When is my fertility window?
Enter the first day of your last period and your shortest and longest recent cycles. The fertility calculator shows the days you are most likely to get pregnant.
- Fertile from
- Sunday, October 4, 2026
Your fertile window is Sunday, October 4, 2026 to Saturday, October 10, 2026, cycle days 10 to 16.
- Fertile until
- Saturday, October 10, 2026
- First fertile cycle day
- 10
- Last fertile cycle day
- 16
- Fertile days
- 7
- Likely ovulation from
- October 9, 2026
- Likely ovulation to
- October 9, 2026
- Next period from
- October 23, 2026
- Next period by
- October 23, 2026
- Standard Days method
- Your cycles are 26 to 32 days, so the Standard Days method also applies: days 8 to 19, October 2, 2026 to October 13, 2026.
Answer for the example date Friday, October 2, 2026. It changes to today's date when the page loads.
Fertile from: Sunday, October 4, 2026. Your fertile window is Sunday, October 4, 2026 to Saturday, October 10, 2026, cycle days 10 to 16.
When are your fertile days?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
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).
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.
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).
- 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
Each example is checked against the calculator on every build.
- First day of your last period 2026-09-01, Shortest cycle (days) 28, Longest cycle (days) 28 gives Fertile from 2026-09-10, Fertile until 2026-09-16, First fertile cycle day 10, Last fertile cycle day 16, Fertile days 7, Likely ovulation from 2026-09-15, Next period from 2026-09-29, Standard Days method 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)
- First day of your last period 2026-02-10, Shortest cycle (days) 26, Longest cycle (days) 32 gives Fertile from 2026-02-17, Fertile until 2026-03-01, First fertile cycle day 8, Last fertile cycle day 20, Fertile days 13, Likely ovulation from 2026-02-22, Likely ovulation to 2026-02-28, Next period from 2026-03-08, Next period by 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)
- First day of your last period 2028-02-20, Shortest cycle (days) 24, Longest cycle (days) 35 gives Fertile from 2028-02-25, Fertile until 2028-03-13, Fertile days 18, Likely ovulation from 2028-03-01, Likely ovulation to 2028-03-12, Standard Days method 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)
How it works
Cycle day 1 is the first day of your last period, so cycle day k is that date plus k − 1 days. Your shortest cycle S and longest cycle L are each 21 to 45 days, with S no more than L (no answer otherwise). Type the same number twice for regular cycles.
Following ACOG, ovulation is 14 days before the next period starts, and pregnancy can follow sex from 5 days before ovulation to 1 day after it:
- Likely ovulation: from cycle day S − 13 to cycle day L − 13.
- Fertile window: from 5 days before the earliest ovulation to 1 day after the latest: cycle day S − 18 to cycle day L − 12, which is L − S + 7 days.
- Next period: from the last period plus S days to the last period plus L days.
- Standard Days method: when S is at least 26 and L at most 32, the page also gives cycle days 8 to 19 and their dates; otherwise it says the method does not apply.
The dates are for the cycle that starts on the date you type; they do not move on to later cycles.
Worked examples by hand
Regular 28-day cycles from September 1, 2026. Ovulation on day 28 − 13 = 15: September 15. Fertile from day 28 − 18 = 10 (September 10) to day 28 − 12 = 16 (September 16), 7 days. Next period September 29. The cycles fit the Standard Days method: days 8 to 19, September 8 to September 19, 2026.
Cycles of 26 to 32 days from February 10, 2026. Likely ovulation from day 13 (February 22) to day 19 (February 28). Fertile from day 26 − 18 = 8 (February 17) to day 32 − 12 = 20 (March 1), 13 days. Next period from March 8 to March 14.
Cycles of 24 to 35 days from February 20, 2028. Fertile from day 6 (February 25) to day 23 (March 13), counting February 29, 2028: 18 days. Likely ovulation from March 1 to March 12. The Standard Days method does not apply.
Other questions people ask
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.