Period Calculator

Estimate future period dates, ovulation timing and a fertile window from a recent period start and average cycle pattern. Results are calendar estimates only.

First day of last period

Average cycle length (days)

Period duration (days)

Calendar predictions cannot diagnose cycle problems or confirm ovulation. Adult cycles outside 21–35 days or bleeding longer than 7 days may merit professional advice, especially if new or symptomatic.

About This Tool

What it does

Adds the entered average cycle length to the last period start, projects the entered bleeding duration, estimates ovulation 14 days before the next period and displays a seven-day fertile window.

Who it's for

People tracking a recent cycle pattern. Predictions are less reliable with irregular cycles, hormonal contraception, postpartum changes, perimenopause or illness.

Your privacy

All calculations happen entirely in your browser. No dates, health data or personal information is transmitted or stored anywhere. ToolBullet never collects your health data.

How period predictions are estimated

The first day of bleeding is cycle day one. The calculator adds the average cycle length to estimate the next start, then estimates ovulation 14 days before that date. The fertile window is shown from five days before through one day after estimated ovulation.

Adult cycles commonly fall around 21–35 days and bleeding commonly lasts up to 7 days, but variation occurs. Persistent or new changes, very heavy bleeding, severe pain or pregnancy concerns require professional assessment.

Authoritative references

Reviewed 23 June 2026: World Health Organization: menstrual health; ACOG: abnormal uterine bleeding; ACOG: fertility-awareness methods.

Helpful guides

Ovulation, fertile windows and due datesSee how the three estimates relate and where their limitations begin.Read guide →

Frequently Asked Questions

How is the next period estimated?

The calculator adds the average cycle length to the first day of the last period. It then applies the entered period duration to estimate the end date.

How are ovulation and the fertile window estimated?

Ovulation is placed 14 days before the next expected period. The displayed fertile window runs from five days before through one day after that estimated date.

What cycle ranges are common in adults?

Adult cycles commonly fall around 21–35 days and bleeding commonly lasts up to 7 days, although individual variation occurs.

Can the calculator diagnose an irregular cycle?

No. It only projects dates from the numbers entered. Persistent changes, severe pain, very heavy bleeding or other concerns require professional assessment.

Can I use the prediction for contraception?

No. Calendar estimates cannot guarantee non-fertile days and should not be the sole method used to prevent pregnancy.