Period Calculator - Track Your Menstrual Cycle Free

What Does This Calculator Do?

Knowing when your period is coming — and planning around it — is something most women wish they could do with more confidence. Whether you’re scheduling a holiday, preparing for an important event, tracking your reproductive health, or trying to conceive, having your upcoming period dates mapped out in advance makes life meaningfully easier. This Period Calculator does exactly that. You enter a few details about your cycle and it predicts up to six future period start dates, along with your ovulation date and fertile window for the first upcoming cycle.

What makes it more than just a basic date adder is the combination of outputs it provides. You don’t just get a list of predicted dates — you get a full event table that lays out every key milestone across your upcoming cycles in one organized view, covering your LMP, fertile window start and end, ovulation date, and each predicted period date. A color-coded timeline chart visualizes the first cycle in full, with the fertile window shown in pink and ovulation marked in blue, so you can see the shape of your cycle rather than just reading dates off a table.

For women with irregular cycles, the calculator automatically builds in a ±2 day buffer around the ovulation and fertile window estimates, giving you a more realistic and useful range rather than a single point estimate that might be off by a few days.

The Formula Behind It

The predictions are based on the standard menstrual cycle model used in reproductive health:

Next Period Date = Last Menstrual Period + Cycle Length

Each subsequent predicted period simply adds another cycle length on top of the previous one:

Period N = LMP + (Cycle Length × N)

For example, with an LMP of April 1 and a 28-day cycle:

• Period 1: April 1 + 28 = April 29
• Period 2: April 1 + 56 = May 27
• Period 3: April 1 + 84 = June 24
• Period 4: April 1 + 112 = July 22
• Period 5: April 1 + 140 = August 19
• Period 6: April 1 + 168 = September 16

For the ovulation and fertile window within the first predicted cycle, the same formula used in the Ovulation Calculator applies:

Ovulation Date = LMP + (Cycle Length − 14)

Using the same example: April 1 + (28 − 14) = April 15

• Fertile Window Start = April 15 − 5 = April 10
• Fertile Window End = April 15 + 1 = April 16

The logic behind subtracting 14 days from the cycle length is that the luteal phase — the time between ovulation and the next period — is relatively consistent at around 14 days across most women, even when overall cycle length varies. So ovulation is estimated 14 days before the next period rather than 14 days after the last one.

For irregular cycles, ±2 days are added to the fertile window and ovulation estimate, widening the window from 6 days to 8 days to account for the unpredictability of the cycle.

How to Use It

It takes just seconds to generate your predictions:

1. Enter your Last Menstrual Period (LMP) — the first day of your most recent period
2. Enter your Cycle Length in days — your average if it varies (21–35 days is the typical range)
3. Select your Cycle Regularity — Regular (±2 days) or Irregular (>±2 days)
4. Choose Cycles to Predict — select how many future periods you want to forecast, from 1 up to 6
5. Click Calculate to see your results
6. Review the event table with all key dates and the timeline chart showing your first cycle’s phases

Why It’s Worth Using

Most period tracking apps require you to log your cycle data over weeks or months before they can make accurate predictions. This calculator gives you useful predictions immediately, right from your first visit, with nothing more than your last period date and your average cycle length. That makes it genuinely helpful when you need to plan ahead quickly — checking if your period will fall during a trip you’re booking, figuring out your cycle dates for a medical appointment, or simply wanting a clearer picture of the next few months.

The multi-cycle view — up to six periods ahead — is particularly useful for longer-term planning. Instead of knowing only when your next period is, you can see the full pattern across the next several months in one view, which makes scheduling around your cycle much more practical.

The ovulation and fertile window data included for the first cycle also makes this tool useful beyond just period tracking — giving women who are trying to conceive or simply staying informed about their fertility a fuller picture from a single calculation.