# When is my fertility window?

Finds the fertile days of a cycle from the first day of the last period and the shortest and longest recent cycles: from 5 days before the earliest likely ovulation to 1 day after the latest (ACOG).

- Page: https://www.acalculator.org/health/fertility-calculator
- JSON spec: https://www.acalculator.org/health/fertility-calculator.json
- Version: bad4716269bf

## Default answer

Example with the default inputs (First day of your last period September 25, 2026, Shortest cycle (days) 28, Longest cycle (days) 28) on the example date Friday, October 2, 2026: Your fertile window is Sunday, October 4, 2026 to Saturday, October 10, 2026, cycle days 10 to 16.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| lmp | First day of your last period | The first day of bleeding of your most recent period: day 1 of this cycle. |
| short | Shortest cycle (days) | Your shortest cycle over the last 6 or more: days from the first day of one period to the first day of the next. |
| long | Longest cycle (days) | Your longest cycle over the same months. Type the same number as the shortest if your cycles are regular. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| fertileStart | Fertile from | The first fertile day: 5 days before the earliest likely ovulation. |
| fertileEnd | Fertile until | The last fertile day: 1 day after the latest likely ovulation. |
| firstDay | First fertile cycle day | The first fertile day as a day of the cycle (day 1 is the first day of the period): shortest − 18. |
| lastDay | Last fertile cycle day | The last fertile day as a day of the cycle: longest − 12. |
| days | Fertile days | How many days the window spans: longest − shortest + 7. |
| ovulationFrom | Likely ovulation from | The earliest likely ovulation: 14 days before a period after the shortest cycle. |
| ovulationTo | Likely ovulation to | The latest likely ovulation: 14 days before a period after the longest cycle. |
| nextFrom | Next period from | The earliest start of the next period: the last period plus the shortest cycle. |
| nextTo | Next period by | The latest start of the next period: the last period plus the longest cycle. |
| standard | Standard Days method | Whether the Standard Days method (days 8 to 19) fits your cycles, and its dates when it does. |

## Method

Cycle day 1 is the first day of the last period. Ovulation falls on day (cycle length − 13), 14 days before the next period; the fertile window runs from day (shortest − 18) to day (longest − 12).

## Assumptions

- Ovulation happens about 14 days before the next period starts (ACOG).
- Pregnancy is possible from 5 days before ovulation to 1 day after it: sperm live up to about 5 days and the egg about 1 day.
- Your next cycle is no shorter than your shortest and no longer than your longest recent cycle.
- This is an estimate for planning, not medical advice and not a way to prevent pregnancy.

## Worked examples

1. lmp = 2026-09-01, short = 28, long = 28 gives fertileStart = 2026-09-10, fertileEnd = 2026-09-16, firstDay = 10, lastDay = 16, days = 7, ovulationFrom = 2026-09-15, nextFrom = 2026-09-29, standard = Your cycles are 26 to 32 days, so the Standard Days method also applies: days 8 to 19, September 8, 2026 to September 19, 2026.. Source: ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02).
2. lmp = 2026-02-10, short = 26, long = 32 gives fertileStart = 2026-02-17, fertileEnd = 2026-03-01, firstDay = 8, lastDay = 20, days = 13, ovulationFrom = 2026-02-22, ovulationTo = 2026-02-28, nextFrom = 2026-03-08, nextTo = 2026-03-14. Source: ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02).
3. lmp = 2028-02-20, short = 24, long = 35 gives fertileStart = 2028-02-25, fertileEnd = 2028-03-13, days = 18, ovulationFrom = 2028-03-01, ovulationTo = 2028-03-12, standard = The Standard Days method (days 8 to 19) is only for cycles that are always 26 to 32 days long.. Source: ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02).

## FAQ

### When am I most fertile?

From about 5 days before ovulation to 1 day after it, because sperm can live up to about 5 days and the egg about 1 day (ACOG). Ovulation is usually about 14 days before your next period, so in a 28-day cycle the fertile days are cycle days 10 to 16.

### How does the calculator handle irregular cycles?

It uses your shortest and your longest cycle. The window starts 5 days before the earliest likely ovulation (after the shortest cycle) and ends 1 day after the latest (after the longest). With cycles of 26 to 32 days that is cycle days 8 to 20.

### How many cycles should I track?

Six or more is a good base for the shortest and longest cycle. The more cycles you record, the less likely a new cycle falls outside them.

### What is the Standard Days method?

A method described by ACOG for cycles that are always 26 to 32 days long: it counts cycle days 8 to 19 as the most fertile. The calculator says whether your cycles fit it.

### How is this different from the ovulation calculator?

The ovulation calculator takes one cycle length and gives one ovulation day. This page takes your shortest and longest cycles, so the window widens to cover irregular cycles.

### Can I use this to avoid pregnancy?

Not on its own. Calendar estimates miss when ovulation shifts. ACOG reports that 12 to 24 of 100 people who rely on fertility awareness methods become pregnant in the first year of typical use. Talk to a doctor about birth control.

## Sources

- American College of Obstetricians and Gynecologists, FAQ024 Fertility Awareness-Based Methods of Family Planning (ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after ovulation; sperm live about 3 and up to 5 days, the egg about 24 hours; Standard Days method days 8–19 for cycles of 26 to 32 days; 12–24 of 100 pregnant in the first year of typical use), retrieved 2026-10-02. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning