# Ovulation Calculator

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

> The ovulation calculator estimates your next ovulation day, fertile window and next period from the first day of your last period and your average cycle length. Ovulation usually happens about 14 days before the next period.

## Inputs

- First day of last period
- Average cycle length (days)
- Luteal phase (days)

## Results

- Estimated ovulation
- Fertile window starts
- Fertile window ends
- Next period

## Formula

```
Next period = LMP + cycle length
Ovulation ≈ next period − 14 days
Fertile window = ovulation − 5 days to ovulation + 1 day
```

## Explanation

Sperm can survive up to five days, so the most fertile days are the five before ovulation and the day of ovulation itself.

### How to use the Ovulation Calculator

- **First day of last period**
- **Average cycle length**
- **Luteal phase**

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

### How it works

The calculator counts forward one cycle and back 14 days for the luteal phase.

### Inputs explained

FieldWhat to enterFirst day of last periodA dateAverage cycle lengthIn daysLuteal phaseIn days

### What each result means

- **Estimated ovulation** = [First day of last period] + [Average cycle length] - [Luteal phase]
- **Fertile window starts** = [Estimated ovulation] - 5
- **Fertile window ends** = [Estimated ovulation] + 1
- **Next period** = [First day of last period] + [Average cycle length]

*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; Average cycle length: 28 days; Luteal phase: 14 days.

Results — **Estimated ovulation = Thu, October 15, 2026**; Fertile window starts = Sat, October 10, 2026; Fertile window ends = Fri, October 16, 2026; Next period = Thu, October 29, 2026.

## Key terms

- **Luteal phase**: Days between ovulation and the next period.

## FAQ

**Can I rely on this for contraception?**
No. Calendar estimates are not reliable enough to prevent pregnancy.

**How can I confirm ovulation?**
Ovulation test kits and basal body temperature tracking help.

**What formula does the Ovulation Calculator use?**
Next period = LMP + cycle length.


## Sources

- [NHS – Trying to get pregnant](https://www.nhs.uk/pregnancy/trying-for-a-baby/)