The short answer: Take the date your last period started and add your usual cycle length. If your last period started on 3 October and your cycle is 29 days, your next period is due around 1 November. Work out your cycle length by counting the days from the first day of one period to the first day of the next, over three to six months, and taking the average. Expect your period a few days either side of the date, because most cycles vary a little.
The terms you need to know
Before you calculate anything, it helps to be clear on three words. Most confusion about period dates comes from mixing them up.
- Day 1: the first day of proper bleeding, the kind that needs a pad or period panty. Light spotting before your flow starts does not count.
- Cycle length: the number of days from one day 1 to the next day 1. It is not the number of days you bleed, and it is not the gap between the end of one period and the start of the next.
- Period length: how many days you actually bleed. The NHS says this is usually about 5 days, anywhere from 2 to 7.
The NHS says the average cycle is about 28 days, but anywhere from 21 to 35 days is common in adults. For teenagers in the first few years after their first period, the American College of Obstetricians and Gynecologists (ACOG) says 21 to 45 days is normal.
How to calculate your next period: step by step
Step 1: Write down day 1 every month
Note the start date of every period. A calendar on the fridge, a diary, your phone's notes app or any period tracking app works. Many Indian families still keep a paper calendar in the kitchen, and marking a small dot on it works just as well as an app.
Step 2: Count your cycle length
Count the days from one day 1 to the next. The easiest way is to count forward on a calendar, counting the start date of the earlier period as day 1. The day before your next period starts is the last day of that cycle.
Step 3: Average three to six cycles
One cycle can be a one-off. Add up the lengths of your last three to six cycles and divide by how many you counted. Round to the nearest whole day.
Step 4: Add the average to your last start date
Last period start date plus your average cycle length gives your expected next period. Treat it as the middle of a window, not a fixed appointment.
Worked examples with real dates
Example 1: A regular 29-day cycle
Priya's last period started on Saturday 3 October 2026. Her cycles are usually 29 days. Add 29 days to 3 October and you reach Sunday 1 November 2026. She can expect her period around 1 November, give or take a few days.
Example 2: Working out the average from your own dates
Ayesha has noted these start dates:
| Cycle | Period started | Next period started | Cycle length |
|---|---|---|---|
| 1 | 9 May 2026 | 6 June 2026 | 28 days |
| 2 | 6 June 2026 | 6 July 2026 | 30 days |
| 3 | 6 July 2026 | 4 August 2026 | 29 days |
| 4 | 4 August 2026 | 4 September 2026 | 31 days |
| 5 | 4 September 2026 | 1 October 2026 | 27 days |
Her five cycles add up to 145 days. 145 divided by 5 is 29, so her average cycle is 29 days. Her latest period started on 1 October 2026, so her next is due around 30 October 2026. Because her cycles ranged from 27 to 31 days, she could reasonably see it any time from 28 October to 1 November.
Example 3: A 28-day cycle that seems to move every month
Meera has a very steady 28-day cycle. Her periods started on 5 January, 2 February, 2 March, 30 March and 27 April 2026. Her cycle never changed, but her period date moved from the 5th to the 2nd and then to the end of the month. That is simply because most months have 30 or 31 days, not 28. A 28-day cycle will arrive two or three days earlier in the calendar each month, which is why "my period came on the 5th last month" is not a good way to predict.
Example 4: An irregular cycle, using a range
Kavya's last six cycles were 27, 33, 30, 35, 28 and 31 days. The average is about 31 days, but a single date would often be wrong. A better method is to use her shortest and longest cycles. Her last period started on 12 September 2026. Adding 27 days gives 9 October, and adding 35 days gives 17 October. Her period is most likely to come between 9 and 17 October 2026, and around 13 October is the midpoint.
Example 5: A teenager in her first years of periods
Riya is 13 and has had periods for less than a year. Her last period started on 14 August 2026. Since cycles from 21 to 45 days are normal at her age, her next period could come any time from 4 September to 28 September 2026. Her mum packs supplies in her school bag for that whole window instead of guessing a date.
Example 6: Crossing into a new year
Sneha's last period started on 20 December 2026 and her cycle is 30 days. Count the 11 remaining days of December, then 19 more into January: her next period is due around 19 January 2027.
