Please enter your details to calculate your next period and ovulation dates.
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 (ovulation). At the same time, hormonal changes prepare the uterus for pregnancy. If ovulation takes place and the egg isn't fertilized by sperm, the lining of the uterus sheds, causing menstrual bleeding (your period).
Key Components of the Calculator:
Date of Last Menstrual Period Start: This is the most crucial starting point. It marks Day 1 of your cycle.
Average Cycle Length (days): This is the total number of days from the start of one period to the start of the next. The average is around 28 days, but it can vary significantly from person to person and even cycle to cycle, often ranging from 21 to 35 days.
Average Period Duration (days): This is how long your actual menstrual bleeding typically lasts. It's usually between 2 to 7 days.
How the Calculator Works (The Math):
This calculator uses basic date arithmetic and the information you provide to estimate key dates:
Next Period Start Date: This is calculated by adding your Average Cycle Length to the Date of Last Menstrual Period Start.
Next Period Start = Last Period Start Date + Average Cycle Length (days)
Estimated Ovulation Date: Ovulation typically occurs about 14 days before the start of your next period. A common approximation is to subtract 14 days from the calculated Next Period Start Date.
Estimated Ovulation = Next Period Start Date - 14 days
Please note that this is an estimation, as ovulation can vary.
Fertile Window: This is the period when pregnancy is possible. 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. Therefore, the fertile window typically includes the 5 days leading up to ovulation and the day of ovulation itself.
Fertile Window = Ovulation Date - 5 days to Ovulation Date
Period End Date: This is calculated by adding your Average Period Duration to the Date of Last Menstrual Period Start.
Period End Date = Last Period Start Date + Average Period Duration (days)
Important Disclaimer: This calculator is for informational and estimation purposes only. It is not a substitute for professional medical advice. Menstrual cycles can be irregular due to various factors including stress, illness, changes in medication, and underlying health conditions. For accurate tracking, fertility planning, or concerns about your cycle, please consult a healthcare provider.
function calculateCycle() {
var startDateInput = document.getElementById("lastPeriodStartDate");
var cycleLengthInput = document.getElementById("cycleLength");
var periodDurationInput = document.getElementById("periodDuration");
var resultDiv = document.getElementById("result");
var startDateStr = startDateInput.value;
var cycleLength = parseInt(cycleLengthInput.value);
var periodDuration = parseInt(periodDurationInput.value);
if (!startDateStr || isNaN(cycleLength) || isNaN(periodDuration) || cycleLength <= 0 || periodDuration <= 0) {
resultDiv.innerHTML = "Please enter valid details for all fields.";
return;
}
var startDate = new Date(startDateStr);
// — Calculations —
// Next Period Start Date
var nextPeriodStartDate = new Date(startDate);
nextPeriodStartDate.setDate(startDate.getDate() + cycleLength);
// Estimated Ovulation Date (approx. 14 days before next period start)
var estimatedOvulationDate = new Date(nextPeriodStartDate);
estimatedOvulationDate.setDate(nextPeriodStartDate.getDate() – 14);
// Fertile Window (5 days before ovulation to ovulation day)
var fertileWindowStart = new Date(estimatedOvulationDate);
fertileWindowStart.setDate(estimatedOvulationDate.getDate() – 5);
var fertileWindowEnd = new Date(estimatedOvulationDate); // Fertile window includes the ovulation day itself
// Period End Date
var periodEndDate = new Date(startDate);
periodEndDate.setDate(startDate.getDate() + periodDuration -1); // Subtract 1 because duration includes start day
// — Formatting Dates —
var options = { year: 'numeric', month: 'long', day: 'numeric' };
var df = new Intl.DateTimeFormat('en-US', options);
var formattedStartDate = df.format(startDate);
var formattedNextPeriodStartDate = df.format(nextPeriodStartDate);
var formattedEstimatedOvulationDate = df.format(estimatedOvulationDate);
var formattedFertileWindowStart = df.format(fertileWindowStart);
var formattedFertileWindowEnd = df.format(fertileWindowEnd);
var formattedPeriodEndDate = df.format(periodEndDate);
// — Display Result —
resultDiv.innerHTML = `
Last Period Start: ${formattedStartDate}
Estimated Period End: ${formattedPeriodEndDate}
Next Period Start: ${formattedNextPeriodStartDate}
Estimated Ovulation: ${formattedEstimatedOvulationDate}
Estimated Fertile Window: ${formattedFertileWindowStart} – ${formattedFertileWindowEnd}
`;
}