Period Calculator

Predict next period, fertile window, and ovulation day from cycle length.
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Use the Period Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only and does not constitute tax, financial, or legal advice. Results are estimates based on the information you provide and current rates. Always consult a qualified tax professional or financial advisor for advice specific to your situation.

How It Works

Our Period Calculator helps you anticipate your next period, identify your fertile window, and pinpoint your ovulation day based on your typical cycle length. Understanding these key dates is invaluable for family planning, managing period symptoms, or simply gaining a better grasp of your body's rhythm. For instance, if your last period started on January 10th, 2026, and you have a 28-day cycle, this tool will predict your next period around February 7th, 2026.

This calculator employs a straightforward, widely accepted methodology. We take your last period start date and add your average cycle length to predict your next period. Ovulation is typically estimated to occur 14 days before your next predicted period, while the fertile window generally spans 5 days before ovulation, the day of ovulation, and one day after, totaling a 6-day window.

Remember, these are predictions and individual cycles can vary due to stress, diet, or other factors. While the calculator provides a valuable estimate, it's not a substitute for professional medical advice or more precise tracking methods like basal body temperature or ovulation predictor kits. Also, an irregular cycle length will yield less accurate predictions, so it's best to use your average over several months.

Example: Planning for a February 2026 Conception

  1. 1 Let's say your last period started on January 15th, 2026, and your average cycle length is 30 days.
  2. 2 Based on these inputs, your next period is predicted for February 14th, 2026. Counting back 14 days, your ovulation day is estimated to be February 1st, 2026. Your fertile window would then be from January 27th to February 2nd, 2026.
  3. 3 Predicted Next Period: February 14th, 2026. Predicted Ovulation Day: February 1st, 2026. Predicted Fertile Window: January 27th - February 2nd, 2026.
  4. 4 This example shows how the calculator quickly provides the crucial dates for someone aiming to conceive in early 2026, allowing them to plan intercourse during their most fertile window. It also gives a heads-up for when to expect their next period, helping with symptom management.

Source: CDC · Last updated: April 2026

Frequently Asked Questions

How do you predict your next period?
Count the days from the first day of one period to the first day of the next (your cycle length). Add your cycle length to the first day of your last period to estimate the next one. Average cycle length is 28 days but 21-35 days is normal.
When is the fertile window in a menstrual cycle?
The fertile window is approximately 5 days before ovulation through 1 day after. Ovulation typically occurs 14 days before the next period starts. For a 28-day cycle, ovulation is around day 14.
What causes irregular periods?
Common causes include stress, weight changes, excessive exercise, hormonal imbalances (PCOS, thyroid issues), perimenopause, and certain medications. Cycles varying by more than 7-9 days are considered irregular.

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