Period Calculator | Menstrual Cycle & Ovulation Tracker

Period Calculator

Track your cycle, predict your next period, and understand your fertile window.

Usually 28 days.

Usually 5 days.

How to Use This Calculator

  1. Select Last Period Date: Click the calendar icon to choose the first day of your most recent menstrual cycle.
  2. Enter Cycle Length: Input the average number of days between your periods. The default is 28 days, which is the global average.
  3. Enter Period Duration: Input how many days your bleeding typically lasts (usually 3-5 days).
  4. Calculate: Click the button to see your forecast for the next three months, including period start dates and ovulation days.

Example: Planning a Vacation

Let's say your last period started on November 1st, your cycle is usually a bit longer than average (30 days), and you want to know if you'll have your period during a holiday trip in late December.

  1. 1 Set First Day of Last Period to November 1.
  2. 2 Change Average Cycle Length to 30 days.
  3. 3 Click Calculate Cycle.
  4. The Result: The timeline will show your first projected period starting Dec 1 and the second one starting Dec 31. This tells you that mid-to-late December should be period-free!

How This Calculator Works

This tool uses the standard "rhythm method" logic to project future dates. Here is the math behind the predictions:

Predicting the Period

We simply add your Cycle Length to the start date of your last period. For example, if your period started on Nov 1 and your cycle is 28 days, your next period is projected for Nov 29.

Calculating Ovulation

Ovulation generally occurs 14 days before the start of the next period. If your next period is due Nov 29, we subtract 14 days to estimate ovulation on Nov 15.

Determining the Fertile Window

Sperm can survive in the body for up to 5 days. Therefore, your "fertile window" is calculated as the 5 days leading up to ovulation plus the day of ovulation itself.

Understanding Your Cycle Phases

1. Menstrual Phase (Days 1-5)

This phase marks the beginning of your cycle. It occurs when an egg from the previous cycle wasn't fertilized. Because pregnancy hasn't taken place, hormone levels (estrogen and progesterone) drop significantly.

Key Events: The thickened lining of the uterus (endometrium) sheds through the vagina. You may experience cramps caused by uterine contractions.

2. Follicular Phase (Days 1-13)

This phase overlaps with menstruation but lasts longer. The pituitary gland releases Follicle Stimulating Hormone (FSH), stimulating the ovaries to produce 5 to 20 tiny sacs called follicles. Each follicle houses an immature egg. Usually, only one follicle matures into an egg, while the others die off.

Key Events: As the follicle matures, it sets off a surge in estrogen that thickens the uterine lining again, preparing a nutrient-rich environment for a potential embryo.

3. Ovulation (Day 14)

Rising estrogen levels trigger the pituitary gland to release a surge of Luteinizing Hormone (LH). This hormone surge causes the mature follicle to rupture and release the egg. The egg travels down the fallopian tube toward the uterus to be fertilized by sperm.

Key Events: This is your most fertile time. While the egg only survives for 12-24 hours, sperm can survive inside the body for up to 5 days, meaning sex in the days leading up to ovulation can lead to pregnancy.

4. Luteal Phase (Days 15-28)

After the egg releases, the empty follicle transforms into a structure called the "corpus luteum," which secretes progesterone and estrogen. Progesterone keeps the uterine lining thick and healthy for a fertilized egg to implant.

Key Events: If the egg is not fertilized, the corpus luteum shrinks and is resorbed. This causes hormone levels to plummet, which triggers the shedding of the uterine lining and the start of your next period. This drop in hormones is also responsible for PMS symptoms.

Credible Sources & Medical References

Cycle Phase Reference Table

A quick guide to what happens during each phase of an average 28-day cycle.

Swipe table to see details
Phase Typical Days Fertility What's Happening?
Menstruation Days 1–5 Low Uterine lining sheds (period starts).
Follicular Days 6–13 Rising Body prepares an egg; estrogen rises.
Ovulation Day 14 Peak Egg is released; best time for conception.
Luteal Days 15–28 Low Body prepares for possible pregnancy or next period.

Frequently Asked Questions

What does this period calculator do?
This period calculator helps you track your menstrual cycle, predict your next periods, and estimate your fertile window. You enter the first day of your last period, your average cycle length (e.g. 28 or 30 days), and your usual period duration (e.g. 5 days). The tool then projects your next period start dates, estimated ovulation day, and fertile days for the coming months.

How does the calculator predict my next period?
The calculator uses a simple formula: It adds your average cycle length to the first day of your last period. For example, if your last period started on November 1 and your cycle is 28 days, your next period is projected to start on November 29. This pattern is then repeated to estimate the following cycles.

How is my ovulation day calculated?
Ovulation is estimated as 14 days before the start of your next period. If your next period is due on November 29, the calculator subtracts 14 days and estimates ovulation on November 15. This is a standard assumption based on the typical menstrual cycle, but actual ovulation day can vary from person to person and from cycle to cycle.

What is the “fertile window” and how is it calculated?
Your fertile window is the time when you’re most likely to get pregnant if you have unprotected sex. Sperm can survive in the female reproductive tract for up to 5 days. The calculator takes the 5 days before ovulation, plus the ovulation day itself, and marks this as your fertile window. This is an estimate and should not be used as the only method for birth control.

How accurate are the predictions from this period calculator?
The predictions are estimates, not guarantees. They are most accurate if your cycles are regular and you enter a realistic average cycle length based on several months of tracking. Factors like stress, illness, travel, weight changes, and hormonal shifts can all cause your cycle to come earlier or later than predicted.

What if my cycle is irregular – can I still use this calculator?
Yes, you can still use it, but results will be less reliable. If your cycle length varies a lot from month to month, the predictions for your next period, ovulation, and fertile window are only very rough estimates. For irregular cycles, it’s especially helpful to track your periods over several months and discuss any concerns with a healthcare provider.

Which cycle length should I enter if my cycles vary?
If your cycles are slightly variable, you can take the average of your last 6–12 cycles and enter that number (for example, if they range from 27–32 days, your average might be about 29–30 days), or use the length that happens most often. This won’t make predictions perfect, but it gives a better baseline than using just one unusual cycle.

Does the period duration change my fertile window?
Not directly. The fertile window and ovulation are calculated based on your cycle length and the timing of your next period, not on how many days you bleed. Period duration is used mainly to show how long your period is likely to last once it starts, so you can plan around those days.

Can I use this period calculator to avoid pregnancy?
No, you should not rely on this calculator alone as a method of contraception. The rhythm method and calendar-based tools are not 100% reliable, especially if your cycles are irregular. If you want to avoid pregnancy, speak with a healthcare professional about reliable birth control options and use this calculator only as an informational tool.

Can I use this calculator if I’m on hormonal birth control or have PCOS?
You can still use it, but the results may be much less accurate. Hormonal birth control (pill, IUD, patch, etc.) can change or suppress ovulation and bleeding patterns, so the usual “28-day cycle” and “ovulation 14 days before period” rules often don’t apply. Conditions like PCOS, thyroid problems, or recent pregnancy can also make cycles irregular. In these cases, treat the calculator as a very rough guide only.

Medical Disclaimer: This calculator is designed for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Cycle predictions are estimates based on statistical averages and should never be used as a primary method of contraception (birth control) or to diagnose health conditions. Always consult with a qualified healthcare provider regarding any concerns about your menstrual cycle or reproductive health.

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