Period Calculator guide
Period Calculator: Predict Your Next Period and Fertile Window
Use our period calculator to predict your next six periods, your ovulation date and fertile window from the date of your last period and cycle length.
By the AllCalculator editorial teamUpdated 16 October 20262 min read
Knowing when your next period is due helps you plan holidays, events and what to pack in your bag. Our Period Calculator predicts your next six periods and your fertile window each month.
How to use the calculator (Period Calculator)
- Enter the first day of your last period.
- Enter your average cycle length: the number of days from the first day of one period to the day before the next.
- Enter how many days your period usually lasts.
You'll see when your next period is due, when you'll probably ovulate, your fertile window and the dates of your next six periods.
How it works
- Your next period is your last period's start date plus your cycle length.
- Ovulation is estimated as 14 days before your next period.
- Your fertile window is the 5 days before ovulation and the day after, because sperm can survive for up to 5 days and an egg for about a day.
A worked example
If your last period started on 1 October 2026 and your cycle is 28 days, your next period is due on 29 October, and you probably ovulated around 15 October.
What's a normal cycle?
The average cycle is 28 days, but anything from 21 to 35 days is normal for adults. Periods usually last 2 to 7 days. It's also normal for your cycle to vary by a few days from month to month.
Frequently asked questions
Can I use this as contraception?
No. Cycles vary, so predicted dates aren't reliable enough to avoid pregnancy. Speak to your GP or a sexual health clinic about contraception.
When should I see a GP?
See your GP if your periods are very heavy or painful, you bleed between periods or after sex, your cycle changes suddenly, or your periods stop for more than 3 months and you're not pregnant.
How can I make the prediction more accurate?
Track your cycles for a few months and use your average cycle length.
If you're trying for a baby, see our Cervical Mucus Calculator for your fertile window, or our Conception Calculator.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.