How to calculate your next period date by hand
You don't need an app to get a rough answer. You need two numbers:
- The first day of your last period. Day one is the first day of real bleeding, not spotting.
- Your average cycle length. Count the days from the first day of one period to the first day of the next. Do it for your last three to six cycles and take the average.
Then add the cycle length to the start date. That's your most likely start date. Because almost every cycle drifts a little, treat it as the middle of a window rather than a deadline.
Is my period late?
Late only means something relative to your usual cycle, not to a 28-day textbook. A period up to about 5–7 days after the date you expected is common, even for people whose cycles are usually regular. Stress, illness, travel, sleep changes and recently stopping birth control are the usual reasons.
If there's any chance you could be pregnant, a home test is the first step; most work from the first day of a missed period. If you're not pregnant and it's been more than about six weeks with no period, that's a sensible point to get checked, and you should see a doctor if you've missed three in a row. We go through every scenario in how late can a period be.
Why this calculator gives a range, not a single date
Most period calculators give you one confident date. That date is an average, and averages hide variation: in a large analysis of more than 600,000 cycles, only around 13% were exactly 28 days. If your cycles move by even three or four days, a single date is wrong more often than it's right, and your app starts calling you "late" when you're simply normal. (More on that in the 28-day cycle myth and why period apps get predictions wrong.)
So this tool shows the most likely day and the window around it. The more your cycle varies, the wider the honest window gets.
Calculating your period with irregular cycles or PCOS
If your cycles regularly differ by more than about a week, choose "a week or more" above, or better, switch to Use my period history and enter at least three start dates. The calculator then measures your real variation instead of assuming one. With PCOS, cycles of 35 days or longer and the occasional skipped month are common; read what cycle length looks like with PCOS and whether you can predict your period with PCOS.
Period calculator FAQ
How do I calculate when my next period is due?
Add your average cycle length to the first day of your last period. If your last period started on March 3 and your cycles average 30 days, your next one is most likely around April 2, give or take a few days.
What is a normal cycle length?
For adults, roughly 21 to 35 days is generally considered normal, and your own cycle can vary by a few days month to month. Teens and people approaching menopause often have longer or more variable cycles.
When is my period considered late?
Up to about 5–7 days past the expected date is common. Take a pregnancy test if pregnancy is possible. If you're not pregnant, more than about six weeks without a period is a sensible point to get checked; see a doctor after three missed periods.
Does this work for irregular periods?
Yes, with honest limits. Pick the higher variation option or use the history mode so the window matches your real cycle. No calculator can give a reliable single date for a cycle that varies a lot.
How accurate is a period calculator?
It's an estimate from averages. It's usually right for consistent cycles and often off by days for variable ones, which is why it shows a range. It can't reliably predict ovulation and isn't a form of contraception.
Is my data stored anywhere?
No. Everything is calculated in your browser and nothing you type is sent or saved.
Built and maintained by Kaushik Prasath, founder of Ebb · Last reviewed October 22, 2026 · Sources: ACOG, NHS, Cleveland Clinic, Mayo Clinic; cycle-length data from Bull et al., npj Digital Medicine (2019). Educational, not medical advice and not contraception.