Period Calculator

To calculate your next period date, you need two pieces of information: the start date of your last menstrual period and your average cycle length. Add your cycle length (usually 28 days) to your last period start date. For example, if your last period started January 1st and your cycle is 28 days, your next period should start around January 29th. This period calculator does this math automatically - just enter your dates and click calculate.

A normal menstrual cycle ranges from 21 to 35 days. The average is 28 days, but only about 15% of people actually have a perfect 28-day cycle. If your cycle falls anywhere between 21 and 35 days and stays relatively consistent (within 7-9 days variation), that's considered normal. Cycles shorter than 21 days or longer than 35 days may need a doctor's attention.

Yes. This period calculator for irregular periods works by using your average cycle length. If your cycles vary (say, between 26 and 32 days), enter the middle number (29 days). The prediction won't be exact, but it gives you a reasonable window. For better accuracy with irregular periods, track your actual period dates for 3-6 months to find your personal average.

For regular cycles, the last menstrual period calculation is typically accurate within 1-3 days. Accuracy depends on how consistent your cycles are. If your period comes like clockwork every 28 days, predictions are very reliable. If your cycle length varies by more than a week each month, expect predictions to be off by several days. This is an estimate, not a guarantee.

Common reasons for a late period include: stress (physical or emotional), significant weight changes, intense exercise, travel across time zones, illness, changes in sleep schedule, starting or stopping birth control, and pregnancy. If your period is more than 7 days late and pregnancy is possible, take a test. If periods are regularly irregular or you miss multiple periods, see a healthcare provider.

If you just started tracking and don't know your average cycle length, use 28 days as a starting point to calculate your next menstrual period. This is the population average. After tracking 2-3 complete cycles, you'll have your personal average. Update the cycle length setting then for more accurate predictions going forward.

It depends on your birth control type. If you're on hormonal pills with a 21/7 or 24/4 schedule, your period (withdrawal bleeding) is predictable - it comes during the placebo week. For hormonal IUDs, implants, or continuous pills, periods may be irregular or stop entirely. This calculator works best for natural cycles or regular pill schedules.

Your period is the bleeding phase - typically 3-7 days when you're actively bleeding. Your menstrual cycle is the entire time from the first day of one period to the first day of the next period - usually 21-35 days. When you calculate next period date, you're calculating when your next cycle begins, which is when bleeding starts again.

Yes. This is a global period calculator that works anywhere in the world. Menstrual cycles follow the same biology regardless of location. The only thing that varies is date format - enter dates in whatever format your browser uses, and the calculator handles the rest. Works in North America, Europe, Asia, and everywhere else.

No. This period calculator runs entirely in your browser. Your last menstrual period date and cycle information are processed locally on your device and never sent to any server. When you close the browser tab, the data disappears. There's no account, no login, no tracking. Your menstrual health information stays completely private.