# Period Calculator

URL: https://speedycalculator.com/calculators/health/period/
Last updated: 2026-10-25

> The period calculator predicts your next three periods from the first day of your last period and your average cycle length, plus the expected end of the current period.

## Inputs

- First day of last period
- Cycle length (days)
- Period length (days)

## Results

- Next period
- Following period
- Third period
- Current period ends

## Formula

```
Next period = LMP + cycle × n
```

## Explanation

Tracking your cycle helps you plan ahead and notice changes worth discussing with a doctor.

### How to use the Period Calculator

- **First day of last period**
- **Cycle length**
- **Period length**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Each future period is predicted by adding your average cycle length.

### Inputs explained

FieldWhat to enterFirst day of last periodA dateCycle lengthIn daysPeriod lengthIn days

### What each result means

- **Next period** = [First day of last period] + [Cycle length]
- **Following period** = [First day of last period] + 2×[Cycle length]
- **Third period** = [First day of last period] + 3×[Cycle length]
- **Current period ends** = [First day of last period] + [Period length] - 1

*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — First day of last period: 2026-10-01; Cycle length: 28 days; Period length: 5 days.

Results — **Next period = Thu, October 29, 2026**; Following period = Thu, November 26, 2026; Third period = Thu, December 24, 2026; Current period ends = Mon, October 5, 2026.

## FAQ

**What is a normal cycle length?**
Between 21 and 35 days in adults.

**When should I see a doctor?**
If periods stop, become very irregular, very heavy or very painful.

**What formula does the Period Calculator use?**
Next period = LMP + cycle × n.