Period Calculator

What this period calculator does

This calculator predicts the next six period starts, the length of each period, and the estimated ovulation window for each cycle. It uses the first day of your last period, your average cycle length, and how many days your period usually lasts.

The prediction assumes a regular cycle. Real cycles vary, and stress, illness, travel, and hormonal changes can shift the dates.

How to use this calculator

1. Enter the first day of your last period.
2. Enter your average cycle length.
3. Enter how many days your period usually lasts.
4. Click Calculate to see the next six cycles.

The default example uses today's date, a 28-day cycle, and a 5-day period.

Period prediction formulas

Cycle start = last period + (cycle number × cycle length)
Period end = cycle start + period length − 1
Ovulation = cycle start + cycle length − 14
Ovulation window = ovulation − 2 to +2 days

What the results mean

• Period: the predicted start and end dates for each cycle.
• Ovulation window: the estimated fertile days for that cycle.

The table is a planning aid. It is not a medical record and does not confirm that ovulation occurred.

Assumptions and limitations

The calculator assumes a regular cycle and a consistent period length. It does not account for irregular cycles, hormonal conditions, pregnancy, breastfeeding, or contraception. It is not a contraceptive method.

If your cycles are consistently shorter than 21 days or longer than 35 days, or if your bleeding pattern changes, discuss it with a clinician.

Frequently asked questions

How accurate is the next-period prediction?

It is a calendar estimate. It is most accurate for regular cycles and less accurate when cycle length varies.

Why does the ovulation window change each cycle?

The window moves with the predicted cycle start. The calculator uses a 14-day luteal phase for each cycle.

Can I use this to avoid pregnancy?

No. Calendar-based predictions are not reliable contraception.

What if my period is late?

A late period can have many causes, including stress, illness, and pregnancy. Follow the advice of a clinician or take a pregnancy test according to its instructions.

Source

References:

• ACOG — Fertility awareness
• NICHD — Pregnancy

Disclaimer

This calculator is for informational and educational purposes only. It is not medical advice or contraception. Discuss cycle changes with a qualified clinician.