Cycle length reference table
This table shows when your next period and your likely fertile time fall if your last period started on 1 October 2026. Ovulation, when an egg is released, usually happens about 12 to 16 days before your next period starts, according to the NHS.
| Cycle length | Next period due | Likely ovulation window | Periods in a year, roughly |
|---|---|---|---|
| 21 days | 22 October | 6 to 10 October | 17 |
| 24 days | 25 October | 9 to 13 October | 15 |
| 26 days | 27 October | 11 to 15 October | 14 |
| 28 days | 29 October | 13 to 17 October | 13 |
| 30 days | 31 October | 15 to 19 October | 12 |
| 32 days | 2 November | 17 to 21 October | 11 |
| 35 days | 5 November | 20 to 24 October | 10 |
For any other start date, just add your cycle length. For the ovulation window, count back 12 to 16 days from your next expected period.
Estimating ovulation and fertile days
Once you know roughly when your next period is due, you can estimate ovulation. In Example 1, Priya's next period is due on 1 November, so she probably ovulates between 16 and 20 October. Sperm can survive for up to 7 days, so the NHS notes that the fertile time starts several days before ovulation.
Many women also notice clear, wet, slippery discharge, a bit like raw egg white, around ovulation. Our guide to normal vaginal discharge through the month explains these changes.
A calendar estimate is not reliable contraception. The NHS says fertility awareness methods, which combine temperature, discharge and cycle tracking, need careful teaching and are only about 76% effective with typical, imperfect use. If you want to avoid pregnancy, talk to your doctor about contraception.
Why your period date changes every month
- Calendar months are not equal. As Example 3 shows, even a perfectly regular cycle drifts across the calendar.
- Cycles naturally vary. A difference of a few days between cycles is normal for most women.
- Stress and routine changes. Exams, a move, a wedding in the family, travel and night shifts can push ovulation, and so your period, later. See period late but not pregnant for the full list.
- Illness, weight change and exercise. A fever, sudden weight loss or gain, or a big jump in training can shift your cycle. Some of these can also bring a period sooner, covered in period came early.
- Hormonal contraception. The pill, injection and hormonal coil change bleeding patterns, so the calculation above does not apply in the same way.
- Life stage. Cycles are often irregular in the first years after periods begin and again in the years before menopause.
Are period tracker apps accurate?
Apps do the same sum you just learned, using the dates you enter. With six months or more of correct dates, their predictions are usually reasonable for a regular cycle. They cannot know about a stressful month or an illness, and they are only as good as your data. If you forget to log a period, the app's average will be wrong.
Whichever method you use, logging the first day of bleeding every month is the most useful habit. It also gives your doctor a clear picture if anything changes.
Use your prediction to stay prepared
Knowing your window means you are rarely caught out at school, college or work. A day or two before your expected date, start wearing a period panty so an early start does not spoil your day; it can be worn for up to 8 hours depending on flow. For a daughter whose cycle has not settled yet, a First Period Kit kept in her school bag covers that whole uncertain window.
See a doctor if
- Your cycle is regularly shorter than 21 days or longer than 35 days as an adult
- Your cycle length changes a lot from month to month, or your usual pattern changes
- Your period lasts longer than 7 days
- You have missed three periods in a row
- You bleed between periods or after sex
- Irregular cycles come with weight gain, tiredness, extra facial hair, acne or dry or oily skin, which can be signs of PCOS or a thyroid problem
- You have not started your periods by 15
- You are trying to get pregnant and your periods are irregular
Take your list of period dates to the appointment. It saves time and helps your doctor decide whether blood tests or a scan are needed.
Frequently asked questions
Do I count the first day of my period as day 1? Yes. Day 1 is the first day of proper bleeding. Count it, and count forward until the day before your next period starts.
Do I count spotting as day 1? No. Count the first day of real flow, the kind that needs a pad or period panty.
Is a 30-day cycle normal? Yes. Anything from 21 to 35 days is common in adults, and 21 to 45 days is normal for teens in the first years of periods.
Why did my period come on a different date this month? Usually because calendar months are longer than most cycles, or because your cycle varied by a few days. Both are normal.
How do I calculate my period if my cycle is irregular? Use your shortest and longest cycles from the last six months to get a range rather than a single date, as in Example 4. If your cycles vary a lot, mention it to your doctor.
How many days late is a period considered late? If your period is more than a few days past the end of your usual range, it is reasonable to call it late. If you have had unprotected sex, take a pregnancy test.
Can I calculate my safe days? You can estimate fertile days, but calendar counting alone is not reliable contraception. Talk to a doctor about safer options.
Related reading
- Irregular Periods: Causes, Tests and What Helps
- Period Late but Not Pregnant? 10 Common Reasons
- When Will I Get My First Period? Age and Signs It's Coming
- Period Came Early? 9 Reasons and When to Worry
This article is for general information and doesn't replace advice from your doctor.
Sources: NHS: Periods, NHS: Irregular periods, NHS: Missed or late periods, ACOG: Your First Period, NHS: Natural family planning