Period Calculator

Medically reviewed by [Reviewer Name, credentials] · Last updated October 22, 2026

How to use this period calculator

Our period calculator is designed to give you an accurate next period calculator estimate in under a minute, with no sign-up required.

1. Enter your last period

Tap the date field and select the first day your last period started.

2. Set your cycle details

Adjust your average cycle and period length — or use the 28-day default.

3. Get instant results

See your next period, ovulation day, fertile window and a 3-month calendar.

How to calculate your next period date manually

You don't need an app to make a rough prediction. The manual method behind every period date calculator is simple arithmetic: add your average cycle length to the first day of your last period.

Worked example: if your last period started on June 1st and your average cycle length is 28 days, your next period is estimated to start on June 29th (June 1 + 28 days). If your cycle length is 30 days instead, you'd add 30 days to land on July 1st. This is exactly what our period calculator automates, while also layering in ovulation and fertile-window estimates.

Understanding your menstrual cycle — the 4 phases

A menstrual cycle has four phases driven by shifting hormone levels: menstrual, follicular, ovulation, and luteal. Knowing which phase you're in helps explain energy, mood, and physical changes throughout the month.

Menstrual phase

The uterine lining sheds (your period). Typically lasts 2–7 days.

Follicular phase

Starts on day 1 of your period and lasts until ovulation. Estrogen rises as the body prepares an egg.

Ovulation phase

An egg is released, usually around day 14 in a 28-day cycle. This is the most fertile time.

Luteal phase

After ovulation until your next period. Progesterone rises; PMS symptoms often appear here.

What is a normal cycle length?

For most adults, a normal menstrual cycle length ranges from 21 to 35 days, counted from the first day of one period to the first day of the next. Teenagers, whose cycles are still regulating, can have cycles up to 45 days and still be within a healthy range. Both a 28-day cycle and a 30-day cycle are perfectly normal — there is no single "correct" length, only a healthy range.

Period calculator for irregular periods

If your cycle length varies from month to month, a single fixed-length prediction isn't very useful. Our period calculator for irregular periods lets you enter your last several cycle lengths; it averages them and widens the predicted period and fertile window into a realistic range rather than one exact date. If your cycles are consistently very short, very long, or absent, it's worth mentioning to a doctor.

Ovulation and fertile window explained

Ovulation typically happens about 14 days before your next period starts — not 14 days after your last one, which is a common misconception in shorter or longer cycles. The fertile window is the stretch of days when pregnancy is possible: roughly five days before ovulation through one day after, since sperm can survive several days and the egg is viable for about 24 hours after release.

Period calculator and pregnancy

A missed period is often the first sign of pregnancy. If your period doesn't arrive on its predicted date, especially alongside other symptoms, a home pregnancy test is the next step. Our calculator also gives an estimated due date, calculated as 280 days (40 weeks) from the first day of your last period, adjusted slightly for cycle lengths that differ from the 28-day average.

Your first period — guidance for teens and parents

A first period (menarche) commonly arrives between ages 10 and 15. Early cycles are often irregular while the body's hormone patterns settle, which can take a year or two. For teens tracking a first period calculator estimate, treat early predictions as rough guides rather than precise dates, and don't worry if timing shifts from month to month at first.

Signs your period is coming

Common PMS (premenstrual syndrome) symptoms in the days before a period include cramping, bloating, breast tenderness, headaches, fatigue, acne, food cravings, and mood swings. Symptoms vary widely between people and even between cycles for the same person.

When to see a doctor

Consider talking to a healthcare provider if you experience: cycles consistently shorter than 21 days or longer than 45 days, a missed period without pregnancy, periods that stop for three or more months, bleeding so heavy you soak through protection hourly, or pain severe enough to disrupt daily life. These can be signs of an underlying condition worth investigating.

Periodly vs. period tracker apps

Many period tracker apps require an account, collect detailed health data, and may share or sell that data to advertisers. Periodly works differently: there's no sign-up, no account, and no server-side storage of your cycle data. Every calculation happens directly in your browser, and nothing about your cycle ever leaves your device unless you explicitly export it yourself.

Frequently asked questions

How can I calculate my period?+

Enter the first day of your last period, your average cycle length (usually 21–35 days), and your typical period length into the period calculator above. It instantly predicts your next period, ovulation day, and fertile window based on that information.

How do I know if my cycle is 28 or 30 days?+

Your cycle length is the number of days from the first day of one period to the first day of the next. Track the start date of two or three consecutive periods and count the days between them — the average is your typical cycle length. Both 28 and 30 days are considered normal.

How to confirm my period is coming?+

Common signs include mild cramping, breast tenderness, bloating, mood changes, fatigue, and skin changes in the days before your period (PMS). A period calculator gives you an estimated date, but your body's own signals are a helpful secondary confirmation.

What age can a girl start her period?+

Most girls get their first period (menarche) between ages 10 and 15, with 12 being the average in many countries. Starting earlier (around 9) or a bit later (up to 16) can still be within a normal range — if a period hasn't started by 16, it's worth checking with a doctor.

How accurate is a period calculator?+

A period calculator is highly accurate for people with regular, predictable cycles, typically landing within a day or two of the actual start date. Accuracy decreases for irregular cycles, which is why we widen the prediction range when you tell us your cycles vary.

Can a period calculator be used as birth control?+

No. Period and fertility calculators provide estimates, not guarantees. Ovulation can shift due to stress, illness, travel, or many other factors, so predictions should never be relied on as a method of contraception. Talk to a healthcare provider about reliable birth control options.

Can I use it with irregular periods?+

Yes. Toggle "My cycles are irregular" and enter your last several cycle lengths. The calculator will use their average and show a wider prediction range (from your shortest to longest recent cycle) instead of a single precise date.

Is my data stored?+

No. All calculations happen in your browser — nothing is sent to a server, and Periodly has no account system or database. If you accept functional cookies, your last inputs are saved only in your own browser's local storage so the form can remember them next time.