🗓️ Menstrual Cycle Length Calculator – Track, Analyse & Predict
The Menstrual Cycle Length Calculator turns a simple list of period start dates — or previously known cycle lengths — into a complete statistical picture of your cycle history. It computes your average, median, shortest, and longest cycle, measures variability, and uses those figures to predict your next period, estimated ovulation date, and fertile window.
What Is Menstrual Cycle Length?
A menstrual cycle is counted from day 1 of one period (the first day of full bleeding, not spotting) to the day before the next period begins. The widely quoted "28-day cycle" is a population average; individual cycles most commonly range from 21 to 35 days, and even a single person's cycle can vary month to month.
How the Calculator Works
You can provide input in two ways:
📅 Date-based mode
Enter the first day of each period in chronological order. The calculator subtracts consecutive dates to derive each cycle length automatically, handling month-boundary and leap-year arithmetic for you.
🔢 Manual mode
If you already know your cycle lengths from a previous app or physical diary, enter them as comma-separated numbers (e.g., 27, 28, 30, 26, 31). Statistics and predictions are computed identically.
Core Formulas
| Metric | Formula |
|---|---|
| Cycle length | Next period date − Previous period date |
| Average | Sum of lengths ÷ Number of cycles |
| Median | Middle value when lengths are sorted |
| Standard deviation | √(average of squared deviations from the mean) |
| Next period estimate | Current cycle start + Average cycle length |
| Estimated ovulation | Next period estimate − Luteal phase length |
| Fertile window | Ovulation date − 5 days → Ovulation date + 1 day |
Understanding Regularity
The calculator classifies cycles as Fairly regular when the spread (longest minus shortest cycle) is within the configurable irregularity threshold (default: 7 days). A wider spread is flagged as Variable. The standard deviation provides a complementary picture — a lower value means cycle lengths cluster tightly around the average, while a higher value signals more month-to-month fluctuation.
Next-Period & Ovulation Predictions
Provide a current cycle start date to unlock predictions. The next period is projected by adding the average cycle length to that date. Ovulation timing is then estimated by subtracting the luteal phase length (the time from ovulation to the next period — typically 12–16 days, defaulting to 14). The fertile window spans from 5 days before ovulation through the day after, reflecting the maximum viable lifespan of sperm.
Cycle Phases Explained
Menstrual
Days 1–5 (avg)
Follicular
Days 1–13 (avg)
Ovulation
~Day 14 (avg)
Luteal
Days 15–28 (avg)
The follicular phase (overlapping with menstruation) sees follicle-stimulating hormone rise to mature an egg. An LH surge triggers ovulation. The luteal phase follows — progesterone rises to prepare the uterine lining. If implantation does not occur, progesterone drops and the next period begins.
Tips for Accurate Tracking
Always record day 1 as the first day of full bleeding, not spotting, to keep entries consistent.
Enter at least 3 cycles for reliable average and regularity statistics; 6+ cycles give a stronger picture.
Use the average cycle override if you already have a well-established average from years of tracking and only want fresh predictions.
Adjust the luteal phase length if you have BBT charting or OPK data suggesting your personal luteal phase differs from 14 days.
If any calculated cycle falls outside 15–90 days, the tool flags a warning — double-check those date entries for typos.
Medical Disclaimer
All outputs — including next-period estimates, ovulation dates, and fertile windows — are statistical projections based on recorded cycle history. They are not a substitute for professional medical advice, diagnosis, or treatment. Cycle predictions should notbe relied upon as a form of contraception. Individuals with very short (< 21 days), very long (> 35 days), or highly irregular cycles are encouraged to consult a gynaecologist or reproductive health specialist.