Period Calculator - Track Your Menstrual Cycle Free

The Period Calculator takes the uncertainty out of cycle tracking by giving you predicted start dates for your upcoming periods — up to six cycles ahead — along with ovulation and fertile window estimates for the first cycle in your forecast. Knowing when your next period is likely to arrive helps with everyday planning, travel scheduling, and preparing for symptoms in advance. And for anyone who is also keeping an eye on their fertility, having the ovulation and fertile window dates alongside the period predictions means you get a fuller picture of your reproductive cycle all from a single calculation. Enter three straightforward details about your cycle and the results come back immediately with both a date-by-date table and a visual timeline.

How the Calculation Works: Future period start dates are calculated by adding your cycle length to the date of your last menstrual period and repeating that forward for however many cycles you choose to predict. Ovulation for the first predicted cycle is estimated at 14 days before the next period, with the fertile window covering the five days before ovulation through to the day after. For irregular cycles, the estimated dates for ovulation and the fertile window are widened by two days in each direction to reflect natural variation.

How to Use the Period Calculator

Four inputs are all that is needed for a complete multi-cycle forecast:

Enter Your Last Menstrual Period (LMP) Date by selecting the first day of your most recent period from the date picker. This is the anchor point the calculator builds all future predictions from, so selecting the right date is the most important step in getting accurate results.

Enter Your Average Cycle Length in days. Cycles typically range between 21 and 35 days, with 28 days being the most commonly cited average. Enter your own personal average here rather than a textbook figure — even if it is an estimate based on recent months — for the most relevant predictions.

Select Your Cycle Regularity by choosing Regular (±2 days) if your cycle tends to follow a fairly consistent pattern, or Irregular (more than ±2 days) if the timing shifts more noticeably between cycles. The irregular setting applies a wider estimate range to the ovulation and fertile window dates to account for that variability honestly.

Choose How Many Cycles to Predict by selecting a number between 1 and 6. Predicting further ahead gives you more time to plan around upcoming periods, though it is worth remembering that predictions naturally become less precise the further they extend from the last known date.

Click Calculate and your results appear straight away — a table listing all predicted period start dates alongside the LMP, fertile window, and ovulation dates for the first cycle, plus a color-coded timeline chart mapping out the key events of the first cycle visually.

Why Use the Period Calculator Online?

Predicts Multiple Cycles at Once: Rather than calculating one period at a time, this tool forecasts up to six future cycles in a single calculation. That multi-cycle view is genuinely useful for anyone who wants to plan ahead — whether for travel, events, medical appointments, or simply having supplies ready in advance.

Includes Ovulation and Fertile Window for the First Cycle: Period prediction and fertility tracking are closely connected, and having both sets of dates in the same result means you do not need to use a separate tool if you are also paying attention to your fertile window. The ovulation date and the six-day fertile window are calculated and displayed automatically alongside your period predictions.

Handles Irregular Cycles Honestly: Cycles that vary more than a couple of days from month to month are common, and applying a fixed calculation to an irregular cycle produces false precision. The irregular setting widens the estimated fertile window and ovulation range to reflect real variability, giving you a more realistic picture rather than misleadingly exact dates.

Color-Coded Timeline Chart for the First Cycle: The visual timeline maps out your menstrual phase, fertile window, and ovulation day across the first predicted cycle in distinct colors, making it immediately easy to see how the different phases relate to each other and where they fall within the cycle.

Free, Private, and Instantly Accessible: There is no account required, no data stored, and no cost attached. Enter your cycle details and receive your full multi-cycle prediction with fertility dates in seconds — straightforward, private, and available on any device.