When to use this calculator
Estimate the next three period start dates from a typical regular cycle length. These dates assume an unchanged cycle length and are not diagnostic. A missed period has many possible causes. This tool does not confirm pregnancy and must not be used to identify safe days for contraception.
Explore the pregnancy & cycle calendars calculators for other questions in this topic. For a related calculation, use the Pregnancy Due Date Calculator: Estimate a pregnancy due date from the last menstrual period or a known conception date.
PUTTING IT INTO PRACTICE
Period: a worked example
The values below are an illustrative example, not a recommendation. Replace them with measurements or assumptions that match your question.
| Input | Example value |
|---|---|
| First day of last period | 2026-01-01 |
| Typical cycle length (integer days) | 28 |
| Result | Period 1: 2026-01-29 Period 2: 2026-02-26 Period 3: 2026-03-26 |
- Check the input labels and enter the example values above.
- Apply the relationship: Estimated next starts = last start + one, two and three cycle lengths..
- The result is Period 1: 2026-01-29 Period 2: 2026-02-26 Period 3: 2026-03-26 , rounded for display.
How to interpret your result
These dates assume an unchanged cycle length and are not diagnostic. A missed period has many possible causes. This tool does not confirm pregnancy and must not be used to identify safe days for contraception.
These calendars cannot confirm pregnancy, identify all fertile days or diagnose cycle changes. They are unsuitable for contraception. A clinician-established pregnancy date takes precedence over a calendar estimate.
Frequently asked questions
Can a cycle calculator identify safe days for contraception?
No. Ovulation can shift and a calendar cannot reliably identify all fertile days. Use qualified advice about contraception.
When is the period calculator useful?
Estimate the next three period start dates from a typical regular cycle length. These dates assume an unchanged cycle length and are not diagnostic. A missed period has many possible causes. This tool does not confirm pregnancy and must not be used to identify safe days for contraception.
How do I use the period calculator?
Enter first day of last period, typical cycle length (integer days). The result updates automatically. Use the units shown beside each input; the example values illustrate the method.
Which formula does this calculator use?
Estimated next starts = last start + one, two and three cycle lengths..
What should I check before using the result?
These dates assume an unchanged cycle length and are not diagnostic. A missed period has many possible causes. This tool does not confirm pregnancy and must not be used to identify safe days for contraception.
How are results rounded?
Scalar numbers display up to six decimal places, with scientific notation for very large or small values. Text, fraction, date and schedule outputs use the format described on the tool. Intermediate calculations use JavaScript floating-point arithmetic. Keep sensible precision for your measurements.
Further reading
For background on this topic, see NHS: periods and fertility in the menstrual cycle ↗. This is a topic reference, not an external review or endorsement of this tool.
Calculators Map editorial process · Updated · Methods and limitations