How to Use Our Period Calculator
To generate your cycle calendar, we only need two pieces of information. Providing accurate inputs will give you the most reliable predictions.
Start Date of Your Last Period: Select the date your last period began. This is “Day 1” of your cycle. It’s important to use the first day of actual menstrual flow, not just light spotting.
Your Average Cycle Length: Enter the number of days your cycle typically lasts. To find this, count the number of days from the start of one period (Day 1) to the day before the next period begins. While the average is 28 days, a normal cycle can range from 21 to 35 days. For best results, average your cycle length over the last three months.
Understanding Your Results
After entering your information, the calculator will provide a calendar showing more than just your next period. It maps out the key events of your entire menstrual cycle.
Predicted Next Period Dates: This shows the estimated start dates for your next several periods, helping you plan for vacations, events, or just be prepared.
Estimated Ovulation Day: This is the single day in your cycle when your ovary is predicted to release an egg. It is calculated to be about 14 days before your next period starts.
Your Fertile Window: This is the 6-day timeframe when you are most fertile and pregnancy is possible. It includes the 5 days leading up to ovulation and the day of ovulation itself.
Here is an example of what your cycle might look like, based on a 28-day cycle starting today, July 26, 2025:
Frequently Asked Questions
What are the phases of the menstrual cycle?
Your menstrual cycle is more than just your period. It’s a month-long process with four distinct phases, driven by hormonal changes.
Menstrual Phase (The Period): Day 1 of your cycle. The uterus sheds its lining, causing bleeding. This typically lasts 2 to 7 days.
Follicular Phase: This phase starts on Day 1 and overlaps with your period. It ends with ovulation. During this time, estrogen rises, and your ovaries prepare an egg for release.
Ovulation: The release of a mature egg from an ovary. This happens around the middle of your cycle and is your most fertile time.
Luteal Phase: This phase begins after ovulation and lasts until your next period. The hormone progesterone rises to prepare your uterus for a potential pregnancy. If pregnancy doesn’t occur, progesterone falls, and your period begins.
My periods are irregular. How does that affect the calculator’s accuracy?
A period calculator is most accurate for those with regular, predictable cycles. If your cycle length varies by more than a few days each month, the calculator’s predictions will be less reliable because it uses a fixed cycle length to forecast future dates.
What to do:
Use your average cycle length from the last 3-6 months.
Use the calculator as a rough guide, but pay closer attention to your body’s physical signs of ovulation (like changes in cervical mucus).
If your cycles are consistently irregular, it’s a good idea to chat with a healthcare provider.
What is PMS and when does it usually start?
PMS, or Premenstrual Syndrome, is a collection of physical and emotional symptoms that many people experience in the days or weeks leading up to their period. These symptoms occur during the luteal phase of your cycle, typically starting 1 to 2 weeks before your period begins. Common symptoms include bloating, headaches, mood swings, breast tenderness, and fatigue. Symptoms usually disappear once your period starts.
Can stress, travel, or diet affect my period?
Absolutely. Your menstrual cycle is regulated by a delicate balance of hormones, which can be easily influenced by external and internal factors.
Stress: High levels of stress can delay ovulation, which in turn delays your period.
Travel: Changing time zones and disrupting your sleep schedule can temporarily throw off your cycle.
Diet & Exercise: Significant changes in your diet, weight, or exercise intensity can also impact your hormonal balance and affect your period’s timing and regularity.
What’s considered a “normal” period?
“Normal” exists within a wide range. What’s most important is understanding what is normal for you. Medically, here are the general guidelines for a normal cycle:
Cycle Length: Anywhere from 21 to 35 days.
Period Length: Bleeding that lasts between 2 and 7 days.
Flow: The amount of blood loss is typically between 30-80ml (about 2-5 tablespoons) over the entire period.
When should I see a doctor about my period?
You should consult a healthcare provider if you experience any of the following, as it could indicate an underlying issue:
Your cycles suddenly become irregular after being regular.
Your cycle is consistently shorter than 21 days or longer than 35 days.
You bleed for more than 7 days.
You bleed so heavily that you have to change your pad or tampon more than every 1-2 hours.
You experience severe pain that isn’t relieved by over-the-counter medication.
You miss a period for more than 90 days (and are not pregnant).
Explore Your Cycle Further
Now that you’ve predicted your period, dive deeper into your fertility. Pinpoint your most fertile days with our dedicated Ovulation Calculator. If you’re planning to become pregnant, our Conception Calculator can help you estimate a due date based on your cycle.
