Tracking your menstrual cycle is a valuable tool for understanding your body's natural rhythms, predicting fertility, and managing your health. This calculator helps you estimate key dates based on your past cycles.
How the Calculation Works:
Last Period Start Date: This is the foundation of the calculation. You need to know the first day of your most recent menstrual period.
Average Cycle Length: This is the number of days from the first day of one period to the first day of your next period. The average length is typically around 28 days, but it can vary significantly from person to person, usually ranging from 21 to 35 days. To calculate this, you'd look at several past cycles and find the average duration.
Average Period Length: This is the number of days your actual period lasts. The average is usually between 3 and 7 days.
Calculating Key Dates:
Our calculator uses the following logic:
Next Period Start Date: Calculated by adding your Average Cycle Length (in days) to your Last Period Start Date.
Period End Date: Calculated by adding your Average Period Length (in days) to your Last Period Start Date.
Estimated Ovulation Date: This is generally estimated to occur around 14 days *before* your next period is due. Since the luteal phase (the time after ovulation until your period starts) is usually more consistent than the follicular phase (from your period start to ovulation), it's more reliable to count backward from the next period's start. So, it's approximately: Next Period Start Date – 14 days.
Fertile Window: This is the period when pregnancy is most likely to occur. It's generally considered to be the 5 days leading up to ovulation and the day of ovulation itself. This is because sperm can live in the female reproductive tract for up to 5 days, and the egg is viable for about 12-24 hours after ovulation.
Why Track Your Cycle?
Family Planning: Identifying your fertile window can help if you are trying to conceive or trying to avoid pregnancy.
Health Monitoring: Irregular cycles, changes in flow, or severe pain can sometimes indicate underlying health conditions. Tracking helps you notice these changes.
Symptom Management: Understanding your cycle can help you anticipate and manage symptoms associated with PMS (premenstrual syndrome) or other hormonal fluctuations.
Personal Awareness: Simply knowing your body better can be empowering.
Disclaimer: This calculator provides estimates based on the data you provide. Individual cycles can vary due to many factors including stress, illness, travel, and hormonal changes. For accurate medical advice or if you have concerns about your cycle, please consult a healthcare professional.
function calculateCycle() {
var lastPeriodStartDateInput = document.getElementById("lastPeriodStart");
var cycleLengthInput = document.getElementById("cycleLength");
var periodLengthInput = document.getElementById("periodLength");
var predictedDatesElement = document.getElementById("predictedDates");
var lastPeriodStartDateStr = lastPeriodStartDateInput.value;
var cycleLength = parseInt(cycleLengthInput.value);
var periodLength = parseInt(periodLengthInput.value);
// Clear previous results and errors
predictedDatesElement.innerHTML = "";
// — Input Validation —
if (!lastPeriodStartDateStr) {
predictedDatesElement.innerHTML = "Please enter your last period start date.";
return;
}
if (isNaN(cycleLength) || cycleLength <= 0) {
predictedDatesElement.innerHTML = "Please enter a valid average cycle length (greater than 0 days).";
return;
}
if (isNaN(periodLength) || periodLength = cycleLength) {
predictedDatesElement.innerHTML = "Period length cannot be greater than or equal to cycle length.";
return;
}
// — Date Calculations —
var lastPeriodStartDate = new Date(lastPeriodStartDateStr);
// Calculate Next Period Start Date
var nextPeriodStartDate = new Date(lastPeriodStartDate);
nextPeriodStartDate.setDate(lastPeriodStartDate.getDate() + cycleLength);
// Calculate Period End Date
var periodEndDate = new Date(lastPeriodStartDate);
periodEndDate.setDate(lastPeriodStartDate.getDate() + periodLength – 1); // -1 because it includes the start day
// Calculate Estimated Ovulation Date (approx. 14 days before next period start)
var estimatedOvulationDate = new Date(nextPeriodStartDate);
estimatedOvulationDate.setDate(nextPeriodStartDate.getDate() – 14);
// Calculate Fertile Window (5 days before ovulation to ovulation day)
var fertileWindowStart = new Date(estimatedOvulationDate);
fertileWindowStart.setDate(estimatedOvulationDate.getDate() – 5);
var fertileWindowEnd = new Date(estimatedOvulationDate); // Ovulation day is the end of the fertile window
// — Formatting Dates —
function formatDate(date) {
var options = { year: 'numeric', month: 'long', day: 'numeric' };
return date.toLocaleDateString(undefined, options);
}
// — Display Results —
var resultHTML = "