Find your estimated fertile days
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How the calendar method works
OWH describes a calendar method: subtract 18 from your shortest cycle to find the start of the fertile window, and 11 from your longest cycle to find the end.[1]
OWH says there are about 6 days in each menstrual cycle when you can get pregnant, called the fertile window.[2] It says fertile windows differ between women, and from month to month in the same woman.[2]
OWH says the average cycle is 28 days, and normal cycles range from 21 to 35 days.[1] It says a period comes 14 to 16 days after ovulation unless pregnancy occurs.[1]
What this calculator cannot do
OWH says its own ovulation calculator gives an estimate of the fertile window and is not a guarantee of pregnancy or of birth control.[2] It says to talk to a doctor or nurse to plan a pregnancy.[2]
Other ways to track ovulation
OWH says that with cervical mucus tracking, you are most fertile at the first sign of wetness after your period.[1] ACOG says cervical mucus becomes thin, slippery and stretchy just before ovulation.[3]
ACOG says a positive LH urine test suggests ovulation in the next 24 to 48 hours.[3]
When to see a doctor
OWH says to see a doctor after 1 year of frequent sex without birth control if you are under 35.[1] It says to see a doctor after 6 months if you are 35 or older.[1]
ACOG recommends an evaluation after 1 year of regular sex without birth control, after 6 months if you are older than 35, and talking with an ob-gyn right away if you are older than 40.[3]
Questions families ask
Can an ovulation calculator prevent pregnancy?
OWH says its own ovulation calculator gives an estimate of the fertile window and is not a guarantee of pregnancy or of birth control.[2]
Does it work with irregular periods?
It says fertile windows differ between women, and from month to month in the same woman.[2] OWH says the average cycle is 28 days, and normal cycles range from 21 to 35 days.[1]
How many fertile days are there in a cycle?
OWH says there are about 6 days in each menstrual cycle when you can get pregnant, called the fertile window.[2]
Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages
Updated 2026-10-24: first published.
Sources
All sources were opened and checked on the date shown.
- Office on Women's Health (HHS). Trying to conceive. Checked October 24, 2026.
- Office on Women's Health (HHS). Ovulation calculator. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. Evaluating Infertility. Checked October 24, 2026.