How to use the period tracker calculator
Prepare the values for this calculation
Summarize recent cycle lengths from entered period-start dates and project the next start using their mean interval. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Period start dates (ISO YYYY-MM-DD, one per line): keep one record per line and preserve the column order shown in the example. Replace the example data with your own complete dataset.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Projected next period start first. With the unchanged example inputs, it is 2026-04-23. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
The result diagram shows the inputs, method and output. A numeric chart is shown only when the calculation supports a meaningful numeric relationship.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Period Tracker Calculator formula
Cycle lengths are day differences between consecutive entered start dates. Next projected start = latest start + rounded mean observed cycle length.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Period start dates (ISO YYYY-MM-DD, one per line) | One row per line | 2026-01-01
2026-01-29
2026-02-26
2026-03-26 |
Worked example
Using the example inputs above, the projected next period start is 2026-04-23.
- Projected next period start
- 2026-04-23
- Mean observed cycle length
- 28 days
- Observed range
- 28–28 days
- Sample SD
- 0 days
- Cycles analyzed
- 3
Results and assumptions
Educational estimate only; do not use this result by itself to diagnose, treat, or make an urgent medical decision. This projection uses only the entered history and does not predict ovulation or guarantee the next start date.
Sources and method references
Fix an input or unexpected result
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.