Home › Health & Education › Period Calculator
Period Calculator
Our free period calculator predicts your next period, your fertile window, and your next three periods. Enter the first day of your last period and your usual cycle length, and it works out the dates so you can plan ahead.
Usual range: 21–35 days. 28 is the average.
How many days your period usually lasts.
If your cycles vary, use the average of your last three. A regular 28-day cycle is the most common.
| Estimated ovulation day | |
| Fertile window | |
| Next period (1st) | |
| Following period (2nd) | |
| Following period (3rd) |
How it works
A menstrual cycle is counted from the first day of one period to the first day of the next period. The calculator adds your cycle length to the first day of your last period to predict the next one. If your cycle is 28 days, your next period starts 28 days after the last one began. It then repeats the same step to predict the two periods after that, so you can see three months ahead at a glance.
Ovulation, when an egg is released, usually happens about 14 days before the next period starts. The calculator finds that date by subtracting 14 days from the next predicted period. The fertile window is the five days before ovulation plus ovulation day itself, a span of about six days. Pregnancy is most likely during this window because sperm can survive in the body for up to five days, while an egg lives for about 24 hours after ovulation.
Ovulation day = Next period − 14 days
Fertile window = Ovulation day − 5 days through Ovulation day
These dates are estimates built on averages, not measurements of your body. Real cycles shift. Stress, illness, travel across time zones, a change in exercise or weight, and hormonal birth control can all move ovulation by a few days, and some months a cycle is simply longer or shorter for no clear reason. That is why the calculator asks for your average cycle length: if your last three cycles were 27, 29, and 28 days, enter 28. The more cycles you track, the better the prediction becomes.
Knowing your dates helps with everyday planning: packing supplies before a trip, scheduling a beach vacation, or noticing early if a period is late. A late period is common now and then, but if your periods stop, become very irregular, or come with severe pain, heavy bleeding, or bleeding between periods, that is worth a conversation with a doctor rather than a calculator.
Worked example
Say your last period started on November 1, 2026, and your cycle is a regular 28 days. Here is how the calculator works it out, step by step.
First, the next period: November 1 plus 28 days is November 29, 2026. Next, ovulation: November 29 minus 14 days is November 15, 2026. Then the fertile window: five days before November 15 is November 10, so the fertile window runs November 10 through November 15, 2026. Finally, the next three periods: add 28 days again and again. November 29 plus 28 days is December 27, 2026, and December 27 plus 28 days is January 24, 2027. So the three predicted period start dates are November 29, 2026, December 27, 2026, and January 24, 2027. If your period usually lasts 5 days, the November period would run about November 29 through December 3.
Try a different cycle to see how it changes. With a 30-day cycle starting November 1, the next period would be December 1, 2026, ovulation would be about November 17, and the fertile window would be November 12 through November 17. A longer cycle pushes everything later, which is why using your own average matters more than using the textbook 28 days.
FAQ
How does a period calculator predict my next period?
It adds your average cycle length to the first day of your last period. For example, if your last period started on November 1, 2026 and your cycle is 28 days, your next period is predicted to start on November 29, 2026.
What is the fertile window?
The fertile window is the roughly six-day span ending on ovulation day, when pregnancy is most likely. Ovulation usually happens about 14 days before your next period. Sperm can survive up to five days, which is why the window starts five days before ovulation.
How accurate is a period calculator?
It is an estimate based on averages. Most cycles vary by a few days from month to month because of stress, illness, travel, or hormonal changes. Track your last few cycles and use your average length for the best estimate.
Can I use this calculator to avoid pregnancy?
No. A period calculator is not a reliable method of birth control, because ovulation can shift from month to month. If you want to avoid pregnancy, use a proven method and talk to a doctor about your options.
What is a normal cycle length?
A typical adult cycle is 21 to 35 days, with 28 days the most common average. Cycles shorter than 21 days or longer than 35 days on a regular basis are worth mentioning to a doctor, especially if they are new for you.