Track your fertile window and optimize your conception planning.
Your Results
Estimated Ovulation Date:–
Most Fertile Window:–
(The 5 days leading up to and including ovulation)
Next Period Starts:–
Approx. Due Date:–
How the Ovulation Date Calculator Works
Predicting your most fertile days is essential whether you are trying to conceive or simply tracking your reproductive health. Our Ovulation Date Calculator uses the standard "Calendar Method" to estimate when your ovaries will release an egg.
The Mathematics of Fertility
For most women, ovulation occurs approximately 14 days before the start of the next menstrual period. While many sources assume a 28-day cycle, our calculator adjusts for your specific average cycle length (which typically ranges from 21 to 35 days).
Ovulation Date: Last Period Start Date + (Cycle Length – 14 Days)
Fertile Window: The 5 days preceding ovulation plus the day of ovulation itself. This is because sperm can live inside the female reproductive tract for up to 5 days.
Due Date (Naegele's Rule): If conception occurs on the ovulation date, the due date is calculated by adding 280 days (40 weeks) to the first day of your last period.
Example Calculations
Example 1: Standard 28-Day Cycle
If your last period started on January 1st and your cycle is 28 days:
– Ovulation: Jan 1 + (28-14) = January 15th.
– Fertile Window: January 10th to January 15th.
Example 2: Longer 32-Day Cycle
If your last period started on January 1st and your cycle is 32 days:
– Ovulation: Jan 1 + (32-14) = January 19th.
– Fertile Window: January 14th to January 19th.
Important Considerations
Please note that this tool provides estimates. Cycle lengths can vary month to month due to stress, diet, or illness. For higher precision, consider combining this calculator with Basal Body Temperature (BBT) tracking or Ovulation Predictor Kits (OPKs) that detect Luteinizing Hormone (LH) surges.
function calculateOvulation() {
var lastPeriodDateInput = document.getElementById('lastPeriodDate').value;
var cycleLengthInput = document.getElementById('cycleLength').value;
if (!lastPeriodDateInput || !cycleLengthInput) {
alert("Please select your last period date and enter your average cycle length.");
return;
}
var cycleLength = parseInt(cycleLengthInput);
if (cycleLength 45) {
alert("Please enter a cycle length between 20 and 45 days.");
return;
}
var lastDate = new Date(lastPeriodDateInput);
// Calculate Ovulation Day (Cycle Length – 14 days from the next expected period)
// Formula: Last Date + (Cycle Length – 14)
var ovulationDate = new Date(lastDate);
ovulationDate.setDate(lastDate.getDate() + (cycleLength – 14));
// Calculate Fertile Window (5 days before ovulation to day of ovulation)
var fertileStart = new Date(ovulationDate);
fertileStart.setDate(ovulationDate.getDate() – 5);
// Calculate Next Period
var nextPeriodDate = new Date(lastDate);
nextPeriodDate.setDate(lastDate.getDate() + cycleLength);
// Calculate Estimated Due Date (40 weeks / 280 days from Last Period Start)
var dueDate = new Date(lastDate);
dueDate.setDate(lastDate.getDate() + 280);
// Format Dates
var options = { weekday: 'short', year: 'numeric', month: 'short', day: 'numeric' };
document.getElementById('ovulationDay').innerText = ovulationDate.toLocaleDateString('en-US', options);
document.getElementById('fertileWindow').innerText = fertileStart.toLocaleDateString('en-US', { month: 'short', day: 'numeric' }) + " – " + ovulationDate.toLocaleDateString('en-US', { month: 'short', day: 'numeric', year: 'numeric' });
document.getElementById('nextPeriod').innerText = nextPeriodDate.toLocaleDateString('en-US', options);
document.getElementById('dueDate').innerText = dueDate.toLocaleDateString('en-US', options);
// Show Results
document.getElementById('resultsArea').style.display = 'block';
}