Period Calculator

About the Period Calculator

Knowing when your next period is due is useful for all sorts of everyday planning — holidays, sport, events, or simply not being caught out — and it also helps you notice when a cycle is unusually early or late.

The prediction itself is simple addition. What makes a period calculator genuinely useful is being honest about how much real cycles vary, so a period a few days off the predicted date is recognised as normal rather than worrying.

This period calculator predicts your next six periods from the first day of your last one, shows your likely ovulation and fertile days for the current cycle, and checks your cycle length against the normal range.

How to Use the Period Calculator

Enter the first day of your last period — the first day of bleeding, not the last.

Enter your usual cycle length. A cycle runs from the first day of one period to the first day of the next. If yours vary, average the last three to six.

Enter how many days your period usually lasts.

How the Dates Are Worked Out

  next period       =  first day of last period + cycle length
  each after that   =  previous period + cycle length
  ovulation         ≈  14 days before the next period

The calculation is the same for every month ahead, but the uncertainty is not. Each month's natural variation adds to the last, so the sixth prediction is much less certain than the first.

Step-by-Step Example

Last period started 1 September 2026. 28-day cycle, 5-day period.

  Next period:   1 September + 28 days  =  29 September to 3 October
  Then:          27 October, 24 November, 22 December, 19 January, 16 February
  Ovulation:     around 14 September
  Fertile days:  9 to 14 September

A period arriving anywhere from about 26 September to 2 October would be well within normal variation.

What Counts as a Normal Cycle

The 28-day cycle is an average, not a rule. Relatively few people have cycles of exactly 28 days, and most vary somewhat from month to month.

For adults, the International Federation of Gynecology and Obstetrics (FIGO) defines:

So a cycle of 26 days one month and 32 the next is still regular. Outside these ranges is not necessarily a problem, but consistently short, long, very irregular or very heavy periods are worth discussing with a doctor.

Teenagers often have longer and more irregular cycles in the first few years after periods begin. Cycles also change approaching menopause, often becoming shorter at first and then more irregular.

Counting the Cycle Correctly

The most common mistake is counting from the end of a period.

Day 1 is the first day of proper bleeding (not spotting beforehand), and the cycle ends the day before the next period starts. A calendar or app where you mark only the start day of each period is all you need to work out your real average cycle length.

Why a Period Might Be Late

If you are sexually active, pregnancy is the first thing to rule out. Most home tests are reliable from the day the period was due.

Otherwise, common reasons include stress, illness, travel and changes in routine, significant weight loss or gain, intense exercise, some medicines including hormonal contraception, and conditions such as polycystic ovary syndrome or thyroid problems. A single late period is common; a pattern of missed or very irregular periods is worth checking.

Fertile Days

Ovulation usually happens about 14 days before the next period — so on day 14 of a 28-day cycle, but day 18 of a 32-day cycle. The fertile window is the six days ending on ovulation. These are estimates: ovulation shifts from month to month, which is why calendar methods alone are not reliable contraception.

Tracking Makes Predictions Better

A prediction is only as good as the cycle length you enter, and most people guess theirs. A few months of simple tracking changes that.

Mark the first day of each period in a calendar, diary or app. Nothing else is needed to work out your cycle length.

Average at least three cycles, ideally six. A single cycle can be thrown off by illness or stress, while an average smooths that out.

Note the range as well as the average. If your cycles run from 27 to 33 days, you know to expect the next period anywhere in that span, which is more useful than a single date.

Watch for patterns such as periods getting steadily heavier, longer or further apart. Those are easier to describe to a doctor with a record in hand.

Understanding Your Result

Next period is the predicted start date.

Next period in full gives the start and end dates, and the few days either side that would still be normal.

The five after that lists later predictions, which become less certain the further ahead they are.

Ovulation and fertile days shows the current cycle's estimate.

Your cycle length says whether it falls within the normal adult range.

Worth knowing covers cycle length, period length, and how to make predictions more reliable.

When Should You Use This Calculator?

For planning — holidays, events, sport, or appointments.

To see whether your cycle is typical, especially if it seems long or short.

To notice changes, such as a period that is later than usual.

Not as contraception. Ovulation varies too much for calendar methods alone.

Common Mistakes

Counting from the end of the period. Day 1 is the first day of bleeding.

Assuming every cycle is 28 days. Use your own average.

Treating a few days early or late as a problem. Variation of up to a week is normal.

Trusting predictions months ahead exactly. Variation accumulates.

Ignoring persistent changes. Very irregular, long, short or heavy periods are worth mentioning to a doctor.

Nothing here is medical advice; for concerns about your cycle, talk to a doctor or nurse.