Health & fitness · FREE ONLINE TOOL

Period Calculator

Project a next-period date from menstrual history and assumed cycle length, with associated cycle estimates and their limitations.

Your inputs

First day of last menstrual period

Calculations run in your browser. No account needed.

EXAMPLE RESULTCalculated locally
Next period estimate
2026-07-29
Estimated ovulation
2026-07-15
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My workspace

Calculate with your inputs to save a result or compare scenarios.

Detailed breakdown

All 6 rows

The complete calculation, row by row.

Swipe or scroll horizontally for more columns.
Detailed breakdown · All 6 rows
CycleNext periodOvulation estimatePossible fertile windowEstimated bleeding end
12026-07-292026-07-152026-07-10 to 2026-07-152026-08-02
22026-08-262026-08-122026-08-07 to 2026-08-122026-08-30
32026-09-232026-09-092026-09-04 to 2026-09-092026-09-27
42026-10-212026-10-072026-10-02 to 2026-10-072026-10-25
52026-11-182026-11-042026-10-30 to 2026-11-042026-11-22
62026-12-162026-12-022026-11-27 to 2026-12-022026-12-20

How it works

A useful distinction

Forecast errors accumulate across cycles

If actual cycles are consistently 30 days but the entered assumption is 28, the sixth projected start is 12 days early. More displayed dates extend the same assumption; they do not increase prediction confidence.

Planning with an estimated period date

A period-date estimate extends the entered cycle pattern into the future. Check that the starting date and cycle length describe the information you intend to use. Cycles can vary, so a predicted date is not a guarantee or a diagnosis when a period arrives earlier or later. The calculation does not identify the reason for a change in a person's cycle.

Cycle estimate: formula and assumptions

Next period = start + cycle length. Estimated ovulation = next period − luteal phase.

Cycle length varies. The fertile window shown is the five days before estimated ovulation plus ovulation day.

Worked example

Using the Cycle estimate example:

Inputs
First day of last menstrual period
2026-07-01
Typical cycle length (days)
28
Assumed luteal phase (days)
14
Number of cycles
6
Typical bleeding duration (days)
5
Method

Next period = start + cycle length. Estimated ovulation = next period − luteal phase.

Result

Example result

Next period estimate: 2026-07-29

Estimated ovulation: 2026-07-15

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Do not use calendar estimates alone to prevent pregnancy. This tool does not confirm ovulation.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

How is the next period date calculated?

The model adds the entered cycle length to the period start date. It assumes that cycle length repeats.

Does a predicted date explain a late or missed period?

No. The calculator projects dates and cannot determine the reason for a change in menstrual timing.

Prepared by Finflo. See our methodology and editorial policy. Read the relevant professional-advice limitations.