Menstrual Cycle Tracker (Simple Calculator Version)

Menstrual cycle tracker to estimate your next period date and fertile window

This menstrual cycle tracker is designed for one decision: estimating your next period start date so you can plan your week with fewer surprises. It also provides an ovulation estimate and a fertile window estimate, because many people search for those dates alongside the next period. This is still a simple calculator, not a medical tool, and it should be treated as an estimate based on averages. If your cycle varies a lot month to month, the results will be less reliable and you should use a longer history to choose a realistic average cycle length.

To use the calculator, enter the first day of your most recent period in the YYYY-MM-DD format. Then enter your average cycle length in days. The cycle length is counted from the first day of one period to the first day of the next period. If you are not sure, 28 days is a common default, but many people are shorter or longer. Add your typical period length in days to estimate the expected end of the next period once it starts. The optional advanced input is luteal phase length. If you leave it blank, the calculator assumes 14 days, which is a common planning assumption. Changing luteal length mainly shifts the ovulation estimate and the fertile window estimate.

The primary output is your estimated next period start date. This is calculated by adding your average cycle length to the first day of your last period. The calculator also shows the estimated next period end date by adding your period length to that predicted start date. For ovulation, the calculator estimates the next ovulation date as the predicted next period start date minus the luteal phase length. The fertile window is shown as the 6 day range that starts 5 days before the ovulation estimate and ends 1 day after it, which is a practical planning window often used for estimates. You will also see a short list of upcoming predicted period start dates, which helps with calendar planning and sanity-checking your cycle length.

Assumptions and how to use this calculator

• Your cycle length is treated as an average and is assumed to be reasonably stable for planning purposes.
• The next period start date is estimated by adding cycle length days to the first day of your last period.
• If luteal phase length is not provided, the calculator uses 14 days as a default planning assumption.
• The fertile window is an estimate only, shown as 5 days before ovulation through 1 day after ovulation.
• This tool does not account for pregnancy, hormonal contraception changes, postpartum changes, perimenopause, illness, travel, or major stress, all of which can shift timing.

Common questions

Why does the calculator ask for the first day of my last period?

Cycle timing is anchored to the first day of bleeding, not the last day. Most cycle tracking and most planning formulas count the cycle from day 1 of bleeding to day 1 of the next period. Using the first day keeps the calculation consistent. If you only remember roughly, choose the best estimate and treat the results as approximate. If you have several months of dates, use the most recent one and set a realistic average cycle length based on your history.

My cycle is not exactly the same every month. Is this still useful?

Yes, as a planning estimate. If your cycle varies by a few days, the next period estimate can still be useful for calendar planning, travel, and scheduling. If your cycle varies widely, the estimate becomes less dependable. In that case, use an average based on at least three to six recent cycles, and consider treating the result as a range around the predicted date. The calculator is intentionally simple and does not try to model variability patterns.

What does the ovulation date mean here?

It is an estimate based on the assumed luteal phase length. Ovulation is estimated as the predicted next period start date minus luteal days. If you do not know your luteal phase length, leaving the default is fine for rough planning. If you do track ovulation using tests or basal body temperature, you can set luteal length to better match your real pattern and make the ovulation estimate align more closely with your history.

What if I do not know my luteal phase length?

Leave it blank and use the default. For many people, luteal length is more stable than the full cycle length, which is why the calculator treats it as an optional refinement rather than a required input. If you later learn your typical luteal length, update it and compare the predicted ovulation date to what you observe. The important point is that this input shifts ovulation timing, not the next period start date, because the next period estimate is driven by cycle length.

When should I not rely on this calculator?

Do not rely on it if you recently started or changed hormonal contraception, recently stopped contraception, are postpartum, are approaching menopause, are recovering from significant illness, or have missed periods. Also, if you suspect pregnancy or have significant pain or unusual bleeding patterns, a calculator is the wrong tool and you should use appropriate medical advice. This page is for calendar planning using typical averages, not diagnosis or fertility treatment planning.