# Fertile window calculator

> The fertile window is the stretch of about 6 days when pregnancy is possible: the 5 days before ovulation plus ovulation day. Ovulation comes about 14 days before the next period. With a period that started November 1 and a 28-day cycle, ovulation falls on November 15 and the window runs from November 10 to November 15. An estimate, not birth control.

Source: https://www.calterix.com/en-us/calculator/fertile-window/ · Updated 11/06/2026 · en-US

## How the fertile window is calculated

The calculator starts with the **first day of your last period** and your average cycle length. The phase after ovulation (the luteal phase) stays fairly steady at about 14 days. That is why ovulation is counted back from the next period instead of fixed on day 14.

```
Ovulation = first day of last period + cycle length − 14 days
Fertile window = 5 days before ovulation through ovulation day
```
Example: period starts November 1, cycle of 28 days. Ovulation = November 1 + 28 − 14 = November 15. Window = November 10 to November 15. Next period around November 29.

The window is about 6 days because sperm can survive up to 5 days in the body, while the egg can be fertilized for 12 to 24 hours. The two days before ovulation and ovulation day carry the highest chance of pregnancy.

To pin down the ovulation day and ways to confirm it, see the [ovulation calculator](https://www.calterix.com/en-us/calculator/ovulation/). To predict upcoming dates, use the [period calculator](https://www.calterix.com/en-us/calculator/period/).

## Ovulation day by cycle length

| Cycle length | Likely ovulation day | Fertile window (cycle days) |
| --- | --- | --- |
| 21 days | Day 7 | Days 2 to 7 |
| 24 days | Day 10 | Days 5 to 10 |
| 26 days | Day 12 | Days 7 to 12 |
| 28 days | Day 14 | Days 9 to 14 |
| 30 days | Day 16 | Days 11 to 16 |
| 32 days | Day 18 | Days 13 to 18 |
| 35 days | Day 21 | Days 16 to 21 |
| 38 days | Day 24 | Days 19 to 24 |

Day 1 is the first day of the period. Ovulation falls on day (cycle length − 14).

## Irregular cycles and other limits

When cycles change from month to month, so does ovulation. The **irregular cycle** tab uses the calendar method: the first fertile day is the shortest cycle minus 18, and the last is the longest cycle minus 11. The resulting window is much wider than 6 days, which shows how much accuracy is lost.

- Stress, illness, travel, breastfeeding, and recently stopping birth control can move ovulation earlier or later.
- Cycles outside 21 to 35 days, or ones that vary a lot, are worth discussing with a gynecologist.
- LH tests, basal body temperature, and cervical mucus help confirm ovulation. The calculated date is only a forecast.

> This calculator is not birth control. Calendar-based methods fail more often than hormonal and barrier methods. To prevent pregnancy, talk with a health care provider.

## Frequently asked questions

### How do I calculate my fertile window?

Add your cycle length to the first day of your last period and subtract 14 days. That is ovulation. The fertile window is the 5 days before it plus ovulation day. With a 28-day cycle and a period starting November 1, the window is November 10 to 15.

### What days am I most fertile after my period?

In a 28-day cycle, the fertile days fall between cycle day 9 and day 14, counting from the first day of the period. Shorter cycles reach the window sooner, and longer cycles later.

### Do I always ovulate on day 14?

No. Ovulation comes about 14 days before the next period. Only in a 28-day cycle does that match day 14. In a 32-day cycle, it falls on day 18.

### Can I get pregnant outside my fertile window?

It is less likely but possible, because ovulation can come early or late and the calculated dates are estimates. For that reason the calculator is not a way to avoid pregnancy.

### How do I find my fertile window with irregular periods?

Use the irregular cycle tab with your shortest and longest cycle from recent months. The window gets wider and less precise. With very irregular cycles, tracking with a gynecologist works better.

### How long does the fertile window last?

About 6 days a cycle: the 5 days before ovulation and ovulation day itself. The egg can be fertilized for 12 to 24 hours after release.