Period Calculator
Predict your next period, ovulation date, and fertile window. Track your menstrual cycle with accurate forecasts for the next 6 months.
Enter Your Cycle Details
Enter the first day of your most recent period
Day 1 of one period to Day 1 of the next (21–45 days)
How many days your period typically lasts (2–10 days)
How to Use the Period Calculator
Our period calculator predicts your next period, ovulation date, and fertile window based on your cycle history. Enter the first day of your last period, your average cycle length, and how long your period typically lasts — then get a 6-month forecast instantly.
What Does the Period Calculator Calculate?
The calculator uses your last menstrual period (LMP) and average cycle length to predict:
- Next period start date — based on your average cycle length
- Ovulation date — typically 14 days before the next period (cycle length minus 14)
- Fertile window — the 5 days before ovulation through the day after (6 days total)
- Most fertile days — the 2 days before and the day of ovulation
- Current cycle phase — menstrual, follicular, ovulatory, or luteal
How Accurate Is the Period Prediction?
Period calculators are most accurate for women with regular cycles. The average cycle is 28 days, but anywhere from 21–35 days is considered normal. If your cycle varies by more than a few days each month, predictions may be less precise. Track your period over 3–6 months to find your true average cycle length for best results.
Can I Use This to Predict Ovulation?
The calculator estimates ovulation as cycle length minus 14 days from the start of your next period. For a 28-day cycle, ovulation is estimated around Day 14. For a 32-day cycle, it would be around Day 18. For the most accurate ovulation tracking, consider using ovulation predictor kits (OPKs) or basal body temperature (BBT) charting alongside this tool.
Frequently Asked Questions
What is the first day of my period?
Day 1 is the first day of full menstrual flow (not spotting). This is the start of a new cycle and the date you enter as your "last period date".
What is a normal menstrual cycle length?
A typical cycle ranges from 21 to 35 days, with the average being 28 days. A cycle shorter than 21 days or longer than 35 days may be worth discussing with your doctor.
When am I most fertile?
You are most fertile in the 2 days before ovulation and on the day of ovulation itself. Sperm can survive in the reproductive tract for up to 5 days, which extends the fertile window back to 5 days before ovulation.
My cycles are irregular — can I still use this calculator?
Yes, but predictions will be less precise. Use your best estimate of your average cycle length. Tracking your cycle over 3–6 months and averaging the lengths gives a more reliable input for the calculator.
What can cause a late period?
Many factors can delay a period: stress, significant weight changes, excessive exercise, illness, thyroid issues, PCOS, perimenopause, or pregnancy. If your period is more than a week late, take a pregnancy test and consult a healthcare provider if it remains irregular.
How does this relate to pregnancy planning?
Understanding your cycle helps with timing if you are trying to conceive. Use our Pregnancy Due Date Calculator once you become pregnant to estimate your due date and milestones.