Track your cycle and predict your next monthly period
Usually 21-35 days
Your Predictions
Next Period Starts:–
Estimated Ovulation:–
Fertile Window:–
How to Calculate Monthly Period Cycles
Understanding how to calculate your monthly period is essential for reproductive health, family planning, and general wellness. A menstrual cycle is measured from the first day of one period to the first day of the next. While the average cycle is 28 days, it is perfectly normal for cycles to range anywhere from 21 to 35 days in adults.
The Calculation Logic
To manually calculate your next period, you need to track your dates for at least three months to find your average cycle length. The formula is:
Next Period Start Date = (Last Period Start Date) + (Average Cycle Length in Days)
Predicting Ovulation
Ovulation typically occurs about 14 days before your next period starts. If you have a 28-day cycle, you likely ovulate on day 14. However, if you have a 32-day cycle, ovulation would occur around day 18.
Example Calculation
Last Period Date: January 1st
Cycle Length: 30 Days
Next Period: January 31st
Ovulation Day: January 17th (31 minus 14)
Fertile Window: January 12th – January 18th
function calculatePeriod() {
var lastDateVal = document.getElementById('lastPeriodDate').value;
var cycleLength = parseInt(document.getElementById('cycleLength').value);
if (!lastDateVal) {
alert('Please select the start date of your last period.');
return;
}
if (isNaN(cycleLength) || cycleLength 50) {
alert('Please enter a valid cycle length (usually 21-35 days).');
return;
}
var lastDate = new Date(lastDateVal);
// Calculate Next Period
var nextPeriod = new Date(lastDate);
nextPeriod.setDate(lastDate.getDate() + cycleLength);
// Calculate Ovulation (approx 14 days before next period)
var ovulationDate = new Date(nextPeriod);
ovulationDate.setDate(nextPeriod.getDate() – 14);
// Calculate Fertile Window (5 days before ovulation + ovulation day)
var fertileStart = new Date(ovulationDate);
fertileStart.setDate(ovulationDate.getDate() – 5);
// Format options
var options = { month: 'short', day: 'numeric', year: 'numeric' };
// Display results
document.getElementById('nextPeriodDateDisplay').innerHTML = nextPeriod.toLocaleDateString(undefined, options);
document.getElementById('ovulationDateDisplay').innerHTML = ovulationDate.toLocaleDateString(undefined, options);
document.getElementById('fertileWindowDisplay').innerHTML = fertileStart.toLocaleDateString(undefined, options) + " – " + ovulationDate.toLocaleDateString(undefined, options);
document.getElementById('periodResults').style.display = 'block';
}
// Set default date to today for convenience
var today = new Date().toISOString().split('T')[0];
document.getElementById('lastPeriodDate').value = today;