Period Calculator

Your next period is estimated by adding your average cycle length to the first day of your last period. Most cycles run 21-35 days, with 28 days the commonly cited average, though it's normal to vary a few days month to month. Enter your dates below for a personalized estimate, upcoming periods, and fertile windows for the next 6 months — or scroll to how late is my period? if that's what brought you here.

Not sure? Most cycles are 26-32 days. Count from the first day of one period to the first day of the next.

This is an estimate based on averages. If your period is unexpectedly late, consider a pregnancy test; if cycles are regularly irregular, missing, or unusually heavy, talk to your healthcare provider.

How late is my period?

"Late" simply means today has passed the date you expected your period to start. To find out how many days late you are, compare two dates: the expected start date (your last period's start date plus your average cycle length — the same math the calculator above uses) and today's date. The gap between them, in days, is how late you are. For example, if your calculated expected date was March 10 and today is March 14, you're 4 days late.

Common reasons a cycle shifts

A late period doesn't automatically mean anything is wrong — cycles shift for a lot of ordinary reasons. Stress, travel across time zones, changes in sleep, a new workout routine or a heavier training block, and shifts in weight or diet can all nudge ovulation later than usual. Since your period date is really set by when you ovulate, anything that delays ovulation delays the whole cycle along with it.

If you want to go deeper on what a late period can mean and how to read your own pattern, see our guide on why your period might be late.

What to track besides your period dates

Dates are the easy part. The signal most people miss — and the thing your doctor actually wants — is everything happening between periods. Even logging two or three of these consistently gives you a meaningfully clearer picture of what's normal for your body.

Flow, cramps, and physical symptoms

Track flow as light, medium, heavy, or very heavy by day. Note any clots and whether you're soaking through a pad/tampon faster than every 1-2 hours (clinical sign of heavy menstrual bleeding). For cramps, log severity (1-10), location (lower abdomen, lower back, radiating to thighs), and what helps. Persistent severe pain that interferes with daily life is the #1 underreported symptom of endometriosis.

Mood, energy, and sleep

Mood and energy shift predictably across the cycle for most people — log them daily and the pattern emerges within 2-3 cycles. PMS affects about 75% of women with mild-to-moderate symptoms; PMDD affects 3-8% and looks similar but is severe enough to interfere with daily life. A written log makes the difference visible.

Headaches, skin, and gut

Cycle-linked headaches (often called menstrual migraines) typically hit 2 days before through 3 days into your period. Breakouts, bloating, constipation, and food cravings also follow hormonal patterns. None of these on its own is concerning — but the connection between them and your cycle is data your doctor can actually use.

What ongoing tracking actually unlocks

One calculator predicts your next period. Months of tracking lets you spot the pattern your doctor needs to diagnose PCOS, endometriosis, thyroid issues, or perimenopause — conditions that are highly treatable once identified but can take years to diagnose without data. See our period tracking guide for the full beginner checklist, or our PMS guide for the symptom side of the cycle.

Up next: Want to know if your cycles are actually regular? Run the cycle length calculator. Trying to conceive — or avoid? The ovulation calculator finds your fertile window. Pregnant? The due date calculator is your next stop.

Averages are a guess. Your history is the answer.

Got irregular cycles, post-pill spotting, or post-baby periods that won't settle? Gaia learns from your actual cycle history and gets sharper every period you log — so the next-period date you see is yours, not a textbook 28-day average.

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Common Questions About Your Period

Your next period date is your last period's start date plus your average cycle length. Most cycles fall somewhere between 21 and 35 days, with 28 days being the commonly cited average. Enter your own last period date and cycle length into the calculator above for a personalized estimate, plus your next 6 upcoming periods and fertile windows.

Add your average cycle length to the first day of your last period. The calculator above does this automatically — enter the first day of your last period and select your average cycle length, and it returns your estimated next period date, current cycle day, and current phase, plus a 6-month lookahead.

Take the first day of your last period and add your average cycle length — that's your estimated next period date. So if your period started on March 1 and your cycle is 28 days, you'd expect your next period around March 29. The tricky part is knowing your actual cycle length, which is why tracking for a few months helps. Count from day 1 of one period to day 1 of the next — that's one full cycle.

A normal menstrual cycle is anywhere from 21 to 35 days, with 28 days being the most commonly cited average. But here's the thing — only about 13% of people actually have a perfect 28-day cycle. Most people fall somewhere between 24 and 32 days, and it's completely normal for your cycle to vary by a few days from month to month. A variation of up to 7-9 days between your shortest and longest cycle is still considered regular.

There's no fixed number — cycles that are normally 28 days can occasionally run a week or more past the expected date without anything being wrong, since your period date shifts whenever ovulation shifts. To find out how many days late you are, compare today's date to the expected date the calculator above gives you (last period start plus average cycle length); the difference in days is how late you are. See the "How late is my period?" section above for common reasons cycles shift.

A late period doesn't always mean pregnancy — there are a bunch of reasons your cycle might shift. Stress is a big one, since cortisol can directly delay ovulation. Other common causes include significant weight changes, starting or stopping birth control, illness, travel across time zones, over-exercising, thyroid issues, and PCOS. If your period is more than a week late and you're sexually active, it's worth taking a pregnancy test. If you're consistently late or skipping periods, check in with your doctor.

Absolutely — and it's not just in your head. When you're stressed, your body produces more cortisol, which can suppress GnRH (the hormone that kicks off your whole cycle). This can delay or even prevent ovulation, which pushes your period back. Acute stress (like a big exam or a move) might delay your period by a few days to a week. Chronic stress can cause more significant disruptions, including skipped periods entirely. This is your body's way of saying "now isn't a great time for reproduction."

Most periods last between 3 and 7 days, with 5 days being the average. The heaviest flow is usually on days 1-2, then it gradually tapers off. If your period consistently lasts longer than 7 days, involves soaking through a pad or tampon every hour for several hours, or includes clots larger than a quarter — that's worth mentioning to your doctor. On the flip side, very light periods (1-2 days) can be totally normal for some people, especially on hormonal birth control.

It depends on how irregular we're talking. Cycles that vary by a few days each month? Totally normal — your body isn't a clock. But if your cycles swing by more than 9 days (say, 24 days one month and 38 the next), or you're regularly skipping periods, it's a good idea to talk to your doctor. Irregular periods can sometimes signal conditions like PCOS, thyroid disorders, or hormonal imbalances that are very treatable once identified. Also worth noting: periods are often irregular during the first few years after starting, after stopping birth control, and during perimenopause.

Educational content, not medical advice. This calculator gives you an estimate, not a diagnosis. Persistent cycle irregularity, missed periods, very heavy bleeding, or severe pain are worth discussing with your healthcare provider — they can indicate conditions like PCOS, thyroid issues, endometriosis, or perimenopause that benefit from proper evaluation. Go Go Gaia is a tracking tool, not a substitute for professional medical care.