Calculators
Period Calculator
Enter the first day of your last period, your cycle length and how long your period lasts to see your next six predicted periods on a calendar.
Estimates only. Predictions use the calendar method and assume every cycle is the same length. They are not a method of contraception and cannot tell you whether you are pregnant. If your cycles vary, the dates can be wrong by several days or more.
Cycle length is counted from the first day of one period to the first day of the next: with a 28-day cycle, the next period starts 28 days later.
Enter the first day of your last period to see your next six predicted periods.
Past cycles (optional)
Add the first day of a few recent periods to work out your average cycle length. Two dates give one cycle; three to six give a better average.
When to see a doctor or nurse
- Your cycles are regularly shorter than 21 days or longer than 35 days, or their length varies by more than 7 to 9 days.
- Your period lasts more than 7 days, or bleeding is very heavy — for example soaking a pad or tampon every hour.
- You bleed between periods, after sex or after the menopause.
- Your periods have stopped for 3 months or more and a pregnancy test is negative.
- Your periods are irregular and you are trying to get pregnant.
If you might be pregnant, take a pregnancy test. For heavy bleeding with dizziness or fainting, get urgent medical help.
Want only the fertile days? The Ovulation Calculator focuses on ovulation dates and the fertile window.
Sources: ACOG, fertility awareness-based methods; ACOG, abnormal uterine bleeding; NHS, periods; NHS, fertility in the menstrual cycle; NHS, irregular periods.
Calculated on your device. Nothing you enter is sent anywhere; past-cycle dates are stored in this browser only if you tick the box above.
Works in your browser. No account needed.
Frequently asked questions
How does the period calculator predict my next period?
It adds your cycle length to the first day of your last period, then keeps adding it to project six periods ahead. Cycle length is counted from the first day of one period to the first day of the next. The prediction assumes every cycle is the same length, so it is most useful when your cycles are regular; real periods often arrive a few days earlier or later.
What is a normal cycle length?
The NHS says periods usually come about every 28 days but commonly range from every 21 to every 35 days, and last 2 to 7 days, usually about 5. ACOG gives the same 21–35 day range for adults and notes that cycles in the first years after periods start can run from 21 to 45 days. The calculator accepts 21–45 days, but a cycle regularly outside 21–35 days in an adult is worth discussing with a doctor.
How are the fertile window and ovulation estimated?
ACOG explains that ovulation happens about 14 days before the next period in a 28-day cycle, because the time from ovulation to the next period is the most consistent part of the cycle. The calculator therefore counts back 14 days from each predicted period and marks that day and the 5 days before it, since sperm can survive up to 5 days and an egg about 24 hours. The NHS notes ovulation can happen anywhere from 10 to 16 days before a period, so treat these dates as rough.
Can I use this to avoid pregnancy?
No. A calendar prediction is not contraception. Even fertility awareness methods that track temperature and cervical mucus daily see 12–24 pregnancies per 100 women in the first year of typical use, according to ACOG; the NHS puts typical use at about 76% effective. Ovulation can shift in any cycle, and irregular cycles make it much less reliable. Talk to a doctor, nurse or sexual health clinic about contraception.
When should I see a doctor about my periods?
ACOG and the NHS advise getting checked if cycles are shorter than 21 or longer than 35 days, if cycle length varies by more than 7 to 9 days, if bleeding lasts more than 7 days or soaks a pad or tampon every hour, if you bleed between periods, after sex or after the menopause, or if your periods stop for 3 months or more and you are not pregnant. Also see a GP if your periods are irregular and you are trying to get pregnant.
Is my period data private?
Yes. Every calculation runs in your browser and nothing is sent to a server. Past-cycle dates are kept only for the current visit unless you tick Remember, which saves them in this browser’s local storage on this device alone. Untick the box or clear site data to delete them. The .ics file is created on your device, and discreet titles keep shared calendars neutral.
About this tool
This period calculator projects your next six periods from three numbers: the first day of your last period, your usual cycle length and the length of your period. Each prediction appears in a table and on a month calendar that also shades the estimated fertile window and ovulation day, so you can plan trips, events or appointments around them and download the dates to your own calendar app.
The method is the calendar method. A period is expected one cycle length after the previous one began, and ovulation is placed about 14 days before each expected period, which is how ACOG describes the average cycle. The fertile window covers ovulation day and the five days before it. All of this assumes a steady cycle. The NHS notes that ovulation usually happens 10 to 16 days before a period rather than exactly 14, and things such as stress, big changes in weight or exercise, hormonal contraception, breastfeeding and perimenopause can change a cycle.
If you are not sure of your cycle length, add the start dates of a few past periods. The calculator averages the gaps, shows your shortest and longest cycles and warns when they vary by more than ACOG’s 7-to-9-day guide for irregular periods or fall outside the usual 21–35 days. For fertility planning in particular, the separate Ovulation Calculator focuses on ovulation dates, and ovulation test kits measure the hormone surge directly.
Predictions are estimates, not medical advice or contraception, and they cannot tell you whether you are pregnant. Irregular cycles make them unreliable. If a period is late and you might be pregnant, take a test. See a doctor or nurse for very heavy or long periods, bleeding between periods or after sex, or periods that stop — changes in your periods are worth mentioning even when the calculator looks fine.