About the Period Calculator
This period calculator predicts when your next periods are likely to start, based on the first day of your last period, the usual length of your cycle and how many days your period lasts. For each of the next six cycles it lists the expected start and end, the estimated day of ovulation and the fertile window around it. It is a planning aid for people with fairly regular cycles, for example to plan a holiday or to spot when a period is late. It is an estimate and not a medical tool or a method of contraception. Your dates are not stored or sent anywhere.
Updated October 2026
How to use the Period Calculator
- 1Pick the first day of your last period.
- 2Enter your usual cycle length, counted from the first day of one period to the first day of the next.
- 3Enter how many days your period usually lasts.
- 4Read the next period and how many days away it is.
- 5Scroll the list for the next six cycles with ovulation and fertile windows.
Making predictions more useful
If your cycles vary, run the calculator twice: once with your shortest cycle and once with your longest. The earliest and latest dates give you a range rather than a single day. Note the actual start date each month and update the first-day box, so predictions stay close to your recent pattern.
What the dates mean
- Period: the expected first and last day of bleeding.
- Ovulation: the likely day an egg is released, about 14 days before the next period.
- Fertile window: the days when pregnancy is most likely, about six days ending on ovulation day plus one.
- A late period by a week or more may warrant a pregnancy test.
Related tools
The Ovulation Calculator focuses on the fertile window, the Pregnancy Due Date Calculator works out a due date, the Date Calculator adds days to any date and the Calendar Maker lets you print a calendar. All of these give estimates only, and a doctor can give personal advice.
Features
- Next six periods predicted
- Estimated ovulation day for each cycle
- Fertile window of six days
- Days until your next period
- Cycles from 18 to 60 days
- Weekday shown for each date
- Runs in your browser
Frequently asked questions
How do I count my cycle length?
Count from the first day of one period to the day before the first day of the next. If your periods start on 1 October and 29 October, your cycle is 28 days. Track a few months and use the average, because cycle length varies by a few days naturally.
How is ovulation estimated?
Ovulation usually happens about 14 days before the next period starts, not 14 days after the last one. For a 28-day cycle that is around day 14, and for a 35-day cycle it is around day 21. The fertile window is the five days before ovulation and the day after.
How accurate is the prediction?
It is a good guide if your cycles are regular, and less reliable if they vary by more than a few days. Stress, illness, travel, weight change, breastfeeding and some medicines can shift a cycle. Do not rely on this tool to avoid pregnancy.
What is a normal cycle length?
Most cycles are between 21 and 35 days, with 28 as the average. Periods usually last 3 to 7 days. Teenagers and people approaching menopause often have longer or more irregular cycles.
When should I see a doctor?
See a health professional if your periods stop for three months and you are not pregnant, are very heavy or painful, are shorter than 21 or longer than 35 days regularly, or if you bleed between periods or after sex. This tool cannot diagnose anything.
Is my information kept?
No. The calculation happens in your browser, nothing is sent to a server, and nothing is saved when you close the page. If you want to keep track of your cycles, you may prefer a private notebook or a trusted tracking app.