*Note: This is an estimate based on cycle averages. Clearblue tests detect the LH surge to confirm these dates.*
How to Use the Clearblue Ovulation Calculator
Trying to conceive requires timing. Our Clearblue Ovulation Calculator helps you identify the best days to use your Clearblue Digital or Advanced Ovulation Tests. By analyzing your last menstrual period (LMP) and your average cycle length, we can predict when your Luteinizing Hormone (LH) is most likely to surge.
Why Timing Matters
There are only a few days in every cycle when you can get pregnant—the day of ovulation and the days leading up to it. Clearblue tests are designed to detect the rise in fertility hormones. To avoid missing your window, you need to start testing early enough.
Understanding the Results
Start Testing Date: This is the day you should begin using your Clearblue sticks. It is calculated based on your cycle length to ensure you don't miss the LH surge.
Estimated Ovulation: Most women ovulate 12 to 16 days before their next period starts.
Fertile Window: Sperm can survive inside the body for up to 5 days, while an egg survives for 12-24 hours. This 6-day window is your opportunity for conception.
Example Calculation
If your last period started on October 1st and you have a 28-day cycle:
Your next period is expected on October 29th.
Ovulation likely occurs around October 15th.
The calculator will recommend you start testing with Clearblue around October 11th (Cycle Day 11) to catch the rise in estrogen or LH.
Tips for Success
For the most accurate results, use your first morning urine, as it contains the highest concentration of hormones. If your cycle varies by more than 3 days each month, use your shortest cycle length from the last 6 months to determine when to start testing.
function calculateClearblueDate() {
var lmpInput = document.getElementById("lmpDate").value;
var cycleLength = parseInt(document.getElementById("cycleLength").value);
var resultDiv = document.getElementById("ovulationResult");
if (!lmpInput || isNaN(cycleLength)) {
alert("Please enter both your last period date and your cycle length.");
return;
}
if (cycleLength 45) {
alert("Please enter a cycle length between 20 and 45 days for accurate calculation.");
return;
}
var lmpDate = new Date(lmpInput);
// Logic for Clearblue Test Start Date
// Clearblue instruction manuals suggest testing based on cycle length
// Generally: Cycle Length – 17 days is a safe start for Advanced Digital
var daysToSubtract = 17;
if (cycleLength <= 21) daysToSubtract = 5;
else if (cycleLength <= 23) daysToSubtract = 6;
else if (cycleLength <= 25) daysToSubtract = 8;
else if (cycleLength <= 27) daysToSubtract = 10;
else if (cycleLength <= 30) daysToSubtract = 13;
else if (cycleLength <= 35) daysToSubtract = 18;
else if (cycleLength <= 40) daysToSubtract = 23;
else daysToSubtract = 25;
// Estimate Ovulation: Cycle Length – 14 days
var ovulationDayOffset = cycleLength – 14;
var testStart = new Date(lmpDate);
testStart.setDate(lmpDate.getDate() + (cycleLength – daysToSubtract));
var ovulationDate = new Date(lmpDate);
ovulationDate.setDate(lmpDate.getDate() + ovulationDayOffset);
var fertileStart = new Date(ovulationDate);
fertileStart.setDate(ovulationDate.getDate() – 5);
// Formating Dates
var options = { month: 'short', day: 'numeric', year: 'numeric' };
document.getElementById("testStartDate").innerHTML = testStart.toLocaleDateString(undefined, options);
document.getElementById("estOvulation").innerHTML = ovulationDate.toLocaleDateString(undefined, options);
document.getElementById("fertileWindow").innerHTML = fertileStart.toLocaleDateString(undefined, options) + " to " + ovulationDate.toLocaleDateString(undefined, options);
resultDiv.style.display = "block";
resultDiv.scrollIntoView({ behavior: 'smooth' });
}