Period Calculator: Predict Your Next Period Dates
Predict your next period and the five after it from the first day of your last period and your cycle length, with ovulation and fertile days for each cycle. Runs in your browser.
Enter the first day of your last period to see your forecast.
About the Period Calculator
The Period Calculator predicts when your next period will start and lists the expected dates of the five after it. Enter the first day of your last period, your usual cycle length and how many days your period normally lasts. The calculator shows the next period first, the dates it is expected to run, which day of your cycle you are on today and a table of six cycles with the period, the fertile window and the estimated ovulation day for each.
Dates are counted as whole calendar days, so the forecast is the same wherever you are and never moves because of a time zone or daylight saving time. If the period you enter was a few cycles ago, the calculator moves forward to the cycle you are in now.
This is an estimate, not medical advice. Few cycles are exactly the same length every month: a difference of a few days is normal, and stress, illness, travel, breastfeeding and hormonal contraception can all shift a period. It is not a method of contraception.
How to use it
Steps
- Pick the first day of your last period, the first day of real bleeding rather than spotting.
- Enter your cycle length, counted from the first day of one period to the first day of the next. The default is 28 days; any whole number from 20 to 45 is accepted.
- Enter how long your period usually lasts, from 2 to 10 days.
- Read the next period date and the six-cycle table.
Finding your cycle length
If your cycles vary, note the start dates of your last three to six periods, count the days between each pair and take the average. Using an average rather than the most recent cycle gives a steadier forecast. A cycle that is regularly shorter than 21 days or longer than 35, or periods that stop for three months, are worth mentioning to a doctor.
The formula
Each period is predicted by adding the cycle length to the start of the one before:
The table also shows ovulation, estimated as the next period minus a luteal phase of about 14 days, and the fertile window, the five days before ovulation and the day itself, as found by Wilcox and colleagues in 1995. The luteal phase can be changed on the page if you know your own.
Worked example
With a last period on 1 January 2026, a 28-day cycle and a 5-day period, the next period is expected on Thursday, 29 January 2026 and should run to 2 February. The one after starts on 26 February, and the sixth cycle in the table begins on 21 May 2026.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. Your dates are never sent anywhere, nothing is saved, and closing the page clears them.
Related tools
For the fertile window and ovulation day first, open the Ovulation Calculator. If a period is late and you might be pregnant, the Due Date Calculator estimates a due date from the same date, and the Date Calculator counts the days between any two dates.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-1521.
- Lenton EA, Landgren BM, Sexton L. Normal variation in the length of the luteal phase of the menstrual cycle. Br J Obstet Gynaecol 1984;91:685-689.
Open-source credits
The calculations are implemented in-house in TypeScript, with no third-party libraries. They follow the calendar method above, with ovulation one luteal phase before the next period and the six-day fertile window published by Wilcox et al.
Frequently asked questions
How is my next period predicted?
The calculator adds your cycle length to the first day of your last period. With a 28-day cycle and a last period on 1 January, the next period is expected on 29 January. Each later period is one more cycle length on.
How do I measure my cycle length?
Count the days from the first day of one period to the first day of the next. If your cycles vary, average the last three to six. The calculator accepts any whole number from 20 to 45 days.
Why is my period late when the calculator says it is due?
The forecast is an estimate, not medical advice. A few days either way is normal, and stress, illness, travel, weight change, breastfeeding and hormonal contraception can all shift a period. If a period is late and you could be pregnant, a pregnancy test is the way to find out.
Does the table show fertile days too?
Yes. Each cycle also shows the estimated ovulation day, one luteal phase (14 days by default) before the next period, and the six-day fertile window ending on that day. It is not a method of contraception.
Are my dates saved?
No. Everything runs in your browser, nothing is sent to a server, and the dates are cleared when you close the page.