period tools

Cycle Length Calculator

Find your personal average cycle length from your last few period dates. The more cycles you log, the more accurate your prediction gets — and the better any other period calculator works for you.

First days of your last 0 periods

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

    How to read your result

    Cycle length is the number of days from the first day of one period to the first day of the next. The first day is when full menstrual flow starts — not spotting.

    Your average is the most useful number for any period calculator. Plug it into our Period Calculator instead of the 28-day default and predictions will jump in accuracy.

    Your shortest and longest cycles tell you the range to expect. If they’re close together (within 7 days), your cycles are regular. If they spread wider, your cycles are irregular and harder to predict.

    What counts as a normal cycle length?

    • 21–35 days: normal range for adults.
    • Under 21 days (polymenorrhoea): shorter than average; worth tracking and discussing with your provider if it persists.
    • Over 35 days (oligomenorrhoea): longer than average; common in PCOS, perimenopause, thyroid imbalance.
    • Variation >7 days between cycles: considered irregular even if individual cycles fall within range.

    More on the science of cycle length: What is a normal menstrual cycle length?

    What changes cycle length

    • Stress, illness, sleep loss
    • Significant weight change (up or down)
    • New or intense exercise routine
    • Hormonal contraception (starting or stopping)
    • Polycystic ovary syndrome (PCOS)
    • Thyroid imbalance (hyper- or hypothyroidism)
    • Perimenopause (typically late 30s to early 50s)
    • Breastfeeding and recent postpartum

    One unusual cycle is normal. A pattern of unusual cycles is worth a chat with your healthcare provider — especially if it shows up alongside other symptoms (fatigue, heavy bleeding, weight change, hair changes).

    Frequently asked questions

    How do I find my cycle length?
    Count the days from the first day of one period to the first day of the next period. The first day of a period is the first day of full menstrual flow, not spotting. Track at least three cycles and take the average for the most representative number.
    What is a normal cycle length?
    21 to 35 days for adults. The popular 28-day cycle is the textbook average, not a rule. What matters more than the exact number is consistency — staying within a few days of your own typical length month after month.
    How much variation between cycles is normal?
    Variation of up to 7 days between cycles is generally considered normal. If your cycles vary by more than 7 to 9 days, the pattern is considered irregular and worth a conversation with your healthcare provider. Common reasons for irregularity include stress, weight changes, PCOS, thyroid imbalance, perimenopause, and recently starting or stopping hormonal contraception.
    How many cycles should I track?
    Three months is the minimum to get a meaningful average. Six months is better — it smooths out any one-off variations and gives a clearer picture of your personal pattern. After that, you can plug your average into our Period Calculator for accurate predictions.
    What if I just came off birth control?
    Cycles often take three to six months to settle after stopping hormonal contraception. Variation during that window is normal. Re-track after six months to find your true post-pill cycle length.

    Related tools