How this calculator works
Known ovulation mode adds the FDA approximate urine hCG detection window of 12 to 15 days after ovulation in a 28-day cycle. That window is not a guarantee for an individual or a test brand. When the next period date is unknown, sex-date mode uses the NHS advice to test at least 21 days after the last unprotected sex. Expected-period mode uses the entered date for the first missed period, without inferring ovulation from cycle length. The tool compares a test date with that reference; it never estimates a personal false-negative probability or hCG concentration. IVF treatment, hCG-containing medicines and uncertain ovulation require advice from the treating team. The result is an estimate for information only and is not medical advice.
Formula
Ovulation reference dates = known ovulation + 12 and + 15 calendar days. Unknown-period reference = last unprotected sex + 21 calendar days. Expected-period reference = entered expected-period date. Elapsed days = test date - selected starting reference.
Worked example
Known ovulation on January 10, 2026 gives an approximate urine detection window of January 22 to 25. A test on January 21 is 11 days after ovulation and earlier than this reference. If the last unprotected sex was February 10, 2028 and the period date is unknown, the 21-day testing reference is March 2.
Advertisement
Frequently asked questions
Does a date in the window guarantee a positive test if I am pregnant?
No. Ovulation estimates, implantation timing, test sensitivity, urine concentration and instructions all affect detection. A calendar cannot confirm or exclude pregnancy.
Can I enter the day of sex as the day of ovulation?
They are different references. Choose sex-date mode when you do not know ovulation or the next period date, and use the most recent unprotected sex date.
What if the test is negative but my period has not arrived?
Follow the kit instructions about repeating the test and contact a health professional if uncertainty persists. Seek prompt assessment for concerning symptoms rather than relying on these dates.
Can a sensitivity threshold predict an exact testing day?
No. A kit threshold alone does not establish an individual hCG trajectory. This tool deliberately does not turn a threshold into a promised positive-test date.