Worked example: Period Calculator
28-day cycle. This fixed reference example shows one complete scenario; the interactive results above use your current inputs.
View the example inputs
- First day of last menstrual period
- 2026-01-01
- Typical cycle length
- 28 days
- Typical bleeding duration
- 5 days
| Calculated quantity | Example result |
|---|---|
| Estimated next period starts | 2026-01-29 |
| Estimated last bleeding day | 2026-02-02 |
| Calendar ovulation estimate | 2026-01-15 |
| Approximate fertile window starts | 2026-01-10 |
| Approximate fertile window ends | 2026-01-16 |
Understanding calendar-based pregnancy and cycle estimates
Know which date is being estimated
The first day of a menstrual period, an estimated ovulation date, a conception estimate and a due date refer to different events. A due-date calculation using a regular-cycle assumption does not observe fertilization or predict the actual delivery day.
Cycle length and dating conventions affect the calendar arithmetic. The page explains whether a result starts from a menstrual date or works backward from an existing due date. Use the same date convention when comparing the result with another source.
Use the right population and clinical context
Calendar windows cannot confirm ovulation, establish paternity or provide reliable contraception. Irregular cycles and clinical ultrasound findings can change dating. Pregnancy weight references also depend on their stated population, such as a singleton pregnancy and pre-pregnancy BMI.
The worked example shows the formula’s arithmetic. Its dates and reference ranges are illustrative inputs, not an assessment of an individual pregnancy. For care decisions, use dating and targets established with a prenatal clinician.
Formula and calculation method
Next period=LMP+cycle length. Calendar ovulation estimate=next period−14 days. Approximate fertile window=5 days before through 1 day after that estimate.
Scope and Limitations
- Cycle timing varies; this is not reliable contraception, a fertility diagnosis or confirmation of ovulation. Irregular cycles require another approach.
Reference Sources
Frequently asked questions
How does this calculator work?
Next period=LMP+cycle length. Calendar ovulation estimate=next period−14 days. Approximate fertile window=5 days before through 1 day after that estimate.
What assumptions should I check?
Cycle timing varies; this is not reliable contraception, a fertility diagnosis or confirmation of ovulation. Irregular cycles require another approach.
Can I save or print this calculation?
Use Copy link with inputs to reproduce the scenario, Download results for a text record, or Print. Where a schedule is shown, its CSV download includes every row.