Health & Fitness

Period Calculator

Predicts next 3 periods

Shows ovulation and fertile window

Based on your cycle length

For planning and educational purposes only. Results are estimates and not medical advice.

Enter Your Cycle DetailsFree · Private

First Day of Last Period

Cycle Length (days)

days

Average is 28 days (range: 21-45)

Period Duration (days)

days

Average is 5 days (range: 3-7)

How the Period Calculator Works

This calculator predicts your next periods and fertility window based on two inputs: the first day of your last period and your average cycle length. It uses the standard menstrual cycle model to estimate key dates.

Next period = Last period start + Cycle length
Ovulation = Next period start - 14 days
Fertile window = Ovulation - 5 days to Ovulation day

Who Is This Calculator For?

Cycle tracking

Keep track of when to expect your period so you can plan ahead and be prepared.

Trying to conceive

See your estimated ovulation date and fertile window to time intercourse more accurately.

Trying to avoid pregnancy

Understand your cycle pattern. Note: this is not a contraceptive method - see an FAQs answer below.

Health awareness

Notice whether your cycles are consistent or irregular, which can signal hormonal changes worth discussing with a doctor.

When Should You Use It?

To predict your next period date for travel, events, or sports planning
To estimate when you will be most fertile if trying to conceive
To check whether your cycle length is within the normal 21-35 day range
To track how consistent your cycles are over several months

How to Use the Calculator

1
Enter the first day of your last period
This is the day bleeding started, not when it ended. Use a date picker or type the date in MM/DD/YYYY format.
2
Enter your average cycle length
Count the days from the first day of one period to the first day of the next. If you are not sure, use 28. If you have been tracking, use your average over the last 3-6 months.
3
Enter your period duration
How many days your period typically lasts. The average is 5 days. This affects the end date shown for each predicted period.
4
Read your results
See your next period date, estimated ovulation day, fertile window, and the next 3 predicted cycles. All dates update instantly as you change inputs.

Example Calculation

Last period started April 1, 2025. Cycle length: 28 days. Period duration: 5 days.

Next period = April 1 + 28 days = April 29, 2025
Period ends = April 29 + 4 = May 3, 2025
Ovulation = April 29 - 14 = April 15, 2025
Fertile window = April 10 - April 15

What Does the Result Mean?

The predicted date is an estimate based on a regular cycle. Most people's cycles vary by a few days from month to month. A period arriving 1-3 days early or late is completely normal.

Medical disclaimer: This tool provides estimates for informational purposes only. It does not diagnose pregnancy, fertility issues, or menstrual disorders. Consult a healthcare provider for medical concerns.

Common Mistakes to Avoid

Entering the last day of your period instead of the first

Always use the first day bleeding started. Using the last day will push all predictions later by your period duration.

Using a single cycle to determine your average

One cycle's length is not necessarily your average. Track 3-6 cycles and calculate the mean for more accurate predictions.

Treating predicted dates as guaranteed

Stress, travel, illness, weight changes, and medications can all shift your cycle. Treat predictions as a window, not an exact date.

Assuming ovulation always occurs on day 14

Day 14 from the last period is only accurate for a 28-day cycle. For longer cycles, ovulation happens later. This calculator adjusts automatically based on your cycle length.

Frequently Asked Questions

This calculator assumes a regular cycle based on your entered cycle length. Actual periods vary due to stress, illness, weight changes, travel, and hormonal shifts. It is a planning tool, not a medical prediction. Cycles shorter than 21 days or longer than 45 days may indicate an underlying condition worth discussing with a doctor.

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Sources & References

ACOG (American College of Obstetricians and Gynecologists) - Menstruation in Girls and Adolescents

Source for normal cycle length range of 21-45 days in adolescents and 21-35 days in adults, and normal period duration of 2-7 days.

Wilcox AJ et al. (2000) - Timing of Sexual Intercourse in Relation to Ovulation, NEJM

Source for the 6-day fertile window calculation: ovulation day plus the 5 days preceding it.

NHS (UK National Health Service) - Periods and the Menstrual Cycle

Source for general cycle guidance including average cycle length of 28 days and the 14-day luteal phase model used for ovulation estimation.

Dr. Sarah Kim, MD

OB-GYN, 8 years in reproductive and women's health

Dr. Kim reviewed the cycle calculation logic, ovulation model, fertile window calculation, and all medical disclaimers on this page. She practices at a women's health clinic and specializes in menstrual health and reproductive planning.

Reviewed: March 2025Last updated: April 2025

Cycle Reference

Metric	Normal Range
Cycle length	21-35 days
Period duration	2-7 days
Luteal phase	10-17 days
Avg cycle length	28 days
Fertile window	6 days/cycle
Egg survival	12-24 hours
Sperm survival	3-5 days

Medical Disclaimer

This calculator provides estimates only. It is not a contraceptive method and does not constitute medical advice. Always consult a licensed healthcare provider for concerns about your cycle or reproductive health.

Pro Tip

Track your cycle start dates over 3-6 months to find your true average. Most people's cycles vary by 1-3 days month to month. A reliable average improves prediction accuracy significantly.

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