Period Calculator

Period calculator at a glance#

A period calculator predicts the start date of your next periods from the date your last period began and your average cycle length. It counts forward from that start date by your cycle length, then repeats: if your last period started on January 1 and your cycle is 28 days, your next period is due around January 29, and the one after around February 26.

The table assumes a last period that started on January 1. To work it out by hand, add your cycle length in days to the date your last period began, then add the same number again for the period after that. A cycle is measured from the first day of one period to the first day of the next.

What counts as a normal cycle length?#

A 28-day cycle is the common reference, but it is an average, not a fixed rule. In adults a normal cycle runs from 21 to 35 days, and in teens it can range from 21 to 45 days. Cycle length often shifts from month to month, so a prediction is an estimate, not a guarantee.

How do I track my cycle accurately?#

Log the first day of each period for a few months, then average the gaps between start dates to find your typical cycle length. Tracking over several cycles gives a clearer picture than a single month and helps you spot patterns or irregularity. If your periods are very irregular or you have a condition such as PCOS, the average is less reliable, so use the prediction as a rough guide and talk to a healthcare provider about persistent changes.

Enter the date your last period started and your average cycle length in the calculator above for your next predicted period and likely ovulation days. The result is an estimate based on your own averages, so individual variation and irregular cycles will shift the actual dates.

How the menstrual cycle works#

The menstrual cycle is measured from the first day of one period to the first day of the next. In adults it usually runs 21 to 35 days, and in teens 21 to 45 days, so the often-quoted 28-day figure is just an average. Each cycle the uterine lining thickens, an egg is released at ovulation, and if the egg is not fertilized the lining sheds as your period. A period itself typically lasts 2 to 7 days.

When you ovulate and your fertile window#

Ovulation is the point in the cycle when an ovary releases an egg, and it is the most fertile part of the month. The fertile window spans roughly the 5 days before ovulation plus the day of ovulation, because sperm can survive several days in the body. If your cycle is regular, ovulation falls about 14 days before your next period starts, so a longer or shorter cycle shifts it earlier or later. The calculator marks these likely ovulation days alongside your predicted period.

What tracking your cycle tells you#

Logging the first day of each period for a few months shows your real cycle length and how much it varies, which makes the prediction more accurate than a single month. A consistent cycle is often a sign of stable hormones, while a sudden change in length, very irregular bleeding, or a missed period can be worth raising with a provider. Tracking also helps you anticipate cramps, mood changes, or PMS, and time intercourse if you are trying to conceive or avoid pregnancy.

Period calculator FAQ#

How does the period calculator predict my next period?#

It adds your average cycle length to the date your last period started. If your last period began on the 1st and your cycle is 28 days, the next is due around the 29th, and it repeats from there. The result is an estimate, since cycle length shifts from month to month.

Can it show my fertile and ovulation days?#

Yes. It estimates ovulation about 14 days before your next predicted period and flags the fertile window as the 5 days before ovulation plus ovulation day. This is the best time to conceive, but it is an estimate, not a precise test.

What does my cycle say about my health?#

A regular, predictable cycle usually reflects stable hormones. Persistent irregularity, very heavy or absent periods, or a sudden change in cycle length can point to issues such as thyroid problems or hormonal imbalance and are worth discussing with a provider.

Does it work with irregular periods or PCOS?#

It still gives an estimate, but it is less reliable when cycles vary a lot, as with PCOS. Track several months to find your average range and treat the prediction as a rough guide rather than a fixed date.

Can I use it for natural family planning?#

You can use the fertile-window estimate to plan or avoid pregnancy, but calendar-based methods are not highly reliable on their own. Combine cycle tracking with other fertility-awareness signs, or a clinically tested method, if you are relying on it for contraception.