What is a period calculator?
A period calculator predicts when your next menstrual period will start, based on the date your last period began and the usual length of your cycle. This free online period calculator also shows how long each period is likely to last, what phase of the cycle you are in today, and a calendar of the next 6 periods with the ovulation day and fertile days for each. It is handy for planning travel, exams, events and sport, spotting a late period early, and understanding your menstrual cycle.
How to calculate your next period date
The next period is simply the start of the last period plus your cycle length:
Next period date = first day of last period + cycle length (days)
- First day of last period = the first day of proper bleeding (day 1 of the cycle)
- Cycle length = the number of days from one period's first day to the next period's first day
Each further period adds another cycle length. Ovulation is estimated as 14 days before the next period, and the fertile window as the five days before ovulation plus the ovulation day.
Worked example
Your last period started on 5 September and your cycle is 28 days with a 5-day period. Your next period starts on 3 October and should end around 7 October. The one after that begins on 31 October, then 28 November, and so on. Ovulation in the September cycle falls around 19 September, with the fertile window from 14 to 20 September.
How to calculate your period cycle length
Many people ask how to calculate the menstrual cycle correctly. Count from the first day of one period (day 1) up to the day before the next period starts. If your period started on 3 March and the next one started on 31 March, your cycle is 28 days. Track at least three cycles and take the average for the best prediction. A normal adult cycle lasts 21 to 35 days, and a normal period lasts 2 to 7 days. Teens often have cycles of 21–45 days in the first few years after their first period.
Phases of the menstrual cycle
| Phase | Days (28-day cycle) | What happens |
|---|---|---|
| Menstrual phase | Days 1 – 5 | The uterine lining sheds; bleeding and cramps are common. |
| Follicular phase | Days 1 – 13 | Oestrogen rises and a follicle matures in the ovary. |
| Ovulation | Around day 14 | An egg is released; this is the most fertile time. |
| Luteal phase | Days 15 – 28 | Progesterone rises; PMS symptoms may appear before the next period. |
The calculator uses these phases to tell you where you are today.
Period calculator for irregular periods
If your cycles vary by more than 7–9 days from month to month, predictions will be rougher. Enter the average of your last three to six cycles and treat the result as a range of a few days either side. Common reasons for irregular periods include stress, big changes in weight, heavy exercise, thyroid problems, PCOS, breastfeeding and perimenopause. See a doctor if your cycles are regularly shorter than 21 days or longer than 35 days, if periods stop for three months, or if bleeding is very heavy.
Period calculator for pregnancy
A period is usually considered late if it has not started 5 days after the expected date. If you might be pregnant, take a home pregnancy test from the day of your missed period. If the test is positive, the due date calculator works out your due date and how many weeks pregnant you are from the same last-period date. If you are trying to conceive, the ovulation calculator focuses on your fertile days.
Can a period calculator be used to avoid pregnancy?
Calendar methods are not reliable contraception on their own. Ovulation can shift by several days in any cycle, and sperm survive up to five days, so "safe days" are not guaranteed. Use a proper contraceptive method if you want to avoid pregnancy.
How to calculate the period of gestation from LMP
Doctors date pregnancy from the first day of the last menstrual period (LMP). The period of gestation is the number of days from the LMP to today, divided by 7, given as weeks and days. For example, 75 days after the LMP is 10 weeks 5 days. Our due date calculator shows this automatically.
Tips for tracking your period
- Note the first day of every period in a calendar or period tracker app.
- Record flow, cramps and mood to spot patterns.
- Keep pads, tampons or a menstrual cup ready a couple of days before the predicted date.
- Update the calculator each month with your latest period date for the most accurate calendar.
What you can do with the results
See the next 6 months (or 12)
The table lists six predicted start and end dates, so it acts as a period calculator for next 6 months. For the next 12 months, take the sixth start date, enter it as your last period and run the calculator again.
Find your ovulation date and fertile days
Every row shows the ovulation day and the fertile days around it, so you get a period calculator with ovulation date without a separate tool. Working backwards is also possible: if you know when you ovulated, add 14 days to find your next period. That is how a calculator based on your ovulation date predicts the period.
Know which phase you are in
The calculator's "current phase" row shows which of the period cycle phases you are in today: menstrual, follicular, fertile or luteal.
Late period: how late am I, according to the calculator
Compare today with the predicted start date. If your period is more than 5 days past the date shown, it counts as late. That is all a "how late is my period" calculator does.
When to take a pregnancy test
Use the calculator for pregnancy test timing: test from the first day of your missed period, which is the next start date in the table. If you are pregnant, doctors count pregnancy weeks from the first day of your last period, so the date you entered in the calculator is also the starting point for pregnancy weeks.
