Cycle Length Calculator: Determine Your Average Cycle Length

Cycle Length Calculator — Add Recent Cycles

Enter cycle start dates or lengths to compute a reliable average and simple predictions.

How to Use Cycle Length Calculator: Determine Your Average Cycle Length

1. Step 1: Choose input mode

   Pick Dates to enter recent period starts or Days to type cycle lengths.

2. Step 2: Add at least 3 cycles

   Enter three to six recent cycles for a stable average.

3. Step 3: Adjust filters (optional)

   Exclude very short or long cycles that do not reflect your typical pattern.

4. Step 4: Review results

   See your average, median, range, standard deviation, and regularity label.

5. Step 5: See predictions

   If a last start date is entered, view estimates for next periods and ovulation.

6. Step 6: Save if needed

   Export a CSV for your personal records; we do not store any data.

Key Features

• Average calculation from cycle history
• Exclude outliers with flexible filters
• Variability metrics (median, range, SD)
• Next period and ovulation estimates
• Local privacy with CSV export

Understanding Results

Formula

Your average cycle length is the sum of all included cycles divided by the number of cycles. The median is the middle value once lengths are sorted. Range shows your shortest and longest included cycles. Standard deviation (SD) measures how much your months tend to vary around the average—the smaller the SD, the more consistent your pattern.

If you entered at least one recent start date, the calculator estimates the next period by adding your average length to your latest start. Because ovulation commonly happens about 14 days before a period begins, we also show a simple ovulation estimate and a short fertile window. These are helpful planning guides, not guarantees.

Reference Ranges & Interpretation

Many adults experience cycles ranging roughly from 21 to 35 days. If your results are consistently within that window—and your SD is low—you likely have a regular pattern. If your lengths vary more widely month to month, consider using the built‑in range filter to exclude clear outliers and focus on what’s typical for you.

Using the cycle length calculator results

Treat the average and variability as guides for planning. Pair them with how you feel and your schedule; revisit after a few more cycles to see trends.

Short‑term changes in routine (sleep, stress, travel) often shift timing by a few days. Long‑term changes, heavy bleeding, skipped periods, or pain that interferes with daily life deserve a professional opinion.

Assumptions & Limitations

This tool is designed for general tracking and planning. It doesn’t replace clinical evaluation or fertility diagnostics. It assumes your entries reflect first‑day bleeding starts and that you’ve recorded a few recent cycles. Hormonal contraception, postpartum shifts, peri‑menopause, and medical conditions can all affect cycle timing and may require specialized guidance.

Complete Guide: Cycle Length Calculator: Determine Your Average Cycle Length

Compute an accurate average from recent dates with the cycle length calculator. Exclude anomalies, view rolling averages, and export your results if needed.

What is cycle length?

Cycle length is the number of days from the first day of one menstrual period to the day before the next period begins. Put simply: count day one when bleeding starts, and keep counting until the day before your next day-one. The result is your cycle length for that interval. Your length may be the same most months, or it may drift slightly—both are common.

The cycle length calculator helps you turn a handful of recent cycles into a single average. When you enter multiple dates or lengths, the tool computes the mean, median, range, and standard deviation so you can see not only the “typical” number but also how much it varies. This provides practical context for planning, whether you’re monitoring changes, managing symptoms, or estimating the timing of your next period.

How the calculator works

You can input either period start dates or cycle lengths in days. When you choose dates, the calculator sorts them chronologically and computes each interval’s length by counting the days between consecutive starts. When you choose days, you simply enter the numbers directly. In both cases, you can optionally exclude values outside a chosen range—by default, cycles shorter than 20 days or longer than 45 days are omitted from the average to reduce the impact of outliers.

After filtering, we calculate your average, median, minimum, maximum, and standard deviation (a measure of variability). If you entered at least one recent start date, we also estimate your next period by adding the average length to your most recent start. You’ll see the next two predicted starts as well. Because ovulation usually happens roughly 14 days before a period begins, the calculator provides a simple ovulation estimate and a short fertile window to help you plan.

How to enter your data for the best average

Aim to enter three to six recent cycles. More data points can give a more stable estimate of your typical length. If you are entering dates, include the first day of bleeding for each period. If you track spotting separately, focus on the first day of full flow for consistency. If you are entering days, round each cycle to a whole number (for example, 27, 28, 29) so the average reflects how you actually experience the calendar month.

You can exclude obvious irregularities—such as a one‑off 15‑day cycle after a medication change—without discarding your entire record. Use the range filters to gently trim extremes. This makes the output more useful for planning while keeping transparency over which values were included (displayed in the table under “Your data”).

