Period Calculator

What This Tool Does

This calculator estimates the date of your next period and your likely fertile window, based on the first day of your last period, how long it lasted, and your average cycle length.

You can also use this XIRR Calculator.

How It’s Calculated

Next Period Start Date = Last Period Start Date + Average Cycle Length

Ovulation typically occurs about 14 days before the next period begins, not 14 days after the last one started — this matters more for longer or shorter cycles.

Estimated Ovulation Date ≈ Next Period Start Date − 14 days

The fertile window is generally estimated as the five days leading up to ovulation through one to two days after.

Worked example: Last period started March 1, lasting 5 days, with an average cycle length of 30 days.

• Next period estimate: March 1 + 30 days = March 31
• Ovulation estimate: March 31 − 14 days = March 17
• Fertile window estimate: roughly March 12 to March 19

Edge Cases and Special Rules

• Regular still means variable: A cycle is generally considered “regular” if the longest and shortest cycles over several months vary by less than about 8 days — not identical every time.
• One cycle isn’t much to go on: Predictions based on a single past period are rough estimates. Accuracy improves as more cycles are tracked over time.
• Irregular cycles reduce accuracy significantly: If cycle length varies widely month to month, predictions become much less reliable, and a calculator can’t account for the underlying cause.
• Some situations make prediction unreliable: Postpartum recovery, breastfeeding, perimenopause, certain medications, and conditions like PCOS can all disrupt normal cycle patterns — a calculator won’t reflect these accurately.
• Not a reliable method of contraception: Estimated fertile windows are approximations, not precise measurements. Calendar-based tracking alone has a meaningfully high failure rate for either avoiding or achieving pregnancy — discuss reliable options with a healthcare provider.

Tracking a First Period

For someone experiencing their first period, there’s no prior cycle data to calculate from, so a calculator can’t yet predict a next date. It’s also common for cycles to be irregular for the first couple of years after a first period, as the body’s hormonal rhythm is still establishing itself. Predictions generally become more meaningful once a few cycles have been tracked.

Who Uses This

• Anyone wanting a general estimate of when their next period will start
• People tracking their cycle to identify a likely fertile window for conception planning
• Those preparing for a medical appointment and wanting to describe recent cycle history
• Anyone noticing their period seems late and wanting a quick reference point

Period Calculator vs. Ovulation Calculator vs. Due-Date Calculator

These tools answer related but different questions. A period calculator estimates your next period and general fertile window from your cycle history. A dedicated ovulation calculator focuses specifically on narrowing down the fertile window, sometimes using additional data like basal body temperature. A pregnancy or due-date calculator works in the opposite direction — starting from a known or estimated conception date (or last period date) to estimate when a pregnancy is likely to reach term.

FAQ

How accurate is a period calculator?

It’s a general estimate based on your input cycle length — accuracy depends heavily on how regular your cycles actually are.

What if my period doesn’t arrive on the estimated date?

A few days’ variation is common and not usually a concern; if it’s significantly late or you’re worried, consider a pregnancy test or speaking with a healthcare provider.

Can I use this to avoid or plan a pregnancy?

It can give a general idea of your fertile window, but it isn’t precise enough to rely on alone for either avoiding or achieving pregnancy.

Why is my cycle irregular?

Many factors can affect cycle regularity, including stress, weight changes, medications, and underlying health conditions — a doctor can help identify the cause if it’s a persistent concern.

Does this work for tracking a first period?

Not for prediction, since there’s no prior cycle to calculate from — but it can help once a few cycles have occurred.

Try It Above

Enter the first day of your last period, how long it lasted, and your average cycle length in the calculator above to get an estimate. Results are for general planning purposes only and aren’t a substitute for medical advice — consult a healthcare provider with any concerns about your cycle.