Period Calculator
Calculate your next period, fertile window and ovulation day for the coming months with this free online period calculator and add them to your calendar.
What next?
Found it useful? Share Period Calculator
Free, no sign-up, works in the browser — send it to a friend or add it to your site.
Related tools
- Conception CalculatorCalculate your conception date from your due date, last period or birth date with this free online conception calculator. See the likely date range.
- Planet Age CalculatorCalculate your age on Mercury, Venus, Mars, Jupiter and other planets from your birthday. Free online space age calculator with your next planet birthdays.
- Percentage CalculatorCalculate percentages with this free online percentage calculator: X% of Y, what percent X is of Y, percent increase or decrease and percent difference.
- Sleep CalculatorCalculate the best time to go to bed or wake up online free using 90-minute sleep cycles. Free online sleep calculator and bedtime planner for all ages.
- Age Difference CalculatorCalculate the age difference between two people in years, months and days with this free online age difference calculator. Includes the half-plus-seven rule.
- Average CalculatorCalculate the average of any list of numbers, plus median, mode, range and sum, or a weighted average. Free online average calculator with steps shown.
How to use Period Calculator
- 1Pick the first day of your last period
- 2Set your usual cycle length and how many days your period lasts
- 3See your next periods, fertile days and estimated ovulation on a 3-month calendar, then download them as a calendar file
Frequently asked questions
How is my next period calculated?▾
Your next period is expected one cycle length after the first day of your last period. The average cycle is 28 days, but anything from 21 to 35 days is common.
When is my fertile window?▾
Ovulation usually happens about 14 days before your next period. The fertile window is the 5 days before ovulation plus the day after it.
Is my data stored anywhere?▾
Your dates are saved only in this browser so they are there next time. Nothing is sent to a server. Predictions are estimates and not a form of birth control.
Embed Period Calculator on your website▾
Paste this snippet into your page. The embedded tool runs in your visitor's browser and shows a small “Powered by clkr.work” link.