What is a menstrual cycle?

A menstrual cycle is the monthly series of changes a body goes through in preparation for pregnancy. Each cycle begins on the first day of menstruation (period) and ends when the next period starts. The average cycle lasts about 28 days, but normal cycles can range from 21 to 45 days depending on the individual.

Understanding your cycle length helps predict when your next period will arrive. The menstrual phase (period) typically lasts 3-7 days, during which the uterine lining sheds. Tracking these patterns over time can help identify irregularities and plan around upcoming periods.

Tool description

This period calculator predicts your next menstrual period based on the start date of your last period and your typical cycle length. Enter your information to see an estimated start and end date for your upcoming period, displayed on an easy-to-read weekly calendar view.

Features

  • Customizable cycle length — Set your personal cycle length anywhere from 21 to 45 days
  • Adjustable period duration — Configure period length from 1 to 10 days for accurate predictions
  • Visual calendar display — See your predicted period highlighted on a weekly calendar
  • Instant calculations — Results update automatically as you adjust your inputs
  • Privacy-focused — All calculations happen locally in your browser with no data stored on servers

Use cases

  • Planning trips or events — Check if your period might coincide with an upcoming vacation, wedding, or important event so you can prepare accordingly
  • Health tracking — Keep track of your cycle patterns to identify irregularities or share accurate information with healthcare providers
  • Athletic and fitness planning — Schedule intense workouts or competitions around your predicted period for optimal comfort and performance

How it works

The calculator uses a simple formula: your next period start date equals your last period start date plus your cycle length. For example, if your last period started on January 10th and your cycle is 28 days, your next period is predicted to start on February 7th.

The period end date is calculated by adding your typical period length to the start date. A 5-day period starting February 7th would end on February 11th.

Tips

  • Track your periods for at least 3 months to determine your average cycle length
  • Cycle lengths can vary by a few days each month — this is completely normal
  • Stress, travel, illness, and lifestyle changes can temporarily affect cycle regularity
  • If your cycles are highly irregular, consider consulting a healthcare provider

Limitations

  • This tool provides estimates based on average patterns and cannot account for individual variations
  • Predictions assume regular cycles — irregular cycles may not follow the calculated pattern
  • This calculator is not intended for use as a contraceptive method or for medical diagnosis
  • Results should be used for planning purposes only and not as medical advice

FAQ

How accurate is this period calculator? Accuracy depends on how regular your cycles are. For people with consistent cycles, predictions are typically within 1-3 days. Irregular cycles will have less predictable results.

What is a normal cycle length? Most cycles fall between 21 and 35 days. Cycles consistently shorter than 21 days or longer than 45 days may warrant a conversation with a healthcare provider.

Why does my cycle length vary? Many factors influence cycle length including stress, weight changes, exercise intensity, sleep patterns, and hormonal fluctuations. Minor variations of a few days between cycles are normal.