates first for more relevant predictions.
Two to six dates works best. More dates can smooth random variation.

Tip: If you forgot a date, check calendar photos, messages, or a health app to jog memory.

What we calculate

Value

Meaning

Average length

The mean days between your entered start dates.

Shortest cycle

Minimum days between any two consecutive starts.

Longest cycle

Maximum days between any two consecutive starts.

Regularity

Quick indicator based on shortest vs longest spread.

Next periods

Predicted starts by adding the average to your last date.

Predictions are estimates and won’t account for stress, illness, travel, or hormonal changes.

How to read the results

Average ~28 days is common, but healthy cycles can vary roughly from 21–35 days.
Large spread between shortest and longest suggests more variability — this is normal for many people.
Predicted dates help planning; always allow a few days of flexibility.

Helpful tips

Log dates as you go to avoid guesswork.
Include dates from months with unusual stress or travel — it reflects real life.
Use the calculator regularly to see patterns over time.

Common mistakes

Entering the end of the period instead of the first day.
Skipping months with a period — this can distort averages.
Using approximate dates without noting uncertainty.

Frequently asked questions

My cycles are irregular. Will this still help?

Yes. The average gives a planning baseline and the spread shows variability. Treat predictions as broad estimates.

Can this predict ovulation?

No. This calculator looks at start dates only. Ovulation timing varies; consider dedicated ovulation tracking methods.

How many months of data should I enter?

Two is the minimum; more (3–6) gives a smoother, more reliable average.

Do medications or life changes affect predictions?

They can. Hormonal birth control, postpartum changes, illness, or high stress can shift cycle length significantly.

When to seek advice

Bleeding much heavier or longer than usual.
Cycles consistently < 21 or > 35 days (not on hormonal methods).
Severe pain, passing large clots, or sudden cycle changes.

Important note

This tool is for information only and doesn’t diagnose conditions. Consult a qualified clinician for personalized care.