Period Calculator

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Period Calculator
Next Period DateOvulation & Fertile WindowEstimated Due Date (Pregnancy)
Analysis Results:
Please enter your dates above.
function calculatePeriod(){var dateInput=document.getElementById('last_period').value;var cycleLen=parseInt(document.getElementById('cycle_length').value);var periodDur=parseInt(document.getElementById('period_duration').value);var calcType=document.getElementById('calc_type').value;var showSteps=document.getElementById('show_steps').checked;if(!dateInput||isNaN(cycleLen)||isNaN(periodDur)){alert('Please fill in all fields with valid data.');return;}var lastDate=new Date(dateInput);var resultHTML=";var options={weekday:'long',year:'numeric',month:'long',day:'numeric'};if(calcType==='next_period'){var nextDate=new Date(lastDate);nextDate.setDate(lastDate.getDate()+cycleLen);resultHTML='Next Period Starts: '+nextDate.toLocaleDateString(undefined,options)+'';if(showSteps){resultHTML+='
Calculation: '+dateInput+' + '+cycleLen+' days cycle.
';}}else if(calcType==='ovulation'){var ovulDate=new Date(lastDate);ovulDate.setDate(lastDate.getDate()+(cycleLen-14));var fertileStart=new Date(ovulDate);fertileStart.setDate(ovulDate.getDate()-3);var fertileEnd=new Date(ovulDate);fertileEnd.setDate(ovulDate.getDate()+1);resultHTML='Estimated Ovulation Day: '+ovulDate.toLocaleDateString(undefined,options)+'
';resultHTML+='Fertile Window: '+fertileStart.toLocaleDateString(undefined,{month:'short',day:'numeric'})+' to '+fertileEnd.toLocaleDateString(undefined,{month:'short',day:'numeric'});}else if(calcType==='due_date'){var dueDate=new Date(lastDate);dueDate.setDate(lastDate.getDate()+280);resultHTML='Estimated Due Date: '+dueDate.toLocaleDateString(undefined,options)+'
';resultHTML+='
Based on a standard 40-week (280 day) gestation period.
';}document.getElementById('resultValue').innerHTML=resultHTML;}

How to Use the Period Calculator

Tracking your menstrual cycle is essential for reproductive health, family planning, and general wellness. This period calculator helps you predict your next cycle start date, identify your fertile window, and estimate a pregnancy due date based on your biological data.

To get the most accurate results, you should have a record of the first day of your last three menstrual cycles to determine your average cycle length.

First Day of Last Period
The date when your most recent bleeding actually began (Day 1 of your cycle).
Average Cycle Length
The number of days between the first day of one period and the first day of the next. The average is 28 days, but 21 to 35 days is considered normal.
Period Duration
How many days your bleeding typically lasts (usually 3 to 7 days).

How It Works: The Science of Your Cycle

The period calculator uses the "LMP" (Last Menstrual Period) method combined with your average cycle length to project future dates. The menstrual cycle consists of four main phases: Menstruation, Follicular Phase, Ovulation, and the Luteal Phase.

Next Period Date = Date of Last Period + Cycle Length (in days)

  • Follicular Phase: Starts on Day 1 and ends with ovulation.
  • Ovulation: Occurs roughly 14 days before your next period starts.
  • Luteal Phase: The time between ovulation and the start of your next period.
  • Fertile Window: The 5 days leading up to ovulation plus the day of ovulation itself.

Calculation Example

Example Scenario: Sarah's last period began on January 1st. She has a consistent 30-day cycle.

Step-by-step prediction:

  1. Last Period Start: January 1st
  2. Cycle Length: 30 days
  3. Calculation (Next Period): January 1 + 30 days = January 31st
  4. Calculation (Ovulation): Next Period (Jan 31) – 14 days = January 17th
  5. Fertile Window: January 14th to January 18th

Common Questions about Menstrual Cycles

How accurate is a period calculator?

A period calculator is highly accurate for individuals with regular cycles. However, stress, diet, exercise, and health conditions (like PCOS) can cause cycle variations. It is a predictive tool, not a medical diagnosis.

What is a "normal" cycle length?

While the global average is 28 days, "normal" ranges from 21 to 35 days in adults and 21 to 45 days in young teens. Consistency is usually more important than the specific number of days.

Can I use this to prevent pregnancy?

While the period calculator identifies your fertile window, it should not be used as your primary method of birth control. Sperm can live inside the body for up to 5 days, and ovulation timing can shift unexpectedly.

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