Period Calculator
Enter the first day of your last period and your usual cycle length to predict when your next period will start, plus your estimated ovulation day and fertile window.
Enter Your Information
How the period calculator works
Your cycle is counted from the first day of one period to the day before the next one starts. The calculator adds your average cycle length to the first day of your last period:
Next period = first day of last period + cycle length
Ovulation usually happens about 14 days before the next period, not always on day 14 of the cycle. The luteal phase (after ovulation) is fairly constant, between 12 and 16 days for most women, so a longer cycle means later ovulation.
Worked example
Your last period started on March 3 and your cycles last 28 days.
- Next period: March 3 + 28 days = March 31
- Estimated ovulation: March 31 minus 14 days = March 17
- Fertile window: about 5 days before ovulation through ovulation day, March 12 to 17
With a 32-day cycle the next period would be April 4 and ovulation around March 21.
What is a normal cycle?
| Measure | Typical range |
|---|---|
| Cycle length (adults) | 21 to 35 days |
| Period length | 2 to 7 days |
| Month-to-month variation | A few days is normal |
| Ovulation | About 12 to 16 days before the next period |
Cycles are often irregular during the first years after the first period, after stopping hormonal contraception, after pregnancy or breastfeeding, and in perimenopause.
Common mistakes
- Counting from the last day of bleeding instead of the first day of the period.
- Using one cycle as the average. Average your last 3 to 6 cycles for a better estimate.
- Assuming ovulation is always on day 14. It depends on your own cycle length.
- Relying on the calculator as contraception. It gives an estimate and cannot prevent pregnancy.
- Using it with very irregular cycles. The prediction will be less reliable, so use the shortest and longest cycle to see a range.
When to see a doctor
Consult a doctor or gynecologist if you miss periods for three months or more without being pregnant, if cycles are shorter than 21 days or longer than 35 days, if bleeding lasts more than 7 days or soaks a pad or tampon every hour or two, if you have severe pain, or if you bleed between periods or after menopause. If your period is late, a pregnancy test is the first step. This tool is informational only.
Frequently Asked Questions
How do I calculate my next period?
Add your usual cycle length to the first day of your last period. For example, if your last period started on March 3 and your cycle is 28 days, your next period is expected around March 31.
How many days is a normal menstrual cycle?
A normal adult cycle lasts between 21 and 35 days, counted from the first day of one period to the first day of the next. A difference of a few days from month to month is common.
When do I ovulate?
Ovulation usually happens around 12 to 16 days before your next period. In a 28-day cycle that is about day 14, but in longer cycles it happens later.
Why is my period late if I am not pregnant?
Stress, illness, weight changes, intense exercise, hormonal conditions such as PCOS or thyroid problems, and some medications can delay a period. If it is late by more than a week or happens often, talk to a doctor.