Pregnancy Test Calculator (when to take a home pregnancy test)

Understanding the pregnancy test calculator — concept and principle

This calculator tells you the earliest day a home pregnancy test is likely to give a reliable result, from either the first day of your last period and cycle length, or the date you had unprotected sex. It also suggests when to test again if the first result is negative and your period has not come.

Test on the missed-period day: with the default inputs the result is 18 Nov (best day to test).

How it is calculated

From the last period: best day to test = first day of last period + cycle length (the day your period is due)

From the date of sex: best day to test = date of sex + 21 days

Retest if negative and no period = best day + 3 days

Estimated ovulation (last-period option) = best day − 14 days

The principle

Home tests detect the hormone hCG in urine. hCG is made only after the embryo implants in the womb, about 6–12 days after ovulation, and then roughly doubles every two days. Before implantation there is nothing to detect, and soon after it the level may still be below the test's threshold, so testing too early can give a false negative. The NHS advises testing from the first day of a missed period, or at least 21 days after unprotected sex if you do not know when your period is due.

Worked example

Default inputs: last period 21 Oct 2026, 28-day cycle (today 10 Nov 2026).

Expected period and best day to test = 21 Oct + 28 = Wed 18 Nov 2026.
Days to wait from today = 18 Nov − 10 Nov = 8.
Retest if negative and no period from 18 Nov + 3 = Sat 21 Nov.
Estimated ovulation = 18 Nov − 14 = Wed 4 Nov.

How to read the result

A positive result on or after the best day is very likely to be correct. A negative result may be too early, especially if ovulation was late; test again after a few days. If a second test is negative and your period still has not started, speak to a doctor. Early-detection tests may show a result a few days earlier, but with more false negatives.

Common mistakes

Testing with diluted urine late in the day; first-morning urine is more concentrated.
Reading the result outside the time window in the instructions.
Assuming irregular cycles follow the average date.

Keep in mind: This is general information, not a diagnosis. Pain, bleeding or feeling faint in early pregnancy needs urgent medical care.

Frequently asked questions

How early can a pregnancy test detect pregnancy?

Some sensitive tests can detect hCG a few days before a missed period, but the most reliable time is from the day the period is due.

Can a test be positive and the pregnancy not continue?

Yes. Early pregnancy loss is common; a doctor can follow up with blood tests or ultrasound.

Why 21 days after sex?

This allows time for ovulation, fertilisation, implantation and hCG to rise to detectable levels.

Sources: NHS, "Doing a pregnancy test" (nhs.uk) · Wilcox AJ, Baird DD, Weinberg CR, "Time of implantation of the conceptus and loss of pregnancy", N Engl J Med 1999;340:1796–1799

Checked 9 Nov 2026. NHS, “Doing a pregnancy test” (nhs.uk, pregnancy – trying for a baby): test from the first day of a missed period, or at least 21 days after unprotected sex; retest after a few days if negative. Implantation timing: Wilcox AJ et al., N Engl J Med 1999;340:1796–1799.

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