Cycle and Fertile Window Calculator

This calculator turns a last-period date and an average cycle length into a calendar estimate. It can help with cycle awareness, but it cannot identify days on which pregnancy is impossible. Do not use the result as your only method of contraception.

What the calculator estimates

The result contains four dates or ranges:

• the estimated start of the next period;
• the most likely ovulation date;
• a six-day estimated fertile window;
• days outside that window, labelled as a lower-fertility estimate, not “safe days.”

The first day of menstrual bleeding is cycle day 1. Enter your usual cycle length in whole days. A cycle average is more useful when it is based on several recent cycles rather than a single month.

Method used

The calculator applies this simplified calendar model:

1. next period = first day of the last period + average cycle length
2. estimated ovulation = next period − 14 days
3. estimated fertile window = ovulation − 5 days through ovulation + 1 day

The six-day range reflects that conception can occur from intercourse in the days before ovulation as well as around ovulation. The actual fertile window can occur earlier or later, even when cycles appear regular.

This is not the same as the Standard Days Method. CDC guidance for that method treats cycle days 8–19 as days requiring abstinence or a barrier method and says it is most suitable for cycles that are usually 26–32 days.

Worked example

Suppose the first day of the last period is January 1, 2026 and the average cycle length is 28 days:

• estimated next period: January 29, 2026;
• estimated ovulation: January 15, 2026;
• estimated fertile window: January 10–16, 2026;
• lower-fertility estimate: January 1–9 and January 17–28, 2026.

Those lower-fertility dates still carry a possibility of pregnancy. For pregnancy prevention, use a contraceptive method appropriate for you and follow its instructions.

How to interpret the result

Use the dates as a planning aid, not as a diagnosis or confirmation of ovulation. Calendar estimates do not observe hormone changes, cervical mucus, basal body temperature, pregnancy, recent childbirth, breastfeeding, perimenopause, illness, medication effects, or sudden cycle changes.

The estimate becomes less dependable when cycles vary substantially. CDC notes that the Standard Days Method may not be appropriate when two or more cycles in a year are shorter than 26 days or longer than 32 days. Fertility-awareness methods also do not protect against sexually transmitted infections.

When to seek appropriate care

Consider speaking with a qualified health professional when periods are repeatedly absent, unusually painful, very heavy, or newly irregular, or when you have questions about fertility, pregnancy, emergency contraception, or choosing birth control. If unprotected intercourse has already occurred and pregnancy is not desired, a calendar result should not delay time-sensitive medical advice.

Sources

• CDC — Standard Days Method
• CDC — Contraception and Birth Control Methods
• World Health Organization — Selected practice recommendations for contraceptive use, fourth edition

Editorial record

Written and manually reviewed by the SoupCalc Editorial Team. Last reviewed: August 10, 2026. No external medical reviewer participated. This educational calculator is not medical advice and is not approved for indexing or advertising until its localized versions complete the same review.