Free period calculator
Predict your next period in two seconds. Enter the first day of your last period and your usual cycle length, and this period calculator counts forward to mark your next expected period — and the next few after that — using next period = LMP + cycle length, updated live, as you type.
On this page13 sections
Prediction based on average cycle length. Periods can vary due to stress, illness, hormonal changes, and other factors.
Results are estimates. Consult a professional.
What is a period calculator?
A period calculator predicts when your next menstrual period is likely to start. You give it two things — the first day of your last period and your usual cycle length — and it counts forward to mark your next expected start date, and the one after that. It turns the calendar math people used to do by hand into an instant estimate, so you can plan around travel, events, or simply know when to expect bleeding.
The arithmetic is deliberately simple: your next period is your last period plus your cycle length. Because the cycle repeats, the calculator can roll the same step forward to project several upcoming periods. That makes it different from an ovulation calculator, which counts back from your next period to place the fertile window — this tool is about the period itself, not conception timing.
How the period calculator predicts your next period
The prediction is plain calendar addition. The calculator takes the first day of your last period and adds your cycle length to find your next start date — then adds the cycle length again to project the period after that, and so on. No account, upload, or hormone data is required.
- It reads your start date. The first day of your last period — the first day of full flow, not spotting — is the anchor.
- It adds your cycle length. Adding your usual cycle length to that date gives your next expected period.
- It rolls the cycle forward. Repeating the step projects the next several periods, so you can see months ahead at a glance.
A worked example using the period calculator
Priya's last period started on June 3 and her cycles usually run 28 days. She wants to know when her next two periods are due. Here is the arithmetic the calculator runs.
Step 1 — Set the anchor (first day of last period)
Priya enters June 3, the first day of full flow last time. That date is the LMP the calculator counts forward from.
Step 2 — Add the cycle length (LMP + cycle length)
Twenty-eight days after June 3 is July 1, so her next period is expected to start around July 1.
Step 3 — Roll the cycle forward for later periods
Adding another 28 days to July 1 gives July 29, and again gives about August 26 — three predicted starts from a single input.
| Period | Expected start date |
|---|---|
| Last period (entered) | June 3 |
| Next period | July 1 |
| Period after that | July 29 |
| Third projected period | August 26 |
Each row is the previous start plus the 28-day cycle length. The calculator shows these instantly.
The four phases of the menstrual cycle
Your period is just one part of a larger monthly cycle. The menstrual cycle runs in four phases, driven by rising and falling hormones, that repeat from the first day of one period to the first day of the next. Understanding them explains why the calculator's simple addition works — the cycle is a loop that resets each time bleeding begins.
| Phase | Cycle days (28-day cycle) | What happens |
|---|---|---|
| Menstrual | Days 1–5 | The uterine lining sheds — this is the bleeding you see as your period. Lasts about 2–7 days. |
| Follicular | Days 1–13 | Overlaps menstruation; FSH stimulates ovarian follicles and estrogen rises, rebuilding the uterine lining. |
| Ovulation | Around day 14 | An LH surge releases a mature egg from the ovary — the single fertile event of the cycle. |
| Luteal | Days 15–28 | The lining thickens for a possible pregnancy; if none occurs, hormones fall and the next period begins. |
The four phases on a textbook 28-day cycle. Day numbers shift with your own cycle length. Sources: Cleveland Clinic; Office on Women's Health.
Two of these phases are worth singling out for the calculator's logic. The follicular phase (before ovulation) is the part that stretches or shrinks with your overall cycle length, while the luteal phase (after ovulation) stays close to 14 days for most people. That is why a longer cycle simply means a later period — the back half barely moves. If you want to pinpoint ovulation and the fertile window inside this cycle, use the ovulation calculator; this tool focuses on when the bleeding returns.
What is a normal menstrual cycle length?
A normal menstrual cycle is anywhere from 21 to 35 days, measured from the first day of one period to the first day of the next. The often-quoted 28 days is just the average — in reality only about 1 in 8 people have an exactly 28-day cycle, and a range of lengths is completely normal.
| Cycle length | What it means |
|---|---|
| 21–35 days | Typical range for adults — anywhere in here is considered normal. |
| Around 28 days | The population average, but only about 13% of people land exactly here. |
| Shorter than 21 days | Outside the typical range; worth mentioning to a clinician if it's your usual pattern. |
| Longer than 35 days | Outside the typical range; consistently long or skipped cycles are worth an evaluation. |
Adult cycle-length ranges. Teens in the first years after their first period and people near menopause naturally vary more. Sources: ACOG; NHS.
Period length vs cycle length
These two numbers are easy to mix up, and the calculator needs the right one. Cycle length is the gap between the start of one period and the start of the next — the figure you enter. Period length is how many days the bleeding itself lasts. They measure different things.
| Measure | What it counts | Typical range |
|---|---|---|
| Cycle length | First day of one period → first day of the next | 21–35 days |
| Period length | Days of actual bleeding within one cycle | 2–7 days |
Cycle length is the long count the calculator uses; period length is the short bleeding window inside it.
A quick way to keep them straight: the period is the bleeding, the cycle is the whole loop from one bleed to the next. The calculator predicts when the next cycle begins, which is the same as when your next period starts — so the input it wants is your cycle length, not how many days you bleed.
Tracking irregular periods
If your cycle length swings widely from month to month, a single-number prediction will be less reliable — the calculator assumes your cycles are reasonably consistent. Periods are considered irregular when the length varies a lot, when they're frequently much shorter than 21 or longer than 35 days, or when they sometimes disappear for months. Tracking still helps; you just lean on your own averaged pattern instead of a textbook 28.
- Log several cycles first. Record the start date of three to six periods, then average the gaps to get a realistic cycle length to enter.
- Use a range, not a single day. If your cycles run 26–34 days, treat the prediction as a window of about a week rather than an exact date.
- Watch for a trend. Cycles that are gradually lengthening, shortening, or becoming erratic are worth noting — the pattern matters more than any one month.
- Note the context. Stress, travel, illness, weight change, breastfeeding, and approaching menopause all shift timing; conditions like PCOS or thyroid issues can too.
When to see a doctor about your periods
A period calculator is for planning, not diagnosis. Most variation in cycle timing is normal, but certain patterns are worth raising with a clinician — especially if they're new or persistent for you.
- Cycles consistently shorter than 21 days or longer than 35 days, or a sudden change from your usual pattern.
- Bleeding that lasts longer than 7 days, or that is very heavy — soaking through a pad or tampon every hour or two.
- Missed periods — no period for 90 days or more when you're not pregnant or in menopause.
- Bleeding between periods, after sex, or after menopause.
- Severe pain that interferes with daily activities, or any abrupt, unexplained change in your cycle.
Data sources and methodology
Your next period is estimated as LMP + cycle length, with later periods projected by adding the cycle length again — plain calendar arithmetic from the inputs you enter. The normal-range figures (cycle length 21–35 days, period length 2–7 days) and the four-phase structure reflect guidance from ACOG, the NHS, Cleveland Clinic, and the US Office on Women's Health. All dates are computed in your browser; nothing you enter is stored or sent anywhere.
Cleveland Clinic — Menstrual cycle: overview and phases.US Office on Women's Health — Your menstrual cycle.Frequently asked questions about the free period calculator
About this period calculator
This period calculator runs entirely in your browser. The dates you enter never leave your device — nothing is sent to a server, logged, or shared. It predicts your next period as LMP + cycle length, rolls the same step forward to project the periods after that, and updates instantly on every change.
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