Ovulation & Period Calculator

Estimate when you are most likely to be fertile using nothing more than the first day of your last period and your average cycle length. This calculator projects your ovulation day, your fertile window and the start date of your next few periods, so you can plan whether you are trying to conceive or simply tracking your cycle. It is an estimate based on average cycles, not a diagnosis or a method of contraception. Everything runs in your browser, with no sign-up and nothing sent to a server.

First day of your last period *

Pick the first day you started bleeding in your most recent period.

Average cycle length

days

Days from the first day of one period to the first day of the next (usually 21–35, default 28).

Period length

days

How many days your bleeding usually lasts (1–10, default 5).

This is only an estimate based on average cycles and is NOT a reliable method of contraception or fertility treatment. Actual ovulation can vary. Talk to a doctor or use tracking methods such as ovulation tests for anything important.

Your fertile window will appear here

Enter the first day of your last period to estimate ovulation and your next period.

Estimated ovulation

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Fertile window

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Next period

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Next cycles

Cycle	Period starts	Fertile window	Ovulation

Projected using the same cycle length for each upcoming cycle. Real cycles vary from month to month.

About "safe days"

Days outside the fertile window are sometimes called "safe days", but sperm can survive up to 5 days and ovulation timing shifts, so no day is truly safe. Do not rely on this to avoid pregnancy — use a proven contraceptive method instead.

How to use

Pick the first day of your most recent period using the date picker.
Enter your average cycle length in days (the default is 28; most cycles fall between 21 and 35).
Enter how many days your period usually lasts (the default is 5).
Read your estimated ovulation day, fertile window and next period at the top of the results.
Check the table to see the projected dates for the next few cycles.
Use Copy summary to save or share the dates, and read the disclaimer before relying on them.

Features

Estimated ovulation day based on your cycle length
Fertile window from about 5 days before to 1 day after ovulation
Next period date, plus a projection for the next three cycles
Adjustable cycle length (21–35 days) and period length
Clear "safe days" explanation and a strong contraception disclaimer
100% in-browser — your cycle data never leaves your device

Frequently asked questions

How is my ovulation day calculated?

The tool assumes ovulation happens about 14 days before your next period starts. It adds your cycle length to the first day of your last period to estimate the next period, then counts back 14 days. This luteal-phase estimate is the standard calendar method, but your real ovulation day can shift by several days.

What is the fertile window?

The fertile window is the range of days when conception is most likely. Because sperm can survive several days and the egg lives about a day, this tool marks the window from roughly 5 days before ovulation to 1 day after it. Having intercourse during this window gives the highest chance of pregnancy.

Can I use this to avoid getting pregnant?

No. This calculator is not a reliable form of birth control. Sperm can live up to 5 days, ovulation timing varies, and irregular cycles make the "safe days" unreliable. If you want to prevent pregnancy, use a proven contraceptive method and talk to a healthcare professional.

What if my cycles are irregular?

The calendar method works best for regular cycles. If your cycle length changes a lot from month to month, the estimates will be less accurate. Consider tracking your cycle over several months, using ovulation predictor kits, or asking a doctor for guidance.

Is my data private?

Yes. The calculation runs entirely in your browser. Your period dates and cycle details are never uploaded, stored, or shared with any server.