Estimate your next period and fertile window based on your cycle history.
Your Estimated Cycle Information:
Understanding Your Menstrual Cycle and Using This Calculator
The menstrual cycle is a natural monthly series of changes a woman's body goes through in preparation for the possibility of pregnancy. Each month, one of the ovaries releases an egg—a process called ovulation. At the same time, hormonal changes prepare the uterus for pregnancy. If ovulation takes place and the egg isn't fertilized by sperm, the lining of the uterus sheds, resulting in menstrual bleeding, commonly known as a period.
The Menstrual Cycle Phases:
Menstruation: The first day of your period is considered Day 1 of your cycle. This is when the uterine lining sheds, causing bleeding. It typically lasts 3 to 7 days.
Follicular Phase: This phase begins on the first day of your period and ends with ovulation. During this time, follicles in the ovary mature, and one follicle typically becomes dominant.
Ovulation: The release of a mature egg from the ovary. This usually occurs about 14 days *before* the start of the next period. For a typical 28-day cycle, ovulation occurs around Day 14.
Luteal Phase: This phase begins after ovulation and lasts until the start of the next period. The corpus luteum (what remains of the follicle) produces hormones that prepare the uterus for a potential pregnancy. If pregnancy doesn't occur, the corpus luteum breaks down, leading to menstruation.
What is an Irregular Menstrual Cycle?
An irregular menstrual cycle is one where the cycle length varies significantly, or periods occur at unpredictable intervals. Factors contributing to irregularity can include:
Stress
Significant weight changes
Hormonal imbalances (e.g., PCOS, thyroid issues)
Certain medications
Perimenopause
Underlying medical conditions
How This Calculator Works:
This calculator provides estimations based on the data you input. It uses a common method for predicting cycle dates:
Next Period Estimate: The calculator adds your average cycle length (in days) to the first day of your last period. This gives an estimated start date for your next period.
Formula: Date of Last Period's First Day + Average Cycle Length (days) = Estimated Next Period Start Date
Ovulation Estimate: Ovulation typically occurs about 14 days *before* the start of the next menstrual period. The calculator subtracts 14 days from the estimated next period start date.
Formula: Estimated Next Period Start Date – 14 days = Estimated Ovulation Date
Fertile Window Estimate: Sperm can survive in the female reproductive tract for up to 5 days, and the egg is viable for about 12-24 hours after ovulation. Therefore, the fertile window generally includes the 5 days leading up to ovulation, plus the day of ovulation itself.
Calculation: The fertile window is estimated as the 6-day period ending on the estimated ovulation date.
Important Disclaimer:
This calculator is a tool for informational purposes only and should not be considered a substitute for professional medical advice. Irregular cycles can sometimes indicate underlying health conditions. If you have concerns about your menstrual cycle, consult with a healthcare provider. The estimations provided are based on averages and may not perfectly reflect your individual cycle patterns.
Example Usage:
Let's say your last period started on October 26, 2023. Your cycle length has varied, but your average is about 32 days, and your period typically lasts 6 days.
Next Period Estimate: October 26, 2023 + 32 days = November 27, 2023
Ovulation Estimate: November 27, 2023 – 14 days = November 13, 2023
Fertile Window Estimate: Approximately November 8, 2023 to November 13, 2023
function calculateCycleDates() {
var lastPeriodStartStr = document.getElementById("lastPeriodStart").value;
var cycleLengthAvg = parseInt(document.getElementById("cycleLengthAvg").value);
var periodDurationAvg = parseInt(document.getElementById("periodDurationAvg").value);
var resultElement = document.getElementById("result");
var nextPeriodEstimateElement = document.getElementById("nextPeriodEstimate");
var fertileWindowEstimateElement = document.getElementById("fertileWindowEstimate");
var ovulationEstimateElement = document.getElementById("ovulationEstimate");
// Clear previous results
nextPeriodEstimateElement.textContent = "";
fertileWindowEstimateElement.textContent = "";
ovulationEstimateElement.textContent = "";
if (!lastPeriodStartStr || isNaN(cycleLengthAvg) || isNaN(periodDurationAvg) || cycleLengthAvg <= 0 || periodDurationAvg <= 0) {
resultElement.innerHTML = "
Error:
Please enter valid dates and positive numbers for cycle length and duration.";
return;
}
var lastPeriodStartDate = new Date(lastPeriodStartStr);
// Calculate Next Period Start Date
var nextPeriodStartDate = new Date(lastPeriodStartDate);
nextPeriodStartDate.setDate(lastPeriodStartDate.getDate() + cycleLengthAvg);
var nextPeriodFormatted = nextPeriodStartDate.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
// Calculate Estimated Ovulation Date (approx. 14 days before next period)
var ovulationDate = new Date(nextPeriodStartDate);
ovulationDate.setDate(nextPeriodStartDate.getDate() – 14);
var ovulationFormatted = ovulationDate.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
// Calculate Fertile Window (approx. 5 days before ovulation + ovulation day)
var fertileWindowStart = new Date(ovulationDate);
fertileWindowStart.setDate(ovulationDate.getDate() – 5);
var fertileWindowStartFormatted = fertileWindowStart.toLocaleDateString('en-US', { year: 'numeric', month: 'long', day: 'numeric' });
var fertileWindowEndFormatted = ovulationFormatted; // Fertile window ends on ovulation day
// Display Results
nextPeriodEstimateElement.textContent = "Estimated Next Period Start: " + nextPeriodFormatted;
ovulationEstimateElement.textContent = "Estimated Ovulation Date: " + ovulationFormatted;
fertileWindowEstimateElement.textContent = "Estimated Fertile Window: " + fertileWindowStartFormatted + " to " + fertileWindowEndFormatted;
}