Can I use this as birth control?

No. Cycle-length-based prediction is not a reliable form of contraception -- individual cycles vary, and this tool doesn't account for that variation. Talk to a healthcare professional about contraception options.

What if my cycles are irregular?

The further your actual cycle length varies from the average you entered, the less reliable every date here becomes -- this is most useful for people with fairly regular cycles.

Why does the fertile window extend before ovulation, not just on the day itself?

Sperm can survive in the reproductive tract for several days, so the fertile window starts before ovulation, not just on the day the egg is released.

Does this tool store my cycle data anywhere?

No -- like every calculator on this site, everything runs client-side in your browser and nothing you enter is transmitted or stored.

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Health & Fitness

Period Calculator

Predict your next period, ovulation day, and fertile window from your cycle.

🔒 Runs entirely in your browser — nothing here is ever uploaded

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Last reviewed 2026-09-05

About the Period Calculator

Predicts your next period start date, estimated ovulation day, and fertile window from your last period's start date, average cycle length, and average period length.

100% Free Runs in Your Browser No Sign-Up Required

How to use it

Enter the first day of your last period.
Enter your average cycle length and period length in days.
Read your next period, ovulation, and fertile-window estimates, plus the two cycles after that.

Formula

Next period = last period start + cycle length
Ovulation ≈ next period − 14 days
Fertile window ≈ 5 days before ovulation through 1 day after

Same luteal-phase convention as this site's Ovulation Calculator, per ACOG's fertility-awareness guidance: the luteal phase (ovulation to next period) is typically 14 days.

Worked example

A last period starting August 1 with a 28-day cycle puts the next period around August 29, estimated ovulation around August 15, and a fertile window of roughly August 10-16.

Recommendations

• Cycle length varies month to month for most people -- longer-range projections (the 2nd and 3rd cycle out) become less reliable the further out they go.
• This calculator assumes a consistent cycle length equal to your entered average -- irregular cycles make every estimate here less reliable.
• Ovulation predictor kits or basal body temperature tracking give more real-time, cycle-specific signals than a date projection alone.

Frequently asked questions

They use the same underlying luteal-phase math, but this tool is framed around period tracking -- it also shows your period's estimated end date and projects out two additional future cycles, which the Ovulation Calculator doesn't.