Pregnancy test calculator for irregular periods

Find the first day a pregnancy test gives a reliable answer, even when you can’t tell when your period is due.

Free, no account. By the Plum team · Updated October 3, 2026

This page is general information, not medical advice. For questions about your own cycle or health, talk to a doctor, nurse, or other clinician who knows you.

Last time you had unprotected sex

First day of your last periodOptional

Your longest cycle latelyOptional, in days. Leave it blank if you don’t know.

Pick the last day you had sex without contraception, or when it failed. This calculator tells you the first day a pregnancy test gives a reliable answer, even if your periods are irregular.

When should I take a pregnancy test if my periods are irregular?

Most tests work from the first day of a missed period. That advice assumes you know when your period was due, and with irregular periods you often don’t. So the NHS gives a second rule. Test at least 21 days after the last time you had unprotected sex. By then a test gives an answer you can trust either way.

That’s the date the calculator shows when you only fill in the first box. It doesn’t depend on your cycle at all, which is the point.

Can I test sooner if I know my cycles?

Sometimes. With irregular periods, a period is only missed once it’s later than your longest cycle. If your cycles have run between 29 and 41 days lately, a period that hasn’t come by day 42 is missed, and a test works from then. Put in the first day of your last period and your longest cycle, and the calculator uses whichever day comes first.

Don’t know your longest cycle? Leave that box empty. The calculator then counts 36 days from the start of your last period, which is what the Office on Women’s Health suggests for irregular periods. If you have a few start dates to hand, the irregular period calculator will tell you your shortest and longest cycle.

What if the test is negative and my period still hasn’t come?

Wait a few days and test again. A negative result is less reliable than a positive one, and testing too early is one reason. The NHS says to see a GP if a second test is negative and your period still hasn’t arrived.

A late period with negative tests usually has another cause. Stress, illness, weight changes, PCOS, thyroid problems and coming off birth control can all push a period back. Our guide to irregular periods goes through the common causes and the signs worth a doctor’s appointment.

When to see a doctor sooner

Don’t wait for a test date if you have a missed period with unusual bleeding and pain low in your tummy, especially on one side. The NHS lists those as possible signs of an ectopic pregnancy. Call NHS 111 or your doctor the same day.

Knowing your longest cycle next time

Plum, a period tracker app for iPhone, keeps every period start date you log, so the gaps between your periods are already counted when you need them. When your cycles vary, it shows a window for your next period instead of pretending to know the day.

Common questions

When can I take a pregnancy test if I have irregular periods?

The NHS says to test at least 21 days after you last had unprotected sex if you don’t know when your period is due. If you know your longest cycle, you can also test the day after it runs out with no period.

Can I take a pregnancy test 2 weeks after sex?

You can, and a positive result then is worth believing. A negative one is less reliable that early. Test again once you reach 21 days after sex.

Is this pregnancy test calculator free?

Yes. There’s no account and no email box. It runs in your browser, and the dates you type are never sent anywhere.

Does it follow the NHS advice?

Yes. The 21-day count comes from the NHS page on doing a pregnancy test. The 36-day count, used when you give a period date but not your longest cycle, comes from the US Office on Women’s Health.

When should I test with PCOS?

The same way. PCOS often makes cycles long or hard to predict, so the 21-day count after sex is usually the one to use.

Track it with PlumGet a period tracker that runs this every month.

Plum is an iPhone app. It learns your own cycle length as you log. It reminds you before your period and the day before your estimated ovulation. Its predictions aren’t contraception. You can also create a group with your friends. Plum has no ads and never sells your data.

Keep reading

Period calculatorWork out your next six periods, fertile windows and ovulation dates from one date.Period calculator for irregular periodsMeasure the real gaps between your periods. See the range your next one is likely to fall in.Late period calculatorCheck whether your period is late, by how many days, and when a pregnancy test will work.Cycle length calculatorCount the days between your periods. See your average cycle length and how it compares with the normal range.Cycle phase calculatorSee which phase of your cycle you’re in on any day, and when each phase starts and ends.PMS calculatorSee when PMS can start before each of your next three periods, and when it usually eases.Irregular periods: common causes and when to see a doctorStress, travel, PCOS and thyroid problems are common reasons periods go irregular. Find out which signs need a doctor and how to track them.How late can a period be?Being a week late once in a while is ordinary. See what counts as late day by day, when a pregnancy test becomes reliable, and when to call a doctor.Tracking your period when you have PCOSPCOS makes cycles long and unpredictable, so app predictions often miss. Learn what to log and what a tracker should do about it.