Period Calculator: Predict Your Next Period

Never be caught off guard again

Whether you are planning a holiday, a big event, or just want to stop raiding your bag for emergency supplies, knowing when your next period is due makes life simpler. Your cycle runs on a rhythm — and once you know the length of yours, the future becomes surprisingly predictable.

The maths is straightforward: your next period arrives roughly one full cycle after the last one began. A “cycle” is counted from the first day of one period to the first day of the next — most women fall between 21 and 35 days, with 28 the famous average that actually applies to a minority.

Enter the first day of your last period and your usual cycle length below, and the calculator will predict your next several periods — plus your estimated fertile window alongside each one.

Cycle length is counted from the first day of one period to the first day of the next — most fall between 21 and 35 days.

The four phases of your cycle

Shown for a typical 28-day cycle — your own days shift with your cycle length, and the calculator adjusts accordingly.

Phase Days (28-day cycle) What is happening
Menstrual 1 – 5 The uterine lining sheds — your period. Oestrogen and progesterone are at their lowest.
Follicular 1 – 13 Oestrogen rises, the lining rebuilds, and follicles mature in the ovaries. Energy often climbs.
Ovulation Around day 14 A mature egg is released — the fertile peak of the cycle, lasting 12–24 hours.
Luteal 15 – 28 Progesterone rises to prepare for a possible pregnancy; PMS symptoms appear here if they come.

What counts as a normal cycle

Normal is wider than most women realise: anywhere from 21 to 35 days, with bleeding lasting 2 to 7 days, counts as typical. What matters most is your own pattern — a cycle that is consistently 33 days is just as normal as one that is consistently 26.

See your doctor if your cycle suddenly changes, if you bleed between periods or after sex, if periods stop for three months (and you are not pregnant or approaching menopause), or if pain regularly disrupts your life. And a note for readers in their late 40s and 50s: perimenopause commonly makes cycles irregular — shorter, longer, heavier, lighter — for several years before periods stop altogether. A calculator can still estimate, but expect more variation.

Getting the most from tracking

  • Track three months minimum. One cycle tells you little; three reveal your true average length and how much it varies.
  • Note more than dates. Flow heaviness, cramps, mood, and PMS symptoms turn a calendar into genuinely useful health data.
  • Count day one correctly. Day 1 is the first day of full flow — spotting beforehand does not count.
  • Bring your data to appointments. A few months of tracked cycles answer half your doctor’s questions before you sit down.

This calculator is for general information only and is not medical advice. If you have concerns about your cycle, please speak to your doctor.

Frequently asked questions

How accurate is a period calculator?
Very good for regular cycles — usually within a day or two. Stress, illness, travel, weight changes, and perimenopause can all shift ovulation and throw predictions off, so treat the dates as informed estimates rather than guarantees, especially if your cycles vary.
Can I use this to avoid pregnancy?
Not reliably on its own. Calendar-based prediction has a high failure rate because ovulation does not always follow the textbook schedule — sperm can also survive up to five days, extending the real fertile window. If avoiding pregnancy matters, use reliable contraception and talk to your doctor.
Why is my cycle irregular in my late 40s?
Almost certainly perimenopause — the years leading up to menopause, when hormone levels fluctuate and ovulation becomes erratic. Cycles may shorten, lengthen, or skip entirely, and flow can change dramatically. It is normal, but mention significant changes to your doctor to rule out other causes.
What is the difference between cycle length and period length?
Cycle length is the full span from the first day of one period to the first day of the next (often 21–35 days). Period length is just the bleeding itself (usually 2–7 days). When a calculator asks for “cycle length,” it wants the first number.
My period is late but the test is negative — what now?
Late ovulation is the most common reason — stress, illness, or travel can delay it by days or weeks, pushing everything back. Test again in a few days with first-morning urine. If your period stays away for three months, or this keeps happening, see your doctor.
Does a regular period mean I am definitely ovulating?
Usually, but not always. It is possible to bleed without ovulating (anovulatory cycles), which become more common in the teens and during perimenopause. If you are tracking for fertility reasons and suspect this, ovulation predictor kits or a conversation with your doctor can confirm what is happening.