Use this free online period calculator to predict your next six expected period start dates based on the first day of your last period and your average cycle length. It's a quick way to plan ahead using your own menstrual cycle history. This calculator offers general estimates for educational purposes only and does not replace advice from a healthcare professional.
How to Use the Period Calculator
Enter the first day of your last period in the "First Day of Last Period" field.
Enter your average cycle length in days (between 15 and 45) in the "Average Cycle Length (days)" field.
Click "Calculate Periods" to generate your predictions.
Review the list of your next 6 predicted period start dates in the result box.
How the Predictions Are Made
The calculator adds your average cycle length to the first day of your last period to get the next start date, then keeps adding the same number of days to get each of the following five. The first prediction is your last period date plus one cycle, the second is plus two cycles, and so on up to six.
Because every prediction assumes the same cycle length, small differences add up. If your cycles really vary between 28 and 31 days, the sixth prediction could be off by a week or more, so the nearer dates are more dependable than the later ones. Update the calculation each time a new period starts.
Worked Example
Your last period began on 5 September 2026 and your cycles average 30 days. The calculator predicts the next six periods on 5 October 2026, 4 November 2026, 4 December 2026, 3 January 2027, 2 February 2027 and 4 March 2027.
Notice the day of the month moves around. That is not an error: months have 28 to 31 days, so a fixed 30-day cycle lands on a slightly different date each month.
How to Work Out Your Average Cycle Length
Count from the first day of one period (the first day of real bleeding, not spotting) to the day before the next one starts. That number of days is one cycle.
Do not measure from the end of one period to the start of the next. That leaves out the bleeding days and makes your cycle look about five days shorter than it is.
Record three to six cycles and average them. For example, cycles of 29, 31, 28 and 30 days average 29.5; round to a whole number of days, here 30, before entering it.
What Is a Typical Menstrual Cycle?
Feature
Typical range
Cycle length in adults
About 21 to 35 days
Cycle length in the first few years after periods start
About 21 to 45 days, often irregular
Length of bleeding
Up to 7 days, most often 3 to 5
Difference between shortest and longest cycle
A variation of up to about 7 to 9 days is common
Cycles can be thrown off by stress, illness, travel, a big change in weight, very intense exercise, breastfeeding and starting or stopping hormonal contraception. Thyroid problems and PCOS can also make periods irregular, and cycles often change in the years before menopause.
When to See a Doctor
Your periods stop for three months or more and you are not pregnant, or you miss a period and a pregnancy test is positive.
Your cycles are regularly shorter than 21 days or longer than 35 days.
Bleeding lasts more than 7 days, or is heavy enough that you need to change a pad or tampon every one to two hours.
You bleed between periods, after sex or after menopause, or have pain that stops you from carrying on with daily life.
Predicted dates are general estimates for planning and are not medical advice.
Frequently Asked Questions
Starting from the first day of your last period, the calculator repeatedly adds your average cycle length in days to generate each subsequent date, producing a list of your next six predicted period start dates.
Count the days from the first day of one period to the day before the next period begins. Day 1 is the first day of proper bleeding. Average this over a few months and enter the result rounded to a whole number of days.
Yes. Most people's cycles vary by a few days from month to month, so treat each predicted date as the middle of a likely range rather than an exact day. Repeated large swings, or a cycle that is often shorter than 21 or longer than 35 days, are worth discussing with a doctor.
Yes. On combined pills, patches or rings, the monthly bleed is triggered by the break in hormones and follows your pack schedule, not your natural cycle, so the pack is a better guide than this calculator. Hormonal IUDs, implants and injections can make bleeding lighter, irregular or stop altogether.
The dates can help you plan, but they are not reliable contraception. To estimate your fertile days, use the ovulation calculator, and talk to a doctor about contraception if you want to avoid pregnancy.