# Period Calculator

> Predict the start and end dates of your next periods from the first day of your last period, your cycle length and how long your period usually lasts.

Category: Health & fitness · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/period-calculator

If your cycle is regular, each period starts one cycle length after the previous one. Enter your last period to see the dates for the coming months.

Also called: Menstrual cycle calculator, Period predictor.

## Inputs
- First day of your last period (default 2026-01-05)
- Usual cycle length (unit days; default 28)
- Period length (unit days; default 5)
- Periods to show (default 6; optional)

## Results
- Next period starts
- Next period ends
- The one after starts

## Formula

```
Period n starts = LMP + n × cycle length;  ends = start + period length − 1;  ovulation ≈ start − 14 days
```

Where:
- LMP: first day of the last menstrual period
- n: 1 for the next period, 2 for the one after, and so on

## Worked example
1. Next period: 5 Jan + 28 days = 2 Feb 2026.
2. It lasts 5 days, so it ends on 6 Feb (2 Feb + 4 days).
3. The one after: 5 Jan + 56 days = 2 Mar 2026.

**Result:** With a 28-day cycle that started on 5 January 2026, the next period is due on 2 February and the one after on 2 March.

## What the result means

A period arriving a few days early or late is common. If periods stop for three months or more, become very irregular or are unusually heavy or painful, it is worth seeing a doctor or nurse.

## Assumptions and limits
- Your cycle length stays the same from month to month.
- Ovulation is estimated 14 days before each period.
- Predictions are less reliable for irregular cycles, in the first years after periods start, around menopause, after pregnancy or when changing contraception.
- Not suitable for avoiding pregnancy.

## Questions

### How do I work out my cycle length?

Count from the first day of one period to the day before the next one starts; average several cycles for a better figure.

### Is a 28-day cycle normal?

It is the average, but longer and shorter cycles are also normal. What matters most is what is typical for you.

## Sources
- [Tommy’s: Ovulation and fertility](https://www.tommys.org/pregnancy-information/planning-pregnancy/how-get-pregnant/ovulation-and-fertility)

## Related calculators
- [Ovulation Calculator](https://calculators.geekmidst.com/calculators/ovulation-calculator)
- [Conception Calculator](https://calculators.geekmidst.com/calculators/conception-calculator)
- [Date Difference Calculator](https://calculators.geekmidst.com/calculators/date-difference-calculator)

_This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/period-calculator