Period Calculator

A period calculator estimates when your next period may start and end based on the first day of your last period, average cycle length, and typical period length. It can also connect period timing to estimated ovulation and fertile window dates.

The period calculator is designed for cycle awareness and planning. It works best when your cycles are reasonably consistent.

How to Use the Period Calculator

1. Enter the first day of your last period.
2. Enter your average cycle length.
3. Enter your usual period length.
4. Choose how many cycles you want to forecast.
5. Review the next period estimate, future period calendar, ovulation date, and fertile window.

Formula Used

Next period start = last period start + average cycle length
Next period end = next period start + period length - 1
Future period start = last period start + (cycle length x cycle number)
Estimated ovulation = last period start + (cycle length - luteal phase length)

The end date uses period length minus one because the start date counts as day one.

Example

If your last period started on June 1, 2026, your cycle is 28 days, and your period usually lasts 5 days, the next period estimate starts on June 29 and ends on July 3.

What Your Result Means

Your result gives a calendar-style estimate for the next period and future periods. It also shows the estimated ovulation and fertile window for cycle context.

Accuracy and Limitations

Period timing can shift because of stress, illness, travel, weight change, medication, postpartum changes, breastfeeding, or health conditions. A late or missed period can have many causes, so seek care if you have symptoms or concerns.

Related Calculators

• Ovulation Calculator
• Fertile Window Calculator
• Pregnancy Test Calculator
• Implantation Calculator

Methodology and Disclaimer

This calculator uses average cycle length and period length to forecast dates. It provides educational estimates only. It is not medical advice, diagnosis, contraception guidance, or a substitute for a qualified healthcare professional.

FAQ

How does the period calculator estimate my next period?

It adds your average cycle length to the first day of your last period.

Why is the period end date calculated with minus one day?

The start date is counted as the first day of bleeding, so a five-day period runs from day one through day five.

Can the calculator predict an exact start date?

No. It estimates timing based on averages, and real cycles can shift.

What if my period is late?

A late period can happen for many reasons. Consider a pregnancy test when relevant and seek professional guidance for concerns.

Can this work for irregular cycles?

It can provide a rough estimate, but irregular cycles need wider date ranges and cautious interpretation.