Irregular cycles
If your cycles vary, enter your average cycle length. A quick period regularity check with the calculator: cycles that differ by less than 7–9 days from month to month count as regular.
Don't know your cycle length?
Without a cycle length, the calculator predicts your next period with 28 days, the default here. Leave it at that and replace it with your real average once you have tracked a few months.
After an abortion or miscarriage
The first period after an abortion or miscarriage is less predictable and usually returns 4–8 weeks later. Count from the procedure date in the calculator, and see a doctor if nothing comes by 8 weeks.
In Hindi and Tamil
Whether you search in Hindi ("masik dharm") or Tamil, the dates work the same way. Period calculator kaise kare: enter the first day of your last period and your cycle length, and read the dates in the table.
Free, online, no app needed
This online calculator is free and works in any browser, so you can use it instead of installing a free period tracker app. It shows your dates as a calendar-style table, and you can come back to it any time as a simple online period tracker.
How do I calculate my next monthly period by hand? Add your cycle length to the first day of your last period. To calculate the cycle length, count the days from the start of one period to the start of the next; to calculate period days, count how many days the bleeding lasts.
What is a period cycle? It is the full span from one period to the next. What is periods in females? Monthly bleeding as the uterus sheds its lining when no pregnancy occurs. The period calculator formula above predicts when it will come.
How do you calculate periods from your last 3 months?
If your cycle varies, use a period calculator with your last 3 months of dates. Write down the first day of each of your last three or four periods and count the days between each start date. For example, starts on 2 January, 31 January and 1 March give gaps of 29 and 29 days, so your average cycle is 29 days. Enter that number as the cycle length above.
How can I calculate my menstrual cycle by hand?
- Day 1 is the first day of full bleeding, not spotting.
- Count every day up to, but not including, the first day of the next period. That count is your cycle length.
- Repeat for several cycles and take the average.
- Add the average to your last start date to get the next one.
Period calculator between two dates: how many days?
To find how many days lie between two period dates, subtract the earlier date from the later one. From 5 March to 2 April is 28 days, so that cycle was 28 days long. A date calculator does the counting for you if the dates cross months of different lengths.
Monthly and yearly period calculator
A monthly period calculator view shows one period per calendar month, but cycles rarely match months exactly. With a 28-day cycle you have about 13 periods a year, so one calendar month each year has two periods. With a 32-day cycle you have about 11 periods in a year.
For a yearly period calculator plan, keep adding your cycle length to the last start date. The table above shows the next 6 periods; run the calculator again from the sixth date to extend it to a full year.
Period calculator graph
If you keep a note of each cycle length, a simple graph of cycle length against month shows whether your cycle is steady or drifting. Most trackers plot this for you. A spread of up to 7–9 days between your shortest and longest cycle is common; a bigger swing is worth discussing with a doctor.
Planning a vacation or event around your period
Use the period vacation calculator approach: enter your last start date, then check whether your trip dates fall inside a predicted period in the table. Predictions become less certain the further ahead they go, so pack supplies anyway if a date falls within a few days of a predicted start.
Period calculator in India and in Marathi
People in India often search for a period calculator in Hindi, Tamil or Marathi. In Marathi a period is called masik pali, and the same method applies: note the first day of your last period (shevtchi pali) and add your cycle length. Dates on this page follow the Indian day–month–year order.
Period calculator quiz: test yourself
- Your last period started on 10 June and your cycle is 30 days. When is the next one due? 10 July.
- With a 28-day cycle, on which cycle day do you usually ovulate? Around day 14.
- How many days late is a period counted as late? Usually 5 or more days after the expected date.
Frequently asked questions
How do I calculate my next period?
Add your cycle length to the first day of your last period. For a 28-day cycle starting on 1 June, the next period starts on 29 June.
How do I calculate my period cycle length?
Count the days from the first day of one period to the day before the next period starts. Average three or more cycles for accuracy.
What is a normal menstrual cycle length?
Between 21 and 35 days for adults. Periods usually last 2 to 7 days.
How late can a period be?
A period that starts up to about 5 days after the expected date is usually considered normal variation. If it is later and you might be pregnant, take a test.
Does the period calculator work for irregular periods?
It gives an estimate using your average cycle, but the dates may be off by several days. Track more cycles to improve accuracy.
Can I see my next 6 periods?
Yes. The calculator lists the start and end dates of your next six periods, with ovulation and fertile days for each cycle.
When will I get my first period?
Most girls get their first period between ages 10 and 15, about two years after breast development begins. A calculator cannot predict this date.
How many days is a normal period cycle?
Adult cycles usually last 21 to 35 days, and bleeding lasts 2 to 7 days. The average cycle is about 28 days.
How do I work out my cycle from the last 3 months?
Count the days between the start of each of your last three periods, then average the gaps. Enter that average as your cycle length.
Last updated 2026-09-18