Next Period Start Date:—Estimated Ovulation Date:—Estimated Period End Date:—
Understanding Your Menstrual Cycle
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 in a process called ovulation. If fertilization doesn't occur, the uterus lining sheds, which is the menstrual period.
This calculator helps estimate key dates in your cycle based on the start date of your last menstrual period (LMP), your average cycle length, and your average period length.
How the Calculations Work:
Next Period Start Date: This is calculated by adding your average cycle length (from the start of one period to the start of the next) to the start date of your last menstrual period (LMP).
Formula: Next Period Start Date = LMP + (Average Cycle Length – 1) days
(We subtract 1 day because the LMP is day 1 of the cycle).
Estimated Ovulation Date: Ovulation typically occurs about 14 days before the start of your next period. This is a common estimation point, though it can vary.
Formula: Estimated Ovulation Date = Next Period Start Date – 14 days
Estimated Period End Date: This is calculated by adding your average period length to the start date of your last menstrual period.
Formula: Estimated Period End Date = LMP + (Average Period Length – 1) days
(We subtract 1 day because the LMP is day 1 of the period).
Important Considerations:
Variability: Menstrual cycles can vary significantly from person to person and even month to month. Factors like stress, illness, diet, and changes in routine can affect your cycle.
Estimation Tool: This calculator provides estimates. It is not a substitute for professional medical advice or for precise fertility tracking.
Cycle Length Definition: Cycle length is typically measured from the first day of bleeding of one period to the first day of bleeding of the next period.
Ovulation Timing: While ovulation often occurs around day 14 of a 28-day cycle, this is just an average. For longer or shorter cycles, the 14-day countdown to the next period is a more reliable indicator for estimation.
Use this calculator as a helpful tool to become more aware of your body's patterns. For personalized health information, always consult with a healthcare provider.
function calculateMenstrualCycle() {
var startDateInput = document.getElementById("startDate");
var cycleLengthInput = document.getElementById("cycleLength");
var periodLengthInput = document.getElementById("periodLength");
var cycleResultSpan = document.getElementById("cycleResult");
var ovulationResultSpan = document.getElementById("ovulationResult");
var periodEndDateSpan = document.getElementById("periodEndDate");
// Clear previous results
cycleResultSpan.textContent = "–";
ovulationResultSpan.textContent = "–";
periodEndDateSpan.textContent = "–";
var startDateStr = startDateInput.value;
var cycleLength = parseInt(cycleLengthInput.value);
var periodLength = parseInt(periodLengthInput.value);
// — Input Validation —
if (!startDateStr) {
alert("Please enter the start date of your last period.");
return;
}
if (isNaN(cycleLength) || cycleLength 40) {
alert("Please enter a valid average cycle length between 14 and 40 days.");
return;
}
if (isNaN(periodLength) || periodLength 7) {
alert("Please enter a valid average period length between 2 and 7 days.");
return;
}
// — Calculations —
var startDate = new Date(startDateStr);
// Calculate Next Period Start Date
var nextPeriodStartDate = new Date(startDate);
nextPeriodStartDate.setDate(startDate.getDate() + cycleLength – 1); // Add (cycleLength – 1) days
// Calculate Estimated Ovulation Date (approx. 14 days before next period)
var ovulationDate = new Date(nextPeriodStartDate);
ovulationDate.setDate(nextPeriodStartDate.getDate() – 14);
// Calculate Estimated Period End Date
var periodEndDate = new Date(startDate);
periodEndDate.setDate(startDate.getDate() + periodLength – 1); // Add (periodLength – 1) days
// — Formatting Dates —
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var formattedNextPeriodStartDate = nextPeriodStartDate.toLocaleDateString(undefined, options);
var formattedOvulationDate = ovulationDate.toLocaleDateString(undefined, options);
var formattedPeriodEndDate = periodEndDate.toLocaleDateString(undefined, options);
// — Display Results —
cycleResultSpan.textContent = formattedNextPeriodStartDate;
ovulationResultSpan.textContent = formattedOvulationDate;
periodEndDateSpan.textContent = formattedPeriodEndDate;
}