Calculator input form
Enter period start dates from oldest to newest. Add at least two dates for a valid calculation. More dates improve the average and variability estimate.
This tool estimates average cycle length and timing patterns from your entered dates. It supports planning, tracking, and discussion with a healthcare professional.
Example data table
| Interval | From start | To start | Cycle length |
|---|---|---|---|
| Cycle 1 | Jan 03, 2026 | Jan 31, 2026 | 28 days |
| Cycle 2 | Jan 31, 2026 | Feb 28, 2026 | 28 days |
| Cycle 3 | Feb 28, 2026 | Mar 29, 2026 | 29 days |
| Cycle 4 | Mar 29, 2026 | Apr 27, 2026 | 29 days |
Formula used
Cycle length = next period start date − current period start date
Average cycle length = sum of all calculated cycle lengths ÷ number of intervals
Cycle range = longest cycle − shortest cycle
Standard deviation = √[ Σ(x − mean)² ÷ n ]
Estimated next period = latest start date + rounded average cycle length
Estimated ovulation = estimated next period − luteal phase length
Estimated fertile window = ovulation date − 5 days through ovulation date + 1 day
How to use this calculator
- Enter at least two period start dates in order from oldest to newest.
- Add up to six start dates for a stronger average and better variability tracking.
- Enter your typical bleeding duration.
- Adjust luteal phase length only if your clinician advised a different value.
- Click Calculate cycle length.
- Review the average cycle, range, variability, next period estimate, ovulation estimate, and fertile window.
- Use the CSV or PDF buttons to save your summary and interval history.
- Repeat monthly to watch trends over time.
Frequently asked questions
1) What does menstrual cycle length mean?
It is the number of days from the first day of one period to the first day of the next period. It is not the number of bleeding days alone.
2) How many dates should I enter?
Two dates are the minimum. Four to six dates usually give a better average, a clearer variability picture, and a more useful next-period estimate.
3) Why is the average different from my latest cycle?
Your latest cycle may be shorter or longer than your usual pattern. The calculator averages all entered intervals, so one unusual month will not fully control the prediction.
4) Is the ovulation date exact?
No. It is an estimate based on the average cycle and luteal phase length. Stress, illness, travel, medication, and hormonal changes can shift ovulation.
5) What counts as an irregular cycle?
A cycle can be considered irregular when the length varies widely from month to month. Large swings, missed periods, or abrupt changes deserve medical attention.
6) Can this calculator confirm pregnancy?
No. A missed or delayed period can happen for many reasons. Use a pregnancy test and follow up with a healthcare professional for confirmation.
7) Should I change the luteal phase value?
Most people can leave the default. Change it only when you have reliable tracking data or professional guidance showing your typical luteal phase is different.
8) When should I seek medical care?
Seek care for very heavy bleeding, fainting, severe pain, cycles that stay highly irregular, bleeding between periods, or periods that suddenly stop without explanation.