What is a “normal” cycle length?

Many adults have cycles around 28 days, but “normal” spans a wider band. Cycles between about 21 and 35 days are commonly considered within a typical range for adults, while teens often have broader variability. What matters most is your personal pattern over time. If your cycles tend to cluster in a narrow window—say, 27 to 30 days—and your average lands around 28, that is normal for you.

Authoritative resources note that cycle characteristics evolve with age and life stage. If you’re curious, you can review guidance from reputable organizations such as the American College of Obstetricians and Gynecologists (ACOG). For general background, seeU.S. Office on Women’s Health: Menstrual Cycle.

Understanding regularity and variability

Our calculator displays standard deviation to quantify how tightly your cycle lengths cluster around the average. A smaller standard deviation means your cycles are more consistent; a larger one means they vary more. We also show a friendly label (for example, “Very regular” or “Variable”) to help you interpret the numbers at a glance. Variability can be affected by stress, travel, sleep, medications, endocrine conditions, and normal life changes.

If your variability increases over several months, look for context. Did your exercise routine change? Are you sleeping less? Are you recovering from illness? The numbers are signals, not judgments. If something feels off or disruptive, consider discussing patterns with a clinician.

Predicting your next period with an average

If you entered at least one recent start date, the calculator adds your average cycle length to your latest day one to estimate the next start. It also projects a couple of periods beyond that so you can plan ahead. Think of these as soft estimates. If your standard deviation is small and your recent inputs are consistent, the predictions often feel accurate; if your cycles vary more, use these dates as a helpful window rather than an exact appointment.

Want a purpose‑built tool just for the next start date? Try our next period calculator. If you track more details—symptoms, duration, or PMS—you might also find our period tracker helpful.

Ovulation and fertile window (simple estimate)

Ovulation typically happens about 14 days before your next period begins, though this can vary from person to person and month to month. Our calculator uses your selected luteal‑phase offset (default 14) to provide a simple ovulation estimate and a short fertile window. If you’re actively trying to conceive—or avoid pregnancy—consider incorporating your own signs (like basal body temperature or cervical mucus) and consult tools designed for fertility timing.

Explore specialized tools: ovulation calculator, fertile window calculator, and luteal phase calculator.

Handling irregular cycles thoughtfully

If your cycles are irregular, averages still offer value, but avoid over‑interpreting one extreme month. Use the range filters to exclude outliers that don’t reflect your typical experience. If cycles are frequently shorter than 21 days or longer than 35–40 days, or if you skip periods for several months, consider asking a clinician to review your pattern—especially if changes are new or disruptive.

For broader planning around your entire menstrual cycle (including period length and symptoms), you can start with our menstrual cycle calculator. It complements the cycle length view by bringing more of your monthly experience into one place.

Tracking tips that make your data more useful

Keep your process simple and consistent. Mark day one when full flow begins (not light spotting). Try to add each month within a day or two so dates remain accurate. If you skip tracking for a month, simply continue the next month—gaps happen. If you switch between dates and days, remember that dates are preferable when possible because they capture your history and enable predictions.

• Enter at least three cycles; six or more is even better.
• Exclude anomalies with the built‑in range filter instead of deleting your data.
• Note any context (stress, travel, new meds) that may explain one odd month.
• Use CSV export to save your data locally if you want a personal record.

Privacy and data ethics

Your entries live only in your browser session. We don’t collect, store, or sell any personal cycle data. If you choose to export a CSV, it downloads directly to your device and remains under your control. We believe sensitive health information belongs to you and should be portable. If you stop using this page, your inputs are gone unless you save them yourself.

When to consider medical care

Seek timely care for patterns like frequent cycles shorter than 21 days or longer than 35–40 days, periods that suddenly stop for multiple months without pregnancy, heavy bleeding that interferes with daily life, or severe pain. These signals don’t always indicate a serious problem, but they are good reasons to check in with a clinician. Our calculator can help you summarize what you’ve observed.

Formula, assumptions, and limitations

The average is the sum of all included cycle lengths divided by the number of included cycles. Median is the middle value once the list is sorted. Range is minimum to maximum. Standard deviation captures typical day‑to‑day spread around your average. Predictions are simple projections: next period ≈ last start + average length; ovulation ≈ next period − luteal days. These are helpful guides, not firm appointments.

Inputs, tracking consistency, health conditions, and life context all shape results. The calculator does not account for hormonal testing, contraceptive use, postpartum shifts, peri‑menopause, or precise fertility markers. For those needs, consider tools designed for fertility charting and consult a clinician if appropriate.

Frequently Asked